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The HTTP server libraries

3 The HTTP server libraries

The HTTP server infra structure consists of a number of small modular libraries that are combined into library(http/http_server). These modules are:

library(http/thread_httpd)
This library is responsible for accepting and managing connections.2In older versions there were two alternative libraries for managing connections based on XPCE and Unix inetd.
library(http/http_dyn_workers)
This library dynamically adds and removes workers based on the workload of the server.
library(http/http_wrapper)
This library takes a connection, parses the HTTP request header and runs a goal that produces a CGI document based on the parsed request. It watches for exceptions and turns these into (error) status pages. The status page generation may be hooked to provide custom pages.
library(http/http_dispatch)
This library associates the path of the HTTP request with a handler that services this particular path. It also manages timeouts and may pass the execution of a request to a dedicated thread with specified resource limits using http_spawn/2. The module supports plugable request rewrite handlers that may be used to implement identification, authorization, input argument processing, etc.
library(http/http_parameters)
This library parses HTTP request parameters, both dealing with GET and POST style parameter passing.
library(http/html_write)
This library translates a Prolog term into an HTML document using Prolog grammar rules (DCG). It provides a modular infrastructure to build pages that are guaranteed to be valid HTML. The HTTP server libraries provide several alternatives for generating HTML ranging from simple printing to current_output to XML-based templates (PWP).
library(http/http_json)
This library parses a POSTed HTTP document into a Prolog dict and formulates an HTTP JSON reply from a Prolog dict and is typically used to implement REST services.

Most server implementation simply load the library(http/http_server) library, which loads the above modules and reexports all predicates except for those used for internal communication and older deprecated predicates. Specific use cases may load a subset of the individual libraries and may decide to replace one or more of them.

A typical skeleton for building a server is given below. If this file is loaded as main file (using e.g., swipl server.pl) it creates a simple server that listens on port 8080. If the root is accessed it redirects to the home page and shows Hello world!. 

:- use_module(library(http/http_server)).

:- initialization
    http_server([port(8080)]).

:- http_handler(root(.),
                http_redirect(moved, location_by_id(home_page)),
                []).
:- http_handler(root(home), home_page, []).

home_page(_Request) :-
    reply_html_page(
        title('Demo server'),
        [ h1('Hello world!')
        ]).

3.1 Creating an HTTP reply

The handler (e.g., home_page/1 above) is called with the parsed request (see section 3.13) as argument and current_output set to a temporary buffer. Its task is closely related to the task of a CGI script; it must write a header declaring at least the Content-type field and a body. Below is a simple body writing the request as an HTML table.3Note that writing an HTML reply this way is deprecated. In fact, the code is subject to injection attacks as the HTTP request field values are literally injected in the output while HTML reserved characters should be properly escaped. 

reply(Request) :-
        format('Content-type: text/html~n~n', []),
        format('<html>~n', []),
        format('<table border=1>~n'),
        print_request(Request),
        format('~n</table>~n'),
        format('</html>~n', []).

print_request([]).
print_request([H|T]) :-
        H =.. [Name, Value],
        format('<tr><td>~w<td>~w~n', [Name, Value]),
        print_request(T).

The infrastructure recognises the header fields described below. Other header lines are passed verbatim to the client. Typical examples are Set-Cookie and authentication headers (see section 3.7).

Content-type: Type
This field is passed to the client and used by the infrastructure to determine the encoding to use for the stream. If type matches text/* or the type matches with UTF-8 (case insensitive), the server uses UTF-8 encoding. The user may force UTF-8 encoding for arbitrary content types by adding ; charset=UTF-8 to the end of the Content-type header.
Transfer-encoding: chunked
Causes the server to use chunked encoding if the client allows for it. See also section 5 and the chunked option in http_handler/3.
Connection: close
Causes the connection to be closed after the transfer. The default is to keep it open‘Keep-Alive’if possible.
Location: URL
This header may be combined with the Status header to force a redirect response to the given URL. The message body must be empty. Handling this header is primarily intended for compatibility with the CGI conventions. Prolog code should use http_redirect/3.
Status: Status
This header can be combined with Location, where Status must be one of 301 (moved), 302 (moved temporary, default) or 303 (see other). Using the status field also allows for formulating replies such as 201 (created).

Note that the handler may send any type of document instead of HTML. After the header has been written, the encoding of the current_output stream encoding is established as follows:

  1. If the content type is text/* the stream is switched to UTF-8 encoding. If the content type does not provide attributes, ; charset=UTF-8 is added.
  2. The content type contains UTF-8 the stream is switched to UTF-8 encoding.
  3. http:mime_type_encoding/2 succeeds the returned encoding is used. The returned encoding must be valid for set_stream/2.
  4. If the content type matches a list of known encodings, this is used. See mime_type_encoding/2 is http_header. The current list deals with JSON, Turtle and SPARQL.
  5. Otherwise the stream uses octed (binary) encoding.

3.1.1 Returning special status codes

Besides returning a page by writing it to the current output stream, the server goal can raise an exception using throw/1 to generate special pages such as not_found, moved, etc. The defined exceptions are:

http_reply(+Reply, +HdrExtra, +Context)
Return a result page using http_reply/3. See http_reply/3 for supported values for Reply and section 3.10 for providing a custom error page.
http_reply(+Reply, +HdrExtra)
Return a result page using http_reply/3. Equivalent to http_reply(Reply, HdrExtra,[]).
http_reply(+Reply)
Equivalent to http_reply(Reply, [],[]).
http(not_modified)
Equivalent to http_reply(not_modified,[]). This exception is for backward compatibility and can be used by the server to indicate the referenced resource has not been modified since it was requested last time.

In addition, the normal "200 OK" reply status may be overruled by writing a CGI Status header prior to the remainder of the message. This is particularly useful for defining REST APIs. The following handler replies with a "201 Created" header: 

handle_request(Request) :-
        process_data(Request, Id),      % application predicate
        format('Status: 201~n'),
        format('Content-type: text/plain~n~n'),
        format('Created object as ~q~n', [Id]).

3.2 library(http/http_dispatch): Dispatch requests in the HTTP server

Most code doesn't need to use this directly; instead use library(http/http_server), which combines this library with the typical HTTP libraries that most servers need.

This module can be placed between http_wrapper.pl and the application code to associate HTTP locations to predicates that serve the pages. In addition, it associates parameters with locations that deal with timeout handling and user authentication. The typical setup is: 

server(Port, Options) :-
        http_server(http_dispatch,
                    [ port(Port)
                    | Options
                    ]).

:- http_handler('/index.html', write_index, []).

write_index(Request) :-
        ...
[det]http_handler(+Path, :Closure, +Options)
Register Closure as a handler for HTTP requests. Path is either an absolute path such as '/home.html' or a term Alias(Relative). Where Alias is associated with a concrete path using http:location/3 and resolved using http_absolute_location/3. Relative can be a single atom or a term‘Segment1/Segment2/...`, where each element is either an atom or a variable. If a segment is a variable it matches any segment and the binding may be passed to the closure. If the last segment is a variable it may match multiple segments. This allows registering REST paths, for example: 
:- http_handler(root(user/User), user(Method, User),
                [ method(Method),
                  methods([get,post,put])
                ]).

user(get, User, Request) :-
    ...
user(post, User, Request) :-
    ...

If an HTTP request arrives at the server that matches Path, Closure is called as below, where Request is the parsed HTTP request. 

call(Closure, Request)

Options is a list containing the following options:

authentication(+Type)
Demand authentication. Authentication methods are pluggable. The library http_authenticate.pl provides a plugin for user/password based Basic HTTP authentication.
chunked
Use Transfer-encoding: chunked if the client allows for it.
condition(:Goal)
If present, the handler is ignored if Goal does not succeed.
content_type(+Term)
Specifies the content-type of the reply. This value is currently not used by this library. It enhances the reflexive capabilities of this library through http_current_handler/3.
id(+Atom)
Identifier of the handler. The default identifier is the predicate name. Used by http_location_by_id/2 and http_link_to_id/3.
hide_children(+Bool)
If true on a prefix-handler (see prefix), possible children are masked. This can be used to (temporary) overrule part of the tree.
method(+Method)
Declare that the handler processes Method. This is equivalent to methods([Method]). Using method(*) allows for all methods.
methods(+ListOfMethods)
Declare that the handler processes all of the given methods. If this option appears multiple times, the methods are combined.
prefix
Call Pred on any location that is a specialisation of Path. If multiple handlers match, the one with the longest path is used. Options defined with a prefix handler are the default options for paths that start with this prefix. Note that the handler acts as a fallback handler for the tree below it: 
:- http_handler(/, http_404([index('index.html')]),
                [spawn(my_pool),prefix]).
priority(+Integer)
If two handlers handle the same path, the one with the highest priority is used. If equal, the last registered is used. Please be aware that the order of clauses in multifile predicates can change due to reloading files. The default priority is 0 (zero).
spawn(+SpawnOptions)
Run the handler in a separate thread. If SpawnOptions is an atom, it is interpreted as a thread pool name (see create_thread_pool/3). Otherwise the options are passed to http_spawn/2 and from there to thread_create/3. These options are typically used to set the stack limits.
time_limit(+Spec)
One of infinite, default or a positive number (seconds). If default, the value from the setting http:time_limit is taken. The default of this setting is 300 (5 minutes). See setting/2.

Note that http_handler/3 is normally invoked as a directive and processed using term-expansion. Using term-expansion ensures proper update through make/0 when the specification is modified.

Errors
- existence_error(http_location, Location)
- permission_error(http_method, Method, Location)
See also
http_reply_file/3 and http_redirect/3 are generic handlers to serve files and achieve redirects.
[det]http_delete_handler(+Spec)
Delete handler for Spec. Typically, this should only be used for handlers that are registered dynamically. Spec is one of:
id(Id)
Delete a handler with the given id. The default id is the handler-predicate-name.
path(Path)
Delete handler that serves the given path.
[det]http_dispatch(Request)
Dispatch a Request using http_handler/3 registrations. It performs the following steps:

  1. Find a matching handler based on the path member of Request. If multiple handlers match due to the prefix option or variables in path segments (see http_handler/3), the longest specification is used. If multiple specifications of equal length match the one with the highest priority is used.
  2. Check that the handler matches the method member of the Request or throw permission_error(http_method, Method, Location)
  3. Expand the request using expansion hooks registered by http_request_expansion/3. This may add fields to the request, such the authenticated user, parsed parameters, etc. The hooks may also throw exceptions, notably using http_redirect/3 or by throwing http_reply(Term, ExtraHeader, Context) exceptions.
  4. Extract possible fields from the Request using e.g. method(Method) as one of the options.
  5. Call the registered closure, optionally spawning the request to a new thread or enforcing a time limit.
http_request_expansion(:Goal, +Rank:number)
Register Goal for expanding the HTTP request handler. Goal is called as below. If Goal fail the request is passed to the next expansion unmodified. 
call(Goal, Request0, Request, Options)

If multiple goals are registered they expand the request in a pipeline starting with the expansion hook with the lowest rank.

Besides rewriting the request, for example by validating the user identity based on HTTP authentication or cookies and adding this to the request, the hook may raise HTTP exceptions to indicate a bad request, permission error, etc. See http_status_reply/4.

Initially, auth_expansion/3 is registered with rank 100 to deal with the older http:authenticate/3 hook.

[semidet]http_current_handler(+Location, :Closure)
[nondet]http_current_handler(-Location, :Closure)
True if Location is handled by Closure.
[semidet]http_current_handler(+Location, :Closure, -Options)
[nondet]http_current_handler(?Location, :Closure, ?Options)
Resolve the current handler and options to execute it.
[det]http_location_by_id(+ID, -Location)
True when Location represents the HTTP path to which the handler with identifier ID is bound. Handler identifiers are deduced from the http_handler/3 declaration as follows:
Explicit id
If a term id(ID) appears in the option list of the handler, ID it is used and takes preference over using the predicate.
Using the handler predicate
ID matches a handler if the predicate name matches ID. The ID may have a module qualification, e.g., Module:Pred

If the handler is declared with a pattern, e.g., root(user/User), the location to access a particular user may be accessed using e.g., user('Bob'). The number of arguments to the compound term must match the number of variables in the path pattern.

A plain atom ID can be used to find a handler with a pattern. The returned location is the path up to the first variable, e.g., /user/ in the example above.

User code is adviced to use http_link_to_id/3 which can also add query parameters to the URL. This predicate is a helper for http_link_to_id/3.

Errors
existence_error(http_handler_id, Id).
See also
http_link_to_id/3 and the library(http/html_write) construct location_by_id(ID) or its abbreviation #(ID)
http_link_to_id(+HandleID, +Parameters, -HREF)
HREF is a link on the local server to a handler with given ID, passing the given Parameters. This predicate is typically used to formulate a HREF that resolves to a handler implementing a particular predicate. The code below provides a typical example. The predicate user_details/1 returns a page with details about a user from a given id. This predicate is registered as a handler. The DCG user_link//1 renders a link to a user, displaying the name and calling user_details/1 when clicked. Note that the location (root(user_details)) is irrelevant in this equation and HTTP locations can thus be moved freely without breaking this code fragment. 
:- http_handler(root(user_details), user_details, []).

user_details(Request) :-
    http_parameters(Request,
                    [ user_id(ID)
                    ]),
    ...

user_link(ID) -->
    { user_name(ID, Name),
      http_link_to_id(user_details, [id(ID)], HREF)
    },
    html(a([class(user), href(HREF)], Name)).
HandleID is either an atom, possibly module qualified predicate or a compound term if the hander is defined using a pattern. See http_handler/3 and http_location_by_id/2.
Parameters is one of

  • path_postfix(File) to pass a single value as the last segment of the HTTP location (path). This way of passing a parameter is commonly used in REST APIs.

    New code should use a path pattern in the handler declaration and a term‘HandleID(Arg, ...)`

  • A list of search parameters for a GET request.

See also
http_location_by_id/2 and http_handler/3 for defining and specifying handler IDs.
[det]http_reload_with_parameters(+Request, +Parameters, -HREF)
Create a request on the current handler with replaced search parameters.
[det]http_reply_file(+FileSpec, +Options, +Request)
Options is a list of
cache(+Boolean)
If true (default), handle If-modified-since and send modification time.
mime_type(+Type)
Overrule mime-type guessing from the filename as provided by file_mime_type/2.
static_gzip(+Boolean)
If true (default false) and, in addition to the plain file, there is a .gz file that is not older than the plain file and the client acceps gzip encoding, send the compressed file with Transfer-encoding: gzip.
cached_gzip(+Boolean)
If true (default false) the system maintains cached gzipped files in a directory accessible using the file search path http_gzip_cache and serves these similar to the static_gzip(true) option. If the gzip file does not exist or is older than the input the file is recreated.
unsafe(+Boolean)
If false (default), validate that FileSpec does not contain references to parent directories. E.g., specifications such as www('../../etc/passwd') are not allowed.
headers(+List)
Provides additional reply-header fields, encoded as a list of Field(Value).

If caching is not disabled, it processes the request headers If-modified-since and Range.

throws
- http_reply(not_modified)
- http_reply(file(MimeType, Path))
[det]http_safe_file(+FileSpec, +Options)
True if FileSpec is considered safe. If it is an atom, it cannot be absolute and cannot have references to parent directories. If it is of the form alias(Sub), than Sub cannot have references to parent directories.
Errors
- instantiation_error
- permission_error(read, file, FileSpec)
[det]http_redirect(+How, +To, +Request)
Redirect to a new location. The argument order, using the Request as last argument, allows for calling this directly from the handler declaration: 
:- http_handler(root(.),
                http_redirect(moved, myapp('index.html')),
                []).
How is one of moved, moved_temporary or see_other
To is an atom, a aliased path as defined by http_absolute_location/3. or a term location_by_id(Id) or its abbreviations #(Id) or #(Id)+Parameters. If To is not absolute, it is resolved relative to the current location.
[det]http_404(+Options, +Request)
Reply using an "HTTP 404 not found" page. This handler is intended as fallback handler for prefix handlers. Options processed are:
index(Location)
If there is no path-info, redirect the request to Location using http_redirect/3.
Errors
http_reply(not_found(Path))
http_switch_protocol(:Goal, +Options)
Send an "HTTP 101 Switching Protocols" reply. After sending the reply, the HTTP library calls call(Goal, InStream, OutStream), where InStream and OutStream are the raw streams to the HTTP client. This allows the communication to continue using an an alternative protocol.

If Goal fails or throws an exception, the streams are closed by the server. Otherwise Goal is responsible for closing the streams. Note that Goal runs in the HTTP handler thread. Typically, the handler should be registered using the spawn option if http_handler/3 or Goal must call thread_create/3 to allow the HTTP worker to return to the worker pool.

The streams use binary (octet) encoding and have their I/O timeout set to the server timeout (default 60 seconds). The predicate set_stream/2 can be used to change the encoding, change or cancel the timeout.

This predicate interacts with the server library by throwing an exception.

The following options are supported:

header(+Headers)
Backward compatible. Use headers(+Headers).
headers(+Headers)
Additional headers send with the reply. Each header takes the form Name(Value).

3.3 library(http/http_dirindex): HTTP directory listings

To be done
Provide more options (sorting, selecting columns, hiding files)

This module provides a simple API to generate an index for a physical directory. The index can be customised by overruling the dirindex.css CSS file and by defining additional rules for icons using the hook http:file_extension_icon/2.

[det]http_reply_dirindex(+DirSpec, :Options, +Request)
Provide a directory listing for Request, assuming it is an index for the physical directrory Dir. If the request-path does not end with /, first return a moved (301 Moved Permanently) reply.

The calling conventions allows for direct calling from http_handler/3.

[det]directory_index(+Dir, :Options)//
Show index for a directory. Options processed:
order_by(+Field)
Sort the files in the directory listing by Field. Field is one of name (default), size or time.
order(+AscentDescent)
Sorting order. Default is ascending. The altenative is descending
name(:RenderName)
DCG used to render a name in the table. The File is passed.
[nondet,multifile]http:mime_type_icon(+MimeType, -IconName)
Multi-file hook predicate that can be used to associate icons to files listed by http_reply_dirindex/3. The actual icon file is located by absolute_file_name(icons(IconName), Path, []).
See also
serve_files_in_directory/2 serves the images.

3.4 library(http/http_files): Serve plain files from a hierarchy

See also
pwp_handler/2 provides similar facilities, where .pwp files can be used to add dynamic behaviour.

Although the SWI-Prolog Web Server is intended to serve documents that are computed dynamically, serving plain files is sometimes necessary. This small module combines the functionality of http_reply_file/3 and http_reply_dirindex/3 to act as a simple web-server. Such a server can be created using the following code sample, which starts a server at port 8080 that serves files from the current directory (’.’). Note that the handler needs a prefix option to specify that it must handle all paths that begin with the registed location of the handler. 

:- use_module(library(http/http_server)).
:- use_module(library(http/http_files)).

:- http_handler(root(.), http_reply_from_files('.', []), [prefix]).

:- initialization(http_server([port(8080)]), main).
http_reply_from_files(+Dir, +Options, +Request)
HTTP handler that serves files from the directory Dir. This handler uses http_reply_file/3 to reply plain files. If the request resolves to a directory, it uses the option indexes to locate an index file (see below) or uses http_reply_dirindex/3 to create a listing of the directory.

Options:

indexes(+List)
List of files tried to find an index for a directory. The default is ['index.html'].
not_found(+Action)
Action defines what happens if the target file was not found. Options: fail makes the handler fail silently. 404 make the handler call http_404/2. Default is fail.

Note that this handler must be tagged as a prefix handler (see http_handler/3 and module introduction). This also implies that it is possible to override more specific locations in the hierarchy using http_handler/3 with a longer path-specifier.

When using http_handler/3 to bind this predicate to an HTTP location, make sure it is bound to a location that ends in a /. When using http:location/3 to define symbolic names to HTTP locations this is written as

:- http_handler(aliasname(.), http_reply_from_files(srcdir, []), [prefix]).
Dir is either a directory or an path-specification as used by absolute_file_name/3. This option provides great flexibility in (re-)locating the physical files and allows merging the files of multiple physical locations into one web-hierarchy by using multiple user:file_search_path/2 clauses that define the same alias.
See also
The hookable predicate file_mime_type/2 is used to determine the Content-type from the file name.

3.5 library(http/http_session): HTTP Session management

This library defines session management based on HTTP cookies. Session management is enabled simply by loading this module. Details can be modified using http_set_session_options/1. By default, this module creates a session whenever a request is processes that is inside the hierarchy defined for session handling (see path option in http_set_session_options/1). Automatic creation of a session can be stopped using the option create(noauto). The predicate http_open_session/2 must be used to create a session if noauto is enabled. Sessions can be closed using http_close_session/1.

If a session is active, http_in_session/1 returns the current session and http_session_assert/1 and friends maintain data about the session. If the session is reclaimed, all associated data is reclaimed too.

Begin and end of sessions can be monitored using library(broadcast). The broadcasted messages are:

http_session(begin(SessionID,Peer))
Broadcasted if a session is started
http_session(end(SessionId,Peer))
Broadcasted if a session is ended. See http_close_session/1.

For example, the following calls end_session(SessionId) whenever a session terminates. Please note that sessions ends are not scheduled to happen at the actual timeout moment of the session. Instead, creating a new session scans the active list for timed-out sessions. This may change in future versions of this library. 

:- listen(http_session(end(SessionId, Peer)),
          end_session(SessionId)).
[det]http_set_session_options(+Options)
Set options for the session library. Provided options are:
timeout(+Seconds)
Session timeout in seconds. Default is 600 (10 min). A timeout of 0 (zero) disables timeout.
cookie(+Cookiekname)
Name to use for the cookie to identify the session. Default swipl_session.
path(+Path)
Path to which the cookie is associated. Default is /. Cookies are only sent if the HTTP request path is a refinement of Path.
route(+Route)
Set the route name. Default is the unqualified hostname. To cancel adding a route, use the empty atom. See route/1.
enabled(+Boolean)
Enable/disable session management. Sesion management is enabled by default after loading this file.
create(+Atom)
Defines when a session is created. This is one of auto (default), which creates a session if there is a request whose path matches the defined session path or noauto, in which cases sessions are only created by calling http_open_session/2 explicitely.
proxy_enabled(+Boolean)
Enable/disable proxy session management. Proxy session management associates the originating IP address of the client to the session rather than the proxy IP address. Default is false.
gc(+When)
When is one of active, which starts a thread that performs session cleanup at close to the moment of the timeout or passive, which runs session GC when a new session is created.
samesite(+Restriction)
One of none, lax (default), or strict - The SameSite attribute prevents the CSRF vulnerability. strict has best security, but prevents links from external sites from operating properly. lax stops most CSRF attacks against REST endpoints but rarely interferes with legitimage operations. none removes the samesite attribute entirely. __Caution: The value none exposes the entire site to CSRF attacks.
granularity(+Integer)
Granularity for updating that the session is active. Default is 60 (seconds). Smaller values lead to more precise session timeout at the cost of more database updates. This may notably a problem when using Redis.

In addition, extension libraries can define session_option/2 to make this predicate support more options. In particular, library(http/http_redis_plugin) defines the following additional options:

redis_db(+DB)
Alias name of the redis database to access. See redis_server/3.
redis_ro(+DB)
Alias name of the redis database for read-only access. See redis_server/3.
redis_prefix(+Atom)
Prefix to use for all HTTP session related keys. Default is 'swipl:http:session'
[nondet]http_session_option(?Option)
True if Option is a current option of the session system.
[semidet]session_setting(+SessionID, ?Setting)
Find setting for SessionID. It is possible to overrule some session settings using http_session_set(Setting).
[det]http_set_session(Setting)
[det]http_set_session(SessionId, Setting)
Overrule a setting for the current or specified session. Currently, the only setting that can be overruled is timeout.
Errors
permission_error(set, http_session, Setting) if setting a setting that is not supported on per-session basis.
[det]http_session_id(-SessionId)
True if SessionId is an identifier for the current session.
SessionId is an atom.
Errors
existence_error(http_session, _)
See also
http_in_session/1 for a version that fails if there is no session.
[semidet]http_in_session(-SessionId)
True if SessionId is an identifier for the current session. The current session is extracted from session(ID) from the current HTTP request (see http_current_request/1). The value is cached in a backtrackable global variable http_session_id. Using a backtrackable global variable is safe because continuous worker threads use a failure driven loop and spawned threads start without any global variables. This variable can be set from the commandline to fake running a goal from the commandline in the context of a session.
See also
http_session_id/1
[det]http_open_session(-SessionID, +Options)
Establish a new session. This is normally used if the create option is set to noauto. Options:
renew(+Boolean)
If true (default false) and the current request is part of a session, generate a new session-id. By default, this predicate returns the current session as obtained with http_in_session/1.
Errors
permission_error(open, http_session, CGI) if this call is used after closing the CGI header.
See also
- http_set_session_options/1 to control the create option.
- http_close_session/1 for closing the session.
[det]http_session_asserta(+Data)
[det]http_session_assert(+Data)
[nondet]http_session_retract(?Data)
[det]http_session_retractall(?Data)
Versions of assert/1, retract/1 and retractall/1 that associate data with the current HTTP session.
[nondet]http_session_data(?Data)
True if Data is associated using http_session_assert/1 to the current HTTP session.
Errors
existence_error(http_session,_)
[det]http_session_asserta(+Data, +SessionID)
[det]http_session_assert(+Data, +SessionID)
[nondet]http_session_retract(?Data, +SessionID)
[det]http_session_retractall(@Data, +SessionID)
[det]http_session_data(?Data, +SessionID)
Versions of assert/1, retract/1 and retractall/1 that associate data with an explicit HTTP session.
See also
http_current_session/2.
[nondet]http_current_session(?SessionID, ?Data)
Enumerate the current sessions and associated data. There are two pseudo data elements:
idle(Seconds)
Session has been idle for Seconds.
peer(Peer)
Peer of the connection.
[det]http_close_session(+SessionID)
Closes an HTTP session. This predicate can be called from any thread to terminate a session. It uses the broadcast/1 service with the message below. 
http_session(end(SessionId, Peer))

The broadcast is done before the session data is destroyed and the listen-handlers are executed in context of the session that is being closed. Here is an example that destroys a Prolog thread that is associated to a thread: 

:- listen(http_session(end(SessionId, _Peer)),
          kill_session_thread(SessionID)).

kill_session_thread(SessionID) :-
        http_session_data(thread(ThreadID)),
        thread_signal(ThreadID, throw(session_closed)).

Succeed without any effect if SessionID does not refer to an active session.

If http_close_session/1 is called from a handler operating in the current session and the CGI stream is still in state header, this predicate emits a Set-Cookie to expire the cookie.

Errors
type_error(atom, SessionID)
See also
listen/2 for acting upon closed sessions
[det]http_session_cookie(-Cookie)
Generate a random cookie that can be used by a browser to identify the current session. The cookie has the format XXXX-XXXX-XXXX-XXXX[.<route>], where XXXX are random hexadecimal numbers and [.<route>] is the optionally added routing information.
[semidet,multifile]hooked
[multifile]hook(+Goal)
These multifile predicates may be used to hook the data storage of this library. An example is implemented by library(http/http_redis_plugin), storing all session data in a redis database.

3.6 library(http/http_cors): Enable CORS: Cross-Origin Resource Sharing

See also
- http://en.wikipedia.org/wiki/Cross-site_scripting for understanding Cross-site scripting.
- http://www.w3.org/TR/cors/ for understanding CORS

This small module allows for enabling Cross-Origin Resource Sharing (CORS) for a specific request. Typically, CORS is enabled for API services that you want to have useable from browser client code that is loaded from another domain. An example are the LOD and SPARQL services in ClioPatria.

Because CORS is a security risc (see references), it is disabled by default. It is enabled through the setting http:cors. The value of this setting is a list of domains that are allowed to access the service. Because * is used as a wildcard match, the value [*] allows access from anywhere.

Services for which CORS is relevant must call cors_enable/0 as part of the HTTP response, as shown below. Note that cors_enable/0 is a no-op if the setting http:cors is set to the empty list ([]). 

my_handler(Request) :-
      ....,
      cors_enable,
      reply_json(Response, []).

If a site uses a Preflight OPTIONS request to find the server's capabilities and access politics, cors_enable/2 can be used to formulate an appropriate reply. For example: 

my_handler(Request) :-
      option(method(options), Request), !,
      cors_enable(Request,
                  [ methods([get,post,delete])
                  ]),
      format('~n').                           % 200 with empty body
[det]cors_enable
Emit the HTTP header Access-Control-Allow-Origin using domains from the setting http:cors. This this setting is [] (default), nothing is written. This predicate is typically used for replying to API HTTP-request (e.g., replies to an AJAX request that typically serve JSON or XML).
[det]cors_enable(+Request, +Options)
CORS reply to a Preflight OPTIONS request. Request is the HTTP request. Options provides:
methods(+List)
List of supported HTTP methods. The default is GET, only allowing for read requests.
headers(+List)
List of headers the client asks for and we allow. The default is to simply echo what has been requested for.

Both methods and headers may use Prolog friendly syntax, e.g., get for a method and content_type for a header.

See also
http://www.html5rocks.com/en/tutorials/cors/

3.7 library(http/http_authenticate): Authenticate HTTP connections using 401 headers

This module provides the basics to validate an HTTP Authorization header. User and password information are read from a Unix/Apache compatible password file.

This library provides, in addition to the HTTP authentication, predicates to read and write password files.

http_authenticate(+Type, +Request, -Fields)
True if Request contains the information to continue according to Type. Type identifies the required authentication technique:
basic(+PasswordFile)
Use HTTP Basic authetication and verify the password from PasswordFile. PasswordFile is a file holding usernames and passwords in a format compatible to Unix and Apache. Each line is record with : separated fields. The first field is the username and the second the password hash. Password hashes are validated using crypt/2.

Successful authorization is cached for 60 seconds to avoid overhead of decoding and lookup of the user and password data.

http_authenticate/3 just validates the header. If authorization is not provided the browser must be challenged, in response to which it normally opens a user-password dialogue. Example code realising this is below. The exception causes the HTTP wrapper code to generate an HTTP 401 reply. 

(   http_authenticate(basic(passwd), Request, Fields)
->  true
;   throw(http_reply(authorise(basic, Realm)))
).
Fields is a list of fields from the password-file entry. The first element is the user. The hash is skipped.
To be done
Should we also cache failures to reduce the risc of DoS attacks?
[semidet]http_authorization_data(+AuthorizeText, ?Data)
Decode the HTTP Authorization header. Data is a term 
Method(User, Password)

where Method is the (downcased) authorization method (typically basic), User is an atom holding the user name and Password is a list of codes holding the password

[nondet]http_current_user(+File, ?User, ?Fields)
True when User is present in the htpasswd file File and Fields provides the additional fields.
Fields are the fields from the password file File, converted using name/2, which means that numeric values are passed as numbers and other fields as atoms. The password hash is the first element of Fields and is a string.
[det]http_read_passwd_file(+Path, -Data)
Read a password file. Data is a list of terms of the format below, where User is an atom identifying the user, Hash is a string containing the salted password hash and Fields contain additional fields. The string value of each field is converted using name/2 to either a number or an atom. 
passwd(User, Hash, Fields)
[det]http_write_passwd_file(+File, +Data:list)
Write password data Data to File. Data is a list of entries as below. See http_read_passwd_file/2 for details. 
passwd(User, Hash, Fields)
To be done
Write to a new file and atomically replace the old one.
[multifile]http:authenticate(+AuthData, +Request, -Fields)
Plugin for library(http_dispatch) to perform basic HTTP authentication.

This predicate throws http_reply(authorise(basic, Realm)).

AuthData must be a term basic(File, Realm)
Request is the HTTP request
Fields describes the authenticated user with the option user(User) and with the option user_details(Fields) if the password file contains additional fields after the user and password.

3.8 library(http/http_digest): HTTP Digest authentication

See also
https://tools.ietf.org/html/rfc2617

This library implements HTTP Digest Authentication as per RFC2617. Unlike Basic Authentication, digest authentication is based on challenge-reponse and therefore does not need to send the password over the (insecure) connection. In addition, it provides a count mechanism that ensure that old credentials cannot be reused, which prevents attackers from using old credentials with a new request. Digest authentication have the following advantages and disadvantages:

  • Advantages

    • Authentication without exchanging the password
    • No re-use of authentication data

  • Disadvantages

    • An extra round trip is needed for the first authentication
    • Server-side storage of the password is the MD5 hash of the user, realm and password. As MD5 hashes are quick to compute, one needs strong passwords. This fixed algorithm also allows for rainbow table attacks, although their value is limited because you need to precompute the rainbow table for every server (realm) and user.
    • The connection is sensitive to man-in-the-middle attack, where the attacker can both change the request and response.
    • Both client and server need to keep an administration of issued nonce values and associated nonce count values.

And, of course, the connection itself remains insecure. Digest based authentication is a viable alternative if HTTPS is not a good option and security of the data itself is not an issue.

This library acts as plugin for library(http/http_dispatch), where the registered handler (http_handler/3) can be given the option below to initiate digest authentication.

  • authentication(digest(PasswdFile, Realm))

Above, PasswdFile is a file containing lines of the from below, where PasswordHash is computed using http_digest_password_hash/4. See also library(http/http_authenticate), http_read_passwd_file/2 and http_write_passwd_file/2. 

User ":" PasswordHash (":" Extra)*

This library also hooks into library(http/http_open) if the option authorization(digest(User, Password)) is given.

http_digest_challenge(+Realm, +Options)//
Generate the content for a 401 WWW-Authenticate: Digest header field.
[det]http_parse_digest_challenge(+Challenge, -Fields)
Parse the value of an HTTP WWW-Authenticate header into a list of Name(Value) terms.
http_digest_response(+Challenge, +User, +Password, -Reply, +Options)
Formulate a reply to a digest authentication request. Options:
path(+Path)
The request URI send along with the authentication. Defaults to /
method(+Method)
The HTTP method. Defaults to 'GET'
nc(+Integer)
The nonce-count as an integer. This is formatted as an 8 hex-digit string.
Challenge is a list Name(Value), normally from http_parse_digest_challenge/2. Must contain realm and nonce. Optionally contains opaque.
User is the user we want to authenticated
Password is the user's password
Options provides additional options
[det]http_digest_password_hash(+User, +Realm, +Password, -Hash)
Compute the password hash for the HTTP password file. Note that the HTTP digest mechanism does allow us to use a seeded expensive arbitrary hash function. Instead, the hash is defined as the MD5 of the following components: 
<user>:<realm>:<password>.

The inexpensive MD5 algorithm makes the hash sensitive to brute force attacks while the lack of seeding make the hashes sensitive for rainbow table attacks, although the value is somewhat limited because the realm and user are part of the hash.

[multifile]http:authenticate(+Digest, +Request, -Fields)
Plugin for library(http_dispatch) to perform basic HTTP authentication. Note that we keep the authentication details cached to avoid a‘nonce-replay’error in the case that the application tries to verify multiple times.

This predicate throws http_reply(authorise(digest(Digest)))

Digest is a term digest(File, Realm, Options)
Request is the HTTP request
Fields describes the authenticated user with the option user(User) and with the option user_details(Fields) if the password file contains additional fields after the user and password.
[semidet,multifile]http:authenticate_client(+URL, +Action)
This hooks is called by http_open/3 with the following Action value:
send_auth_header(+AuthData, +Out, +Options)
Called when sending the initial request. AuthData contains the value for the http_open/3 option authorization(AuthData) and Out is a stream on which to write additional HTTP headers.
auth_reponse(+Headers, +OptionsIn, -Options)
Called if the server replies with a 401 code, challenging the client. Our implementation adds a request_header(authorization=Digest) header to Options, causing http_open/3 to retry the request with the additional option.

3.9 library(http/http_dyn_workers): Dynamically schedule HTTP workers.

Most code doesn't need to use this directly; instead use library(http/http_server), which combines this library with the typical HTTP libraries that most servers need.

This module defines hooks into the HTTP framework to dynamically schedule worker threads. Dynamic scheduling relieves us from finding a good value for the size of the HTTP worker pool.

The decision to add a worker follows these rules:

  • If the load average caused by the worker threads exceeds http:max_load, no worker is added.
  • Wait for some time, depending on how close we are to the http:max_workers limit.

    • If the worker is still needed, add it.

The policy depends on three settings:

http:max_workers
The maximum number of workers that will be created. Default is 100.
http:worker_idle_limit
The number of seconds a dynamic worker waits for a new job. If no job arrives in time it terminates. Default is 10 seconds.
http:max_load
Max load average created by the HTTP server, i.e. the amount of CPU time consumed per second. Default is 10.
[multifile]http:schedule_workers(+Dict)
Called if there is no immediately free worker to handle the incomming request. The request is forwarded to the thread __http_scheduler as the hook is called in time critical code.

3.9.1 Providing Server-Sent Events (sse)

Server-Sent Events allows for setting up a simple event stream from the server to the client. It can serve roles similar to long polling and web sockets, enabling the server to notify its clients on some event. Long polling uses a normal HTTP (usually) GET request that blocks for a long time on the server. The server finishes the request when it wants to notify the client or after some time (e.g., a minute) to avoid a timeout on the client or some proxy. After receiving an event or timeout, the client repeats the request. Web sockets upgrade a the socket used for a normal HTTP request to create a bi-directional open communication channel that exchanges encapsulated messages in both directions. Server-Sent Events open a normal HTTP channel over which the server can sent simple text messages using a format similar to the HTTP header: a sequence of Name: Value lines followed by two newlines. Unlike long polling, the request does not complete after a message.

Following the MDN documentation above, and SSE request can be served using the simple example below that generates an event, counting every minute. Note the handler declaration that processes the request on a new thread and disables timeout for this location. Note that this implementation uses a thread per client. This design limits the scalability. 

:- http_handler(root(events), events,
                [ spawn([]),
                  time_limit(infinite)
                ]).

events(_Request) :-
    format('X-Accel-Buffering: no\r\n\c
            Content-Type: text/event-stream\r\n\c
            Cache-Control: no-cache\r\n\r\n'),

    between(1, infinite, Min),
        format('event: minute~n'),
        format('data: {"minute": ~d}~n~n', [Min]),
        flush_output,
        sleep(60),
        fail.

Of course, rather than sleep/1 to decide when to fire the next event this thread typically has to wait for events in the application. This can be achieved using thread_wait/2 or message queues.

3.10 Custom Error Pages

It is possible to create arbitrary error pages for responses generated when a http_reply term is thrown. Currently this is only supported for status 403 (authentication required). To do this, instead of throwing http_reply(authorise(Term)) throw http_reply(authorise(Term), [], Key), where Key is an arbitrary term relating to the page you want to generate. You must then also define a clause of the multifile predicate http:status_page_hook/3:

http:status_page_hook(+TermOrCode, +Context, -CustomHTML)
TermOrCode is either the first argument of the http_reply exception or the HTTP status code, i.e., the hook is called twice. New code should using the Term. Context is the third argument of the http_reply exception which was thrown, and CustomHTML is a list of HTML tokens. A page equivalent to the default page for 401 is generated by the example below. 
:- multifile http:status_page_hook/3.

http:status_page_hook(authorise(Term), _Context, HTML) :-
    phrase(page([ title('401 Authorization Required')
                ],
                [ h1('Authorization Required'),
                  p(['This server could not verify that you ',
                     'are authorized to access the document ',
                     'requested.  Either you supplied the wrong ',
                     'credentials (e.g., bad password), or your ',
                     'browser doesn\'t understand how to supply ',
                     'the credentials required.'
                     ]),
                  \address
                ]),
           HTML).

3.11 library(http/http_openid): OpenID consumer and server library

This library implements the OpenID protocol (http://openid.net/). OpenID is a protocol to share identities on the network. The protocol itself uses simple basic HTTP, adding reliability using digitally signed messages.

Steps, as seen from the consumer (or relying partner).

  1. Show login form, asking for openid_identifier
  2. Get HTML page from openid_identifier and lookup <link rel="openid.server" href="server">
  3. Associate to server
  4. Redirect browser (302) to server using mode checkid_setup, asking to validate the given OpenID.
  5. OpenID server redirects back, providing digitally signed conformation of the claimed identity.
  6. Validate signature and redirect to the target location.

A consumer (an application that allows OpenID login) typically uses this library through openid_user/3. In addition, it must implement the hook http_openid:openid_hook(trusted(OpenId, Server)) to define accepted OpenID servers. Typically, this hook is used to provide a white-list of acceptable servers. Note that accepting any OpenID server is possible, but anyone on the internet can setup a dummy OpenID server that simply grants and signs every request. Here is an example: 

:- multifile http_openid:openid_hook/1.

http_openid:openid_hook(trusted(_, OpenIdServer)) :-
    (   trusted_server(OpenIdServer)
    ->  true
    ;   throw(http_reply(moved_temporary('/openid/trustedservers')))
    ).

trusted_server('http://www.myopenid.com/server').

By default, information who is logged on is maintained with the session using http_session_assert/1 with the term openid(Identity). The hooks login/logout/logged_in can be used to provide alternative administration of logged-in users (e.g., based on client-IP, using cookies, etc.).

To create a server, you must do four things: bind the handlers openid_server/2 and openid_grant/1 to HTTP locations, provide a user-page for registered users and define the grant(Request, Options) hook to verify your users. An example server is provided in in <plbase>/doc/packages/examples/demo_openid.pl

[multifile]openid_hook(+Action)
Call hook on the OpenID management library. Defined hooks are:
login(+OpenID)
Consider OpenID logged in.
logout(+OpenID)
Logout OpenID
logged_in(?OpenID)
True if OpenID is logged in
grant(+Request, +Options)
Server: Reply positive on OpenID
trusted(+OpenID, +Server)
True if Server is a trusted OpenID server
ax(Values)
Called if the server provided AX attributes
x_parameter(+Server, -Name, -Value)
Called to find additional HTTP parameters to send with the OpenID verify request.
[det]openid_login(+OpenID)
Associate the current HTTP session with OpenID. If another OpenID is already associated, this association is first removed.
[det]openid_logout(+OpenID)
Remove the association of the current session with any OpenID
[semidet]openid_logged_in(-OpenID)
True if session is associated with OpenID.
[det]openid_user(+Request:http_request, -OpenID:url, +Options)
True if OpenID is a validated OpenID associated with the current session. The scenario for which this predicate is designed is to allow an HTTP handler that requires a valid login to use the transparent code below. 
handler(Request) :-
      openid_user(Request, OpenID, []),
      ...

If the user is not yet logged on a sequence of redirects will follow:

  1. Show a page for login (default: page /openid/login), predicate reply_openid_login/1)
  2. By default, the OpenID login page is a form that is submitted to the verify, which calls openid_verify/2.
  3. openid_verify/2 does the following:

    • Find the OpenID claimed identity and server
    • Associate to the OpenID server
    • redirects to the OpenID server for validation

  4. The OpenID server will redirect here with the authetication information. This is handled by openid_authenticate/4.

Options:

login_url(Login)
(Local) URL of page to enter OpenID information. Default is the handler for openid_login_page/1
See also
openid_authenticate/4 produces errors if login is invalid or cancelled.
[det]openid_login_form(+ReturnTo, +Options)//
Create the OpenID form. This exported as a separate DCG, allowing applications to redefine /openid/login and reuse this part of the page. Options processed:
action(Action)
URL of action to call. Default is the handler calling openid_verify/1.
buttons(+Buttons)
Buttons is a list of img structures where the href points to an OpenID 2.0 endpoint. These buttons are displayed below the OpenID URL field. Clicking the button sets the URL field and submits the form. Requires Javascript support.

If the href is relative, clicking it opens the given location after adding’openid.return_to’and‘stay’.

show_stay(+Boolean)
If true, show a checkbox that allows the user to stay logged on.
openid_verify(+Options, +Request)
Handle the initial login form presented to the user by the relying party (consumer). This predicate discovers the OpenID server, associates itself with this server and redirects the user's browser to the OpenID server, providing the extra openid.X name-value pairs. Options is, against the conventions, placed in front of the Request to allow for smooth cooperation with http_dispatch.pl. Options processes:
return_to(+URL)
Specifies where the OpenID provider should return to. Normally, that is the current location.
trust_root(+URL)
Specifies the openid.trust_root attribute. Defaults to the root of the current server (i.e., http://host[.port]/).
realm(+URL)
Specifies the openid.realm attribute. Default is the trust_root.
ax(+Spec)
Request the exchange of additional attributes from the identity provider. See http_ax_attributes/2 for details.

The OpenId server will redirect to the openid.return_to URL.

throws
http_reply(moved_temporary(Redirect))
[nondet]openid_server(?OpenIDLogin, ?OpenID, ?Server)
True if OpenIDLogin is the typed id for OpenID verified by Server.
OpenIDLogin ID as typed by user (canonized)
OpenID ID as verified by server
Server URL of the OpenID server
[det]openid_current_url(+Request, -URL)
Find the public URL for Request that we can make available to our identity provider. This must be an absolute URL where we can be contacted. Before trying a configured version through http_public_url/2, we try to see wether the login message contains a referer parameter or wether the browser provided one.
openid_current_host(Request, Host, Port)
Find current location of the server.
deprecated
New code should use http_current_host/4 with the option global(true).
ssl_verify(+SSL, +ProblemCert, +AllCerts, +FirstCert, +Error)
Accept all certificates. We do not care too much. Only the user cares s/he is not entering her credentials with a spoofed side. As we redirect, the browser will take care of this.
[semidet]openid_authenticate(+Request, -Server:url, -OpenID:url, -ReturnTo:url)
Succeeds if Request comes from the OpenID server and confirms that User is a verified OpenID user. ReturnTo provides the URL to return to.

After openid_verify/2 has redirected the browser to the OpenID server, and the OpenID server did its magic, it redirects the browser back to this address. The work is fairly trivial. If mode is cancel, the OpenId server denied. If id_res, the OpenId server replied positive, but we must verify what the server told us by checking the HMAC-SHA signature.

This call fails silently if their is no openid.mode field in the request.

throws
- openid(cancel) if request was cancelled by the OpenId server
- openid(signature_mismatch) if the HMAC signature check failed
openid_server(+Options, +Request)
Realise the OpenID server. The protocol demands a POST request here.
openid_grant(+Request)
Handle the reply from checkid_setup_server/3. If the reply is yes, check the authority (typically the password) and if all looks good redirect the browser to ReturnTo, adding the OpenID properties needed by the Relying Party to verify the login.
[det]openid_associate(?URL, ?Handle, ?Assoc)
Calls openid_associate/4 as 
openid_associate(URL, Handle, Assoc, []).
[det]openid_associate(+URL, -Handle, -Assoc, +Options)
[semidet]openid_associate(?URL, +Handle, -Assoc, +Options)
Associate with an open-id server. We first check for a still valid old association. If there is none or it is expired, we esstablish one and remember it. Options:
ns(URL)
One of http://specs.openid.net/auth/2.0 (default) or http://openid.net/signon/1.1.
To be done
Should we store known associations permanently? Where?

3.12 Get parameters from HTML forms

The library library(http/http_parameters) provides two predicates to fetch HTTP request parameters as a type-checked list easily. The library transparently handles both GET and POST requests. It builds on top of the low-level request representation described in section 3.13.

http_parameters(+Request, ?Parameters)
The predicate is passes the Request as provided to the handler goal by http_wrapper/5 as well as a partially instantiated lists describing the requested parameters and their types. Each parameter specification in Parameters is a term of the format Name(-Value, +Options) . Options is a list of option terms describing the type, default, etc. If no options are specified the parameter must be present and its value is returned in Value as an atom.

If a parameter is missing the exception error(existence_error(http_parameter, Name), _) is thrown which. If the argument cannot be converted to the requested type, a error(existence_error(Type, Value), _) is raised, where the error context indicates the HTTP parameter. If not caught, the server translates both errors into a 400 Bad request HTTP message.

Options fall into three categories: those that handle presence of the parameter, those that guide conversion and restrict types and those that support automatic generation of documention. First, the presence-options:

default(Default)
If the named parameter is missing, Value is unified to Default.
optional(true)
If the named parameter is missing, Value is left unbound and no error is generated.
list(Type)
The same parameter may not appear or appear multiple times. If this option is present, default and optional are ignored and the value is returned as a list. Type checking options are processed on each value.
zero_or_more
Deprecated. Use list(Type).

The type and conversion options are given below. The type-language can be extended by providing clauses for the multifile hook http:convert_parameter/3.

;(Type1, Type2)
Succeed if either Type1 or Type2 applies. It allows for checks such as (nonneg;oneof([infinite])) to specify an integer or a symbolic value.
oneof(List)
Succeeds if the value is member of the given list.
length > N
Succeeds if value is an atom of more than N characters.
length >= N
Succeeds if value is an atom of more than or equal to N characters.
length < N
Succeeds if value is an atom of less than N characters.
length =< N
Succeeds if value is an atom of length less than or equal to N characters.
atom
No-op. Allowed for consistency.
string
Convert value to a string.
between(+Low, +High)
Convert value to a number and if either Low or High is a float, force value to be a float. Then check that the value is in the given range, which includes the boundaries.
boolean
Translate =true=, =yes=, =on= and’1’into =true=; =false=, =no=, =off= and’0’into =false= and raises an error otherwise.
float
Convert value to a float. Integers are transformed into float. Throws a type-error otherwise.
integer
Convert value to an integer. Throws a type-error otherwise.
nonneg
Convert value to a non-negative integer. Throws a type-error of the value cannot be converted to an integer and a domain-error otherwise.
number
Convert value to a number. Throws a type-error otherwise.

The last set of options is to support automatic generation of HTTP API documentation from the sources.4This facility is under development in ClioPatria; see http_help.pl.

description(+Atom)
Description of the parameter in plain text.
group(+Parameters, +Options)
Define a logical group of parameters. Parameters are processed as normal. Options may include a description of the group. Groups can be nested.

Below is an example 

reply(Request) :-
        http_parameters(Request,
                        [ title(Title, [ optional(true) ]),
                          name(Name,   [ length >= 2 ]),
                          age(Age,     [ between(0, 150) ])
                        ]),
        ...

Same as http_parameters(Request, Parameters,[])

http_parameters(+Request, ?Parameters, +Options)
In addition to http_parameters/2, the following options are defined.
form_data(-Data)
Return the entire set of provided Name=Value pairs from the GET or POST request. All values are returned as atoms.
attribute_declarations(:Goal)
If a parameter specification lacks the parameter options, call call(Goal, +ParamName, -Options) to find the options. Intended to share declarations over many calls to http_parameters/3. Using this construct the above can be written as below. 
reply(Request) :-
        http_parameters(Request,
                        [ title(Title),
                          name(Name),
                          age(Age)
                        ],
                        [ attribute_declarations(param)
                        ]),
        ...

param(title, [optional(true)]).
param(name,  [length >= 2 ]).
param(age,   [integer]).

3.13 Request format

The body-code (see section 3.1) is driven by a Request. This request is generated from http_read_request/2 defined in library(http/http_header).

http_read_request(+Stream, -Request)
Reads an HTTP request from Stream and unify Request with the parsed request. Request is a list of Name(Value) elements. It provides a number of predefined elements for the result of parsing the first line of the request, followed by the additional request parameters. The predefined fields are:
host(Host)
If the request contains Host: Host, Host is unified with the host-name. If Host is of the format <host>:<port> Host only describes <host> and a field port(Port) where Port is an integer is added.
input(Stream)
The Stream is passed along, allowing to read more data or requests from the same stream. This field is always present.
method(Method)
Method is the HTTP method represented as a lower-case atom (i.e., delete, get, head, options, patch, post, put, trace). This field is present if the header has been parsed successfully.
path(Path)
Path associated to the request. This field is always present.
peer(Peer)
Peer is a term ip(A,B,C,D) containing the IP address of the contacting host.
port(Port)
Port requested. See host for details.
request_uri(RequestURI)
This is the untranslated string that follows the method in the request header. It is used to construct the path and search fields of the Request. It is provided because reconstructing this string from the path and search fields may yield a different value due to different usage of percent encoding.
search(ListOfNameValue)
Search-specification of URI. This is the part after the ?, normally used to transfer data from HTML forms that use the HTTP GET method. In the URL it consists of a www-form-encoded list of Name=Value pairs. This is mapped to a list of Prolog Name=Value terms with decoded names and values. This field is only present if the location contains a search-specification.

The URL specification does not demand the query part to be of the form name=value. If the field is syntactically incorrect, ListOfNameValue is bound the the empty list ([]).

http_version(Major-Minor)
If the first line contains the HTTP/Major.Minor version indicator this element indicate the HTTP version of the peer. Otherwise this field is not present.
cookie(ListOfNameValue)
If the header contains a Cookie line, the value of the cookie is broken down in Name=Value pairs, where the Name is the lowercase version of the cookie name as used for the HTTP fields.
set_cookie(set_cookie(Name, Value, Options))
If the header contains a SetCookie line, the cookie field is broken down into the Name of the cookie, the Value and a list of Name=Value pairs for additional options such as expire, path, domain or secure.

If the first line of the request is tagged with HTTP/Major.Minor, http_read_request/2 reads all input upto the first blank line. This header consists of Name:Value fields. Each such field appears as a term Name(Value) in the Request, where Name is canonicalised for use with Prolog. Canonisation implies that the Name is converted to lower case and all occurrences of the - are replaced by _. The value for the Content-length fields is translated into an integer.

Here is an example: 

?- http_read_request(user_input, X).
|: GET /mydb?class=person HTTP/1.0
|: Host: gollem
|:
X = [ input(user),
      method(get),
      search([ class = person
             ]),
      path('/mydb'),
      http_version(1-0),
      host(gollem)
    ].

3.13.1 Handling POST requests

Where the HTTP GET operation is intended to get a document, using a path and possibly some additional search information, the POST operation is intended to hand potentially large amounts of data to the server for processing.

The Request parameter above contains the term method(post). The data posted is left on the input stream that is available through the term input(Stream) from the Request header. This data can be read using http_read_data/3 from the HTTP client library. Here is a demo implementation simply returning the parsed posted data as plain text (assuming pp/1 pretty-prints the data). 

reply(Request) :-
        member(method(post), Request), !,
        http_read_data(Request, Data, []),
        format('Content-type: text/plain~n~n', []),
        pp(Data).

If the POST is initiated from a browser, content-type is generally either application/x-www-form-urlencoded or multipart/form-data.

3.14 Running the server

The functionality of the server should be defined in one Prolog file (of course this file is allowed to load other files). Depending on the wanted server setup this‘body’is wrapped into a small Prolog file combining the body with the appropriate server interface. There are three supported server-setups. For most applications we advise the multi-threaded server. Examples of this server architecture are the PlDoc documentation system and the SeRQL Semantic Web server infrastructure.

All the server setups may be wrapped in a reverse proxy to make them available from the public web-server as described in section 3.14.7.

  • Using library(thread_httpd) for a multi-threaded server
    This server exploits the multi-threaded version of SWI-Prolog, running the users body code parallel from a pool of worker threads. As it avoids the state engine and copying required in the event-driven server it is generally faster and capable to handle multiple requests concurrently.

    This server is harder to debug due to the involved threading, although the GUI tracer provides reasonable support for multi-threaded applications using the tspy/1 command. It can provide fast communication to multiple clients and can be used for more demanding servers.

  • Using library(inetd_httpd) for server-per-client
    In this setup the Unix inetd user-daemon is used to initialise a server for each connection. This approach is especially suitable for servers that have a limited startup-time. In this setup a crashing client does not influence other requests.

    This server is very hard to debug as the server is not connected to the user environment. It provides a robust implementation for servers that can be started quickly.

3.14.1 Common server interface options

All the server interfaces provide http_server(:Goal, +Options) to create the server. The list of options differ, but the servers share common options:

port(?Port)
Specify the port to listen to for stand-alone servers. Port is either an integer or unbound. If unbound, it is unified to the selected free port.

3.14.2 Multi-threaded Prolog

The library(http/thread_httpd.pl) provides the infrastructure to manage multiple clients using a pool of worker-threads. This realises a popular server design, also seen in Java Tomcat and Microsoft .NET. As a single persistent server process maintains communication to all clients startup time is not an important issue and the server can easily maintain state-information for all clients.

In addition to the functionality provided by the inetd server, the threaded server can also be used to realise an HTTPS server exploiting the library(ssl) library. See option ssl(+SSLOptions) below.

http_server(:Goal, +Options)
Create the server. Options must provide the port(?Port) option to specify the port the server should listen to. If Port is unbound an arbitrary free port is selected and Port is unified to this port-number. The server consists of a small Prolog thread accepting new connection on Port and dispatching these to a pool of workers. Defined Options are:
port(?Address)
Address to bind to. Address is either a port (integer) or a term Host:Port. The port may be a variable, causing the system to select a free port and unify the variable with the selected port. See also tcp_bind/2.
workers(+N)
Defines the number of worker threads in the pool. Default is to use five workers. Choosing the optimal value for best performance is a difficult task depending on the number of CPUs in your system and how much resources are required for processing a request. Too high numbers makes your system switch too often between threads or even swap if there is not enough memory to keep all threads in memory, while a too low number causes clients to wait unnecessary for other clients to complete. See also http_workers/2.
timeout(+SecondsOrInfinite)
Determines the maximum period of inactivity handling a request. If no data arrives within the specified time since the last data arrived, the connection raises an exception, and the worker discards the client and returns to the pool-queue for a new client. If it is infinite, a worker may wait forever on a client that doesn't complete its request. Default is 60 seconds.
keep_alive_timeout(+SecondsOrInfinite)
Maximum time to wait for new activity on Keep-Alive connections. Choosing the correct value for this parameter is hard. Disabling Keep-Alive is bad for performance if the clients request multiple documents for a single page. This may ---for example-- be caused by HTML frames, HTML pages with images, associated CSS files, etc. Keeping a connection open in the threaded model however prevents the thread servicing the client servicing other clients. The default is 2 seconds.
local(+KBytes)
Size of the local-stack for the workers. Default is taken from the commandline option.
global(+KBytes)
Size of the global-stack for the workers. Default is taken from the commandline option.
trail(+KBytes)
Size of the trail-stack for the workers. Default is taken from the commandline option.
ssl(+SSLOptions)
Use SSL (Secure Socket Layer) rather than plain TCP/IP. A server created this way is accessed using the https:// protocol. SSL allows for encrypted communication to avoid others from tapping the wire as well as improved authentication of client and server. The SSLOptions option list is passed to ssl_context/3. The port option of the main option list is forwarded to the SSL layer. See the library(ssl) library for details.
http_server_property(?Port, ?Property)
True if Property is a property of the HTTP server running at Port. Defined properties are:
goal(:Goal)
Goal used to start the server. This is often http_dispatch/1.
scheme(-Scheme)
Scheme is one of http or https.
start_time(-Time)
Time-stamp when the server was created. See format_time/3 for creating a human-readable representation.
http_workers(+Port, ?Workers)
Query or manipulate the number of workers of the server identified by Port. If Workers is unbound it is unified with the number of running servers. If it is an integer greater than the current size of the worker pool new workers are created with the same specification as the running workers. If the number is less than the current size of the worker pool, this predicate inserts a number of‘quit’requests in the queue, discarding the excess workers as they finish their jobs (i.e. no worker is abandoned while serving a client).

This can be used to tune the number of workers for performance. Another possible application is to reduce the pool to one worker to facilitate easier debugging.

http_add_worker(+Port, +Options)
Add a new worker to the HTTP server for port Port. Options overrule the default queue options. The following additional options are processed:
max_idle_time(+Seconds)
The created worker will automatically terminate if there is no new work within Seconds.
http_stop_server(+Port, +Options)
Stop the HTTP server at Port. Halting a server is done gracefully, which means that requests being processed are not abandoned. The Options list is for future refinements of this predicate such as a forced immediate abort of the server, but is currently ignored.
http_current_worker(?Port, ?ThreadID)
True if ThreadID is the identifier of a Prolog thread serving Port. This predicate is motivated to allow for the use of arbitrary interaction with the worker thread for development and statistics.
http_spawn(:Goal, +Spec)
Continue handling this request in a new thread running Goal. After http_spawn/2, the worker returns to the pool to process new requests. In its simplest form, Spec is the name of a thread pool as defined by thread_pool_create/3. Alternatively it is an option list, whose options are passed to thread_create_in_pool/4 if Spec contains pool(Pool) or to thread_create/3 of the pool option is not present. If the dispatch module is used (see section 3.2), spawning is normally specified as an option to the http_handler/3 registration.

We recomment the use of thread pools. They allow registration of a set of threads using common characteristics, specify how many can be active and what to do if all threads are active. A typical application may define a small pool of threads with large stacks for computation intensive tasks, and a large pool of threads with small stacks to serve media. The declaration could be the one below, allowing for max 3 concurrent solvers and a maximum backlog of 5 and 30 tasks creating image thumbnails. 

:- use_module(library(thread_pool)).

:- thread_pool_create(compute, 3,
                      [ local(20000), global(100000), trail(50000),
                        backlog(5)
                      ]).
:- thread_pool_create(media, 30,
                      [ local(100), global(100), trail(100),
                        backlog(100)
                      ]).

:- http_handler('/solve',     solve,     [spawn(compute)]).
:- http_handler('/thumbnail', thumbnail, [spawn(media)]).

3.14.3 library(http/http_unix_daemon): Run SWI-Prolog HTTP server as a Unix system daemon

See also
The file <swi-home>/doc/packages/examples/http/linux-init-script provides a /etc/init.d script for controlling a server as a normal Unix service.
To be done
Cleanup issues wrt. loading and initialization of xpce.

This module provides the logic that is needed to integrate a process into the Unix service (daemon) architecture. It deals with the following aspects, all of which may be used/ignored and configured using commandline options:

  • Select the port(s) to be used by the server
  • Run the startup of the process as root to perform privileged tasks and the server itself as unpriviledged user, for example to open ports below 1000.
  • Fork and detach from the controlling terminal
  • Handle console and debug output using a file and/or the syslog daemon.
  • Manage a pid file

The typical use scenario is to write a file that loads the following components:

  1. The application code, including http handlers (see http_handler/3).
  2. This library

In the code below, ?- [load]. loads the remainder of the webserver code. This is often a sequence of use_module/1 directives. 

:- use_module(library(http/http_unix_daemon)).

:- [load].

The program entry point is http_daemon/0, declared using initialization/2. This may be overruled using a new declaration after loading this library. The new entry point will typically call http_daemon/1 to start the server in a preconfigured way. 

:- use_module(library(http/http_unix_daemon)).
:- initialization(run, main).

run :-
    ...
    http_daemon(Options).

Now, the server may be started using the command below. See http_daemon/0 for supported options. 

% [sudo] swipl mainfile.pl [option ...]

Below are some examples. Our first example is completely silent, running on port 80 as user www. 

% swipl mainfile.pl --user=www --pidfile=/var/run/http.pid

Our second example logs HTTP interaction with the syslog daemon for debugging purposes. Note that the argument to --debug= is a Prolog term and must often be escaped to avoid misinterpretation by the Unix shell. The debug option can be repeated to log multiple debug topics. 

% swipl mainfile.pl --user=www --pidfile=/var/run/http.pid \
        --debug='http(request)' --syslog=http

Broadcasting The library uses broadcast/1 to allow hooking certain events:

http(pre_server_start)
Run after fork, just before starting the HTTP server. Can be used to load additional files or perform additional initialisation, such as starting additional threads. Recall that it is not possible to start threads before forking.
http(post_server_start)
Run after starting the HTTP server.
http_daemon
Start the HTTP server as a daemon process. This predicate processes the commandline arguments below. Commandline arguments that specify servers are processed in the order they appear using the following schema:

  1. Arguments that act as default for all servers.
  2. --http=Spec or --https=Spec is followed by arguments for that server until the next --http=Spec or --https=Spec or the end of the options.
  3. If no --http=Spec or --https=Spec appears, one HTTP server is created from the specified parameters.

    Examples: 

    --workers=10 --http --https
--http=8080 --https=8443
--http=localhost:8080 --workers=1 --https=8443 --workers=25

--port=Port
Start HTTP server at Port. It requires root permission and the option --user=User to open ports below 1000. The default port is 80. If --https is used, the default port is 443.
--ip=IP
Only listen to the given IP address. Typically used as --ip=localhost to restrict access to connections from localhost if the server itself is behind an (Apache) proxy server running on the same host.
--debug=Topic
Enable debugging Topic. See debug/3.
--syslog=Ident
Write debug messages to the syslog daemon using Ident
--user=User
When started as root to open a port below 1000, this option must be provided to switch to the target user for operating the server. The following actions are performed as root, i.e., before switching to User:

  • open the socket(s)
  • write the pidfile
  • setup syslog interaction
  • Read the certificate, key and password file (--pwfile=File)
--group=Group
May be used in addition to --user. If omitted, the login group of the target user is used.
--pidfile=File
Write the PID of the daemon process to File.
--output=File
Send output of the process to File. By default, all Prolog console output is discarded.
--fork[=Bool]
If given as --no-fork or --fork=false, the process runs in the foreground.
--http[=(Bool|Port|BindTo:Port)]
Create a plain HTTP server. If the argument is missing or true, create at the specified or default address. Else use the given port and interface. Thus, --http creates a server at port 80, --http=8080 creates one at port 8080 and --http=localhost:8080 creates one at port 8080 that is only accessible from localhost.
--https[=(Bool|Port|BindTo:Port)]
As --http, but creates an HTTPS server. Use --certfile, --keyfile, -pwfile, --password and --cipherlist to configure SSL for this server.
--certfile=File
The server certificate for HTTPS.
--keyfile=File
The server private key for HTTPS.
--pwfile=File
File holding the password for accessing the private key. This is preferred over using --password=PW as it allows using file protection to avoid leaking the password. The file is read before the server drops privileges when started with the --user option.
--password=PW
The password for accessing the private key. See also‘--pwfile`.
--cipherlist=Ciphers
One or more cipher strings separated by colons. See the OpenSSL documentation for more information. Starting with SWI-Prolog 7.5.11, the default value is always a set of ciphers that was considered secure enough to prevent all critical attacks at the time of the SWI-Prolog release.
--interactive[=Bool]
If true (default false) implies --no-fork and presents the Prolog toplevel after starting the server.
--gtrace=[Bool]
Use the debugger to trace http_daemon/1.
--sighup=Action
Action to perform on kill -HUP <pid>. Default is reload (running make/0). Alternative is quit, stopping the server.

Other options are converted by argv_options/3 and passed to http_server/1. For example, this allows for:

--workers=Count
Set the number of workers for the multi-threaded server.

http_daemon/0 is defined as below. The start code for a specific server can use this as a starting point, for example for specifying defaults or additional options. This uses guided options processing from argv_options/3 from library(main). The option definitions are available as http_opt_type/3, http_opt_help/2 and http_opt_meta/2 

http_daemon :-
    current_prolog_flag(argv, Argv),
    argv_options(Argv, _RestArgv, Options),
    http_daemon(Options).
See also
http_daemon/1
http_opt_type(?Flag, ?Option, ?Type)
http_opt_help(?Option, ?Help)
http_opt_meta(?Option, ?Meta)
Allow reusing http option processing
http_daemon(+Options)
Start the HTTP server as a daemon process. This predicate processes a Prolog option list. It is normally called from http_daemon/0, which derives the option list from the command line arguments.

Error handling depends on whether or not interactive(true) is in effect. If so, the error is printed before entering the toplevel. In non-interactive mode this predicate calls halt(1).

[semidet,multifile]http_certificate_hook(+CertFile, +KeyFile, -Password)
Hook called before starting the server if the --https option is used. This hook may be used to create or refresh the certificate. If the hook binds Password to a string, this string will be used to decrypt the server private key as if the --password=Password option was given.
[semidet,multifile]http_server_hook(+Options)
Hook that is called to start the HTTP server. This hook must be compatible to http_server(Handler, Options). The default is provided by start_server/1.
[multi,multifile]http:sni_options(-HostName, -SSLOptions)
Hook to provide Server Name Indication (SNI) for TLS servers. When starting an HTTPS server, all solutions of this predicate are collected and a suitable sni_hook/1 is defined for ssl_context/3 to use different contexts depending on the host name of the client request. This hook is executed before privileges are dropped.

3.14.4 From (Unix) inetd

All modern Unix systems handle a large number of the services they run through the super-server inetd or one of its descendants (xinetd, systemd etc.) Such a program reads a configuration file (for example /etc/inetd.conf) and opens server-sockets on all ports defined in this file. As a request comes in it accepts it and starts the associated server such that standard I/O is performed through the socket. This approach has several advantages:

  • Simplification of servers
    Servers don't have to know about sockets and -operations.

  • Centralised authorisation
    Using tcpwrappers and similar tools, simple and effective firewalling of all services can be realised.

  • Automatic start and monitor
    The inetd automatically starts the server‘just-in-time’and starts additional servers or restarts a crashed server according to its configuration.

The very small generic script for handling inetd based connections is in inetd_httpd, defining http_server/1:

http_server(:Goal)
Initialises and runs http_wrapper/5 in a loop until failure or end-of-file. This server does not support the Port option as the port is specified with the inetd configuration. The only supported option is After.

Here is the example from demo_inetd 

#!/usr/bin/pl -t main -q -f
:- use_module(demo_body).
:- use_module(inetd_httpd).

main :-
        http_server(reply).

With the above file installed in /home/jan/plhttp/demo_inetd, the following line in /etc/inetd enables the server at port 4001 guarded by tcpwrappers. After modifying inetd, send the daemon the HUP signal to make it reload its configuration. For more information, please check inetd.conf(5). 

4001 stream tcp nowait nobody /usr/sbin/tcpd /home/jan/plhttp/demo_inetd

3.14.5 MS-Windows

There are rumours that inetd has been ported to Windows.

3.14.6 As CGI script

To be done.

3.14.7 Using a reverse proxy

There are several options for public deployment of a web service. The main decision is whether to run it on a standard port (port 80 for HTTP, port 443 for HTTPS) or a non-standard port such as for example 8000 or 8080. Using a standard port below 1000 requires root access to the machine, and prevents other web services from using the same port. On the other hand, using a non-standard port may cause problems with intermediate proxy- and/or firewall policies that may block the port when you try to access the service from some networks. In both cases, you can either use a physical or a virtual machine running ---for example--- under VMWARE or XEN to host the service. Using a dedicated (physical or virtual) machine to host a service isolates security threats. Isolation can also be achieved using a Unix chroot environment, which is however not a security feature.

To make several different web services reachable on the same (either standard or non-standard) port, you can use a so-called reverse proxy. A reverse proxy uses rules to relay requests to other web services that use their own dedicated ports. This approach has several advantages:

  • We can run the service on a non-standard port, but still access it (via the proxy) on a standard port, just as for a dedicated machine. We do not need a separate machine though: We only need to configure the reverse proxy to relay requests to the intended target servers.
  • As the main web server is doing the front-line service, the Prolog server is normally protected from malformed HTTP requests that could result in denial of service or otherwise compromise the server. In addition, the main web server can transparently provide encodings such as compression to the outside world.

Proxy technology can be combined with isolation methods such as dedicated machines, virtual machines and chroot jails. The proxy can also provide load balancing.

Setting up an Apache reverse proxy

The Apache reverse proxy setup is really simple. Ensure the modules proxy and proxy_http are loaded. Then add two simple rules to the server configuration. Below is an example that makes a PlDoc server on port 4000 available from the main Apache server at port 80. 

ProxyPass        /pldoc/ http://localhost:4000/pldoc/
ProxyPassReverse /pldoc/ http://localhost:4000/pldoc/

Apache rewrites the HTTP headers passing by, but using the above rules it does not examine the content. This implies that URLs embedded in the (HTML) content must use relative addressing. If the locations on the public and Prolog server are the same (as in the example above) it is allowed to use absolute locations. I.e. /pldoc/search is ok, but http://myhost.com:4000/pldoc/search is not. If the locations on the server differ, locations must be relative (i.e. not start with /.

This problem can also be solved using the contributed Apache module proxy_html that can be instructed to rewrite URLs embedded in HTML documents. In our experience, this is not troublefree as URLs can appear in many places in generated documents. JavaScript can create URLs on the fly, which makes rewriting virtually impossible.

3.15 The wrapper library

The body is called by the module library(http/http_wrapper.pl). This module realises the communication between the I/O streams and the body described in section 3.1. The interface is realised by http_wrapper/5:

http_wrapper(:Goal, +In, +Out, -Connection, +Options)
Handle an HTTP request where In is an input stream from the client, Out is an output stream to the client and Goal defines the goal realising the body. Connection is unified to ’Keep-alive’ if both ends of the connection want to continue the connection or close if either side wishes to close the connection.

This predicate reads an HTTP request-header from In, redirects current output to a memory file and then runs call(Goal, Request), watching for exceptions and failure. If Goal executes successfully it generates a complete reply from the created output. Otherwise it generates an HTTP server error with additional context information derived from the exception.

http_wrapper/5 supports the following options:

request(-Request)
Return the executed request to the caller.
peer(+Peer)
Add peer(Peer) to the request header handed to Goal. The format of Peer is defined by tcp_accept/3 from the clib package.
http:request_expansion(+RequestIn, -RequestOut)
This multifile hook predicate is called just before the goal that produces the body, while the output is already redirected to collect the reply. If it succeeds it must return a valid modified request. It is allowed to throw exceptions as defined in section 3.1.1. It is intended for operations such as mapping paths, deny access for certain requests or manage cookies. If it writes output, these must be HTTP header fields that are added before header fields written by the body. The example below is from the session management library (see section 3.5) sets a cookie. 
        ...,
        format('Set-Cookie: ~w=~w; path=~w~n', [Cookie, SessionID, Path]),
        ...,
http_current_request(-Request)
Get access to the currently executing request. Request is the same as handed to Goal of http_wrapper/5 after applying rewrite rules as defined by http:request_expansion/2. Raises an existence error if there is no request in progress.
http_relative_path(+AbsPath, -RelPath)
Convert an absolute path (without host, fragment or search) into a path relative to the current page, defined as the path component from the current request (see http_current_request/1). This call is intended to create reusable components returning relative paths for easier support of reverse proxies.

If ---for whatever reason--- the conversion is not possible it simply unifies RelPath to AbsPath.

3.16 library(http/http_host): Obtain public server location

This library finds the public address of the running server. This can be used to construct URLs that are visible from anywhere on the internet. This module was introduced to deal with OpenID, where a request is redirected to the OpenID server, which in turn redirects to our server (see http_openid.pl).

The address is established from the settings http:public_host and http:public_port if provided. Otherwise it is deduced from the request.

[det]http_public_url(+Request, -URL)
True when URL is an absolute URL for the current request. Typically, the login page should redirect to this URL to avoid losing the session.
[det]http_public_host_url(+Request, -URL)
True when URL is the public URL at which this server can be contacted. This value is not easy to obtain. See http_public_host/4 for the hardest part: find the host and port.
[det]http_public_host(?Request, -Hostname, -Port, +Options)
Current global host and port of the HTTP server. This is the basis to form absolute address, which we need for redirection based interaction such as the OpenID protocol. Options are:
global(+Bool)
If true (default false), try to replace a local hostname by a world-wide accessible name.

This predicate performs the following steps to find the host and port:

  1. Use the settings http:public_host and http:public_port
  2. Use X-Forwarded-Host header, which applies if this server runs behind a proxy.
  3. Use the Host header, which applies for HTTP 1.1 if we are contacted directly.
  4. Use gethostname/1 to find the host and http_current_server/2 to find the port.
Request is the current request. If it is left unbound, and the request is needed, it is obtained with http_current_request/1.
[det]http_current_host(?Request, -Hostname, -Port, +Options)
deprecated
Use http_public_host/4 (same semantics)

3.17 library(http/http_log): HTTP Logging module

Simple module for logging HTTP requests to a file. Logging is enabled by loading this file and ensure the setting http:logfile is not the empty atom. The default file for writing the log is httpd.log. See library(settings) for details.

The level of logging can be modified using the multifile predicate http_log:nolog/1 to hide HTTP request fields from the logfile and http_log:password_field/1 to hide passwords from HTTP search specifications (e.g. /topsecret?password=secret).

[semidet]http_log_stream(-Stream)
True when Stream is a stream to the opened HTTP log file. Opens the log file in append mode if the file is not yet open. The log file is determined from the setting http:logfile. If this setting is set to the empty atom (” ), this predicate fails.

If a file error is encountered, this is reported using print_message/2, after which this predicate silently fails. Opening is retried every minute when a new message arrives.

Before opening the log file, the message http_log_open(Term) is broadcasted. This message allows for creating the directory, renaming, deleting or truncating an existing log file.

[det]http_log_close(+Reason)
If there is a currently open HTTP logfile, close it after adding a term server(Reason, Time). to the logfile. This call is intended for cooperation with the Unix logrotate facility using the following schema:

  • Move logfile (the HTTP server keeps writing to the moved file)
  • Inform the server using an HTTP request that calls http_log_close/1
  • Compress the moved logfile
author
Suggested by Jacco van Ossenbruggen
[det]http_log(+Format, +Args)
Write message from Format and Args to log-stream. See format/2 for details. Succeed without side effects if logging is not enabled.
[semidet,multifile]password_field(+Field)
Multifile predicate that can be defined to hide passwords from the logfile.
[multifile]nolog(+HTTPField)
Multifile predicate that can be defined to hide request parameters from the request logfile.
[semidet,multifile]nolog_post_content_type(+Type)
Multifile hook called with the Content-type header. If the hook succeeds, the POST data is not logged. For example, to stop logging anything but application/json messages: 
:- multifile http_log:nolog_post_content_type/1.

http_log:nolog_post_content_type(Type) :-
   Type \= (application/json).
Type is a term MainType/SubType
[det]post_data_encoded(?Bytes:string, ?Encoded:string)
Encode the POST body for inclusion into the HTTP log file. The POST data is (in/de)flated using zopen/3 and base64 encoded using base64//1. The encoding makes long text messages shorter and keeps readable logfiles if binary data is posted.
[det]http_logrotate(+Options)
Rotate the available log files. Note that there are two ways to deal with the rotation of log files:

  1. Use the OS log rotation facility. In that case the OS must (1) move the logfile and (2) have something calling http_log_close/1 to close the (moved) file and make this server create a new one on the next log message. If library(http/http_unix_daemon) is used, closing is achieved by sending SIGHUP or SIGUSR1 to the process.
  2. Call this predicate at scheduled intervals. This can be achieved by calling http_schedule_logrotate/2 in the context of library(http/http_unix_daemon) which schedules the maintenance actions.

Options:

min_size(+Bytes)
Do not rotate if the log file is smaller than Bytes. The default is 1Mbytes.
keep_logs(+Count)
Number of rotated log files to keep (default 10)
compress_logs(+Format)
Compress the log files to the given format.
background(+Boolean)
If true, rotate the log files in the background.
http_schedule_logrotate(When, Options)
Schedule log rotation based on maintenance broadcasts. When is one of:
daily(Hour:Min)
Run each day at Hour:Min. Min is rounded to a multitude of 5.
weekly(Day, Hour:Min)
Run at the given Day and Time each week. Day is either a number 1..7 (1 is Monday) or a weekday name or abbreviation.
monthly(DayOfTheMonth, Hour:Min)
Run each month at the given Day (1..31). Note that not all months have all days.

This must be used with a timer that broadcasts a maintenance(_,_) message (see broadcast/1). Such a timer is part of library(http/http_unix_daemon).

3.18 library(http/http_server_health): HTTP Server health statistics

This module defines an HTTP handler for /health. The handler returns a JSON document with elementary health statistics on the running instance. The location can be changed using http_handler/3. Keys may be added using additional clauses for health/2 or hidden using hide/1.

This library defines an HTTP handler and defines two multifile predicates (health/2 and hide/1) to control the information presented.

server_health(+Request)
HTTP handler that replies with the overall health of the server. Returns a JSON object from all solutions of health/2.

Processes an optional parameter fields to specify the fields that should be returned. The fields content is "," or white space delimited.

[nondet,multifile]health(-Key, -Value)
Multifile extensible. True when Key/Value can be reported as a health statistics. Keys may be added by adding clauses to this multifile predicate. Keys may be filtered using hide/1. Predefined Key values are:
up
Defined to be true.
epoch
Starting time of the server in seconds after Jan 1, 1970 UTC.
cpu_time
Total process CPU usage in seconds.
threads
Number of active threads
workers
Number of HTTP worker threads.
requests
Number of HTTP requests processed.
bytes_sent
Number of bytes send in reply to HTTP requests.
open_files
Number of open file streams. This includes physical files as well as sockets (except for Windows). On Linux we count the file handles in /proc/self/fd. Otherwise we use stream_property/2 with the file_no(Fd) property.
loadavg
An array holding the load average over the last [1,5,15] minutes. This key is only supported on Linux machines. It is based on /proc/loadavg
heap
When compiled with TCMalloc, this provides two properties:
inuse:Bytes
Total amount of in-use memory in bytes
size:Bytes
Same as inuse, but including the TCMalloc overhead and (thus) memory that has been freed and is not (yet) reused.
Key is the name of the JSON key. Must be an atom
Value is the Prolog representation for a JSON (dict) value.
[nondet,multifile]hide(?Key)
Multifile hook. If true for a specific Key, hide this statistics from the output. This may be used to hide keys that are considered a security risk.

3.19 Debugging HTTP servers

The library library(http/http_error) defines a hook that decorates uncaught exceptions with a stack-trace. This will generate a 500 internal server error document with a stack-trace. To enable this feature, simply load this library. Please do note that providing error information to the user simplifies the job of a hacker trying to compromise your server. It is therefore not recommended to load this file by default.

The example program calc.pl has the error handler loaded which can be triggered by forcing a divide-by-zero in the calculator.

3.20 library(http/http_header): Handling HTTP headers

The library library(http/http_header) provides primitives for parsing and composing HTTP headers. Its functionality is normally hidden by the other parts of the HTTP server and client libraries.

[det]http_read_request(+FdIn:stream, -Request)
Read an HTTP request-header from FdIn and return the broken-down request fields as +Name(+Value) pairs in a list. Request is unified to end_of_file if FdIn is at the end of input.
http_read_reply_header(+FdIn, -Reply)
Read the HTTP reply header. Throws an exception if the current input does not contain a valid reply header.
[det]http_reply(+Data, +Out:stream)
[det]http_reply(+Data, +Out:stream, +HdrExtra)
[det]http_reply(+Data, +Out:stream, +HdrExtra, -Code)
[det]http_reply(+Data, +Out:stream, +HdrExtra, +Context, -Code)
[det]http_reply(+Data, +Out:stream, +HdrExtra, +Context, +Request, -Code)
Compose a complete HTTP reply from the term Data using additional headers from HdrExtra to the output stream Out. ExtraHeader is a list of Field(Value). Data is one of:
html(HTML)
HTML tokens as produced by html//1 from html_write.pl
file(+MimeType, +FileName)
Reply content of FileName using MimeType
file(+MimeType, +FileName, +Range)
Reply partial content of FileName with given MimeType
tmp_file(+MimeType, +FileName)
Same as file, but do not include modification time
bytes(+MimeType, +Bytes)
Send a sequence of Bytes with the indicated MimeType. Bytes is either a string of character codes 0..255 or list of integers in the range 0..255. Out-of-bound codes result in a representation error exception.
stream(+In, +Len)
Reply content of stream.
cgi_stream(+In, +Len)
Reply content of stream, which should start with an HTTP header, followed by a blank line. This is the typical output from a CGI script.
Status
HTTP status report as defined by http_status_reply/4.
HdrExtra provides additional reply-header fields, encoded as Name(Value). It can also contain a field content_length(-Len) to retrieve the value of the Content-length header that is replied.
Code is the numeric HTTP status code sent
To be done
Complete documentation
[det]http_status_reply(+Status, +Out, +HdrExtra, -Code)
[det]http_status_reply(+Status, +Out, +HdrExtra, +Context, -Code)
[det]http_status_reply(+Status, +Out, +HdrExtra, +Context, +Request, -Code)
Emit HTML non-200 status reports. Such requests are always sent as UTF-8 documents.

Status can be one of the following:

authorise(Method)
Challenge authorization. Method is one of

  • basic(Realm)
  • digest(Digest)
authorise(basic, Realm)
Same as authorise(basic(Realm)). Deprecated.
bad_request(ErrorTerm)
busy
created(Location)
forbidden(Url)
moved(To)
moved_temporary(To)
no_content
not_acceptable(WhyHtml)
not_found(Path)
method_not_allowed(Method, Path)
not_modified
resource_error(ErrorTerm)
see_other(To)
switching_protocols(Goal, Options)
server_error(ErrorTerm)
unavailable(WhyHtml)
[semidet,multifile]http:serialize_reply(+Reply, -Body)
Multifile hook to serialize the result of http:status_reply/3 into a term
body(Type, Encoding, Content)
In this term, Type is the media type, Encoding is the required wire encoding and Content a string representing the content.
http_join_headers(+Default, +Header, -Out)
Append headers from Default to Header if they are not already part of it.
http_update_encoding(+HeaderIn, -Encoding, -HeaderOut)
Allow for rewrite of the header, adjusting the encoding. We distinguish three options. If the user announces‘text’, we always use UTF-8 encoding. If the user announces charset=utf-8 we use UTF-8 and otherwise we use octet (raw) encoding. Alternatively we could dynamically choose for ASCII, ISO-Latin-1 or UTF-8.
[semidet,multifile]http:mime_type_encoding(+MimeType, -Encoding)
Encoding is the (default) character encoding for MimeType. This is used for setting the encoding for HTTP replies after the user calls format('Content-type: <MIME type>~n'). This hook is called before mime_type_encoding/2. This default defines utf8 for JSON and Turtle derived application/ MIME types.
http_update_connection(+CGIHeader, +Request, -Connection, -Header)
Merge keep-alive information from Request and CGIHeader into Header.
http_update_transfer(+Request, +CGIHeader, -Transfer, -Header)
Decide on the transfer encoding from the Request and the CGI header. The behaviour depends on the setting http:chunked_transfer. If never, even explitic requests are ignored. If on_request, chunked encoding is used if requested through the CGI header and allowed by the client. If if_possible, chunked encoding is used whenever the client allows for it, which is interpreted as the client supporting HTTP 1.1 or higher.

Chunked encoding is more space efficient and allows the client to start processing partial results. The drawback is that errors lead to incomplete pages instead of a nicely formatted complete page.

[det]http_post_data(+Data, +Out:stream, +HdrExtra)
Send data on behalf on an HTTP POST request. This predicate is normally called by http_post/4 from http_client.pl to send the POST data to the server. Data is one of:

  • html(+Tokens) Result of html//1 from html_write.pl
  • json(+Term) Posting a JSON query and processing the JSON reply (or any other reply understood by http_read_data/3) is simple as http_post(URL, json(Term), Reply, []), where Term is a JSON term as described in json.pl and reply is of the same format if the server replies with JSON, when using module :- use_module(library(http/http_json)). Note that the module is used in both http server and http client, see library(http/http_json).
  • xml(+Term) Post the result of xml_write/3 using the Mime-type text/xml
  • xml(+Type, +Term) Post the result of xml_write/3 using the given Mime-type and an empty option list to xml_write/3.
  • xml(+Type, +Term, +Options) Post the result of xml_write/3 using the given Mime-type and option list for xml_write/3.
  • file(+File) Send contents of a file. Mime-type is determined by file_mime_type/2.
  • file(+Type, +File) Send file with content of indicated mime-type.
  • memory_file(+Type, +Handle) Similar to file(+Type, +File), but using a memory file instead of a real file. See new_memory_file/1.
  • codes(+Codes) As codes(text/plain, Codes).
  • codes(+Type, +Codes) Send Codes using the indicated MIME-type.
  • bytes(+Type, +Bytes) Send Bytes using the indicated MIME-type. Bytes is either a string of character codes 0..255 or list of integers in the range 0..255. Out-of-bound codes result in a representation error exception.
  • atom(+Atom) As atom(text/plain, Atom).
  • atom(+Type, +Atom) Send Atom using the indicated MIME-type.
  • string(+String)
  • string(+Type, +String) Similar to atom(Atom) and atom(Type,Atom), accepting a SWI-Prolog string.
  • cgi_stream(+Stream, +Len) Read the input from Stream which, like CGI data starts with a partial HTTP header. The fields of this header are merged with the provided HdrExtra fields. The first Len characters of Stream are used.
  • form(+ListOfParameter) Send data of the MIME type application/x-www-form-urlencoded as produced by browsers issuing a POST request from an HTML form. ListOfParameter is a list of Name=Value or Name(Value).
  • form_data(+ListOfData) Send data of the MIME type multipart/form-data as produced by browsers issuing a POST request from an HTML form using enctype multipart/form-data. ListOfData is the same as for the List alternative described below. Below is an example. Repository, etc. are atoms providing the value, while the last argument provides a value from a file. 
    ...,
http_post([ protocol(http),
            host(Host),
            port(Port),
            path(ActionPath)
          ],
          form_data([ repository = Repository,
                      dataFormat = DataFormat,
                      baseURI    = BaseURI,
                      verifyData = Verify,
                      data       = file(File)
                    ]),
          _Reply,
          []),
...,

  • List If the argument is a plain list, it is sent using the MIME type multipart/mixed and packed using mime_pack/3. See mime_pack/3 for details on the argument format.
[det]http_reply_header(+Out:stream, +What, +HdrExtra)
Create a reply header using reply_header//3 and send it to Stream.
[semidet]http_parse_header_value(+Field, +Value, -Prolog)
Translate Value in a meaningful Prolog term. Field denotes the HTTP request field for which we do the translation. Supported fields are:
content_length
Converted into an integer
status
Converted into an integer
cookie
Converted into a list with Name=Value by cookies//1.
set_cookie
Converted into a term set_cookie(Name, Value, Options). Options is a list consisting of Name=Value or a single atom (e.g., secure)
host
Converted to HostName:Port if applicable.
range
Converted into bytes(From, To), where From is an integer and To is either an integer or the atom end.
accept
Parsed to a list of media descriptions. Each media is a term media(Type, TypeParams, Quality, AcceptExts). The list is sorted according to preference.
content_disposition
Parsed into disposition(Name, Attributes), where Attributes is a list of Name=Value pairs.
content_type
Parsed into media(Type/SubType, Attributes), where Attributes is a list of Name=Value pairs.
expires
Parsed into a time stamp using http_timestamp/2.

As some fields are already parsed in the Request, this predicate is a no-op when called on an already parsed field.

Value is either an atom, a list of codes or an already parsed header value.
[det]http_timestamp(?Time:timestamp, ?Text:atom)
Convert between a SWI-Prolog time stamp and a string in HTTP format (RFC1123). When parsing, it accepts RFC1123, RFC1036 and ASCTIME formats. See parse_time/3.
Errors
syntax_error(http_timestamp(Text)) if the string cannot be parsed.
[det]http_read_header(+Fd, -Header)
Read Name: Value lines from FD until an empty line is encountered. Field-name are converted to Prolog conventions (all lower, _ instead of -): Content-Type: text/html --> content_type(text/html)
[det]http_parse_header(+Text:codes, -Header:list)
Header is a list of Name(Value)-terms representing the structure of the HTTP header in Text.
Errors
domain_error(http_request_line, Line)
[det,multifile]http://(http_address)
HTML-rule that emits the location of the HTTP server. This hook is called from address//0 to customise the server address. The server address is emitted on non-200-ok replies.
[semidet,multifile]http:status_page(+Status, +Context, -HTMLTokens)
Hook called by http_status_reply/4 and http_status_reply/5 that allows for emitting custom error pages for the following HTTP page types:

  • 201 - created(Location)
  • 301 - moved(To)
  • 302 - moved_temporary(To)
  • 303 - see_other(To)
  • 400 - bad_request(ErrorTerm)
  • 401 - authorise(AuthMethod)
  • 403 - forbidden(URL)
  • 404 - not_found(URL)
  • 405 - method_not_allowed(Method,URL)
  • 406 - not_acceptable(Why)
  • 500 - server_error(ErrorTerm)
  • 503 - unavailable(Why)

The hook is tried twice, first using the status term, e.g., not_found(URL) and than with the code, e.g. 404. The second call is deprecated and only exists for compatibility.

Context is the 4th argument of http_status_reply/5, which is invoked after raising an exception of the format http_reply(Status, HeaderExtra, Context). The default context is [] (the empty list).
HTMLTokens is a list of tokens as produced by html//1. It is passed to print_html/2.

3.21 The library(http/html_write) library

Producing output for the web in the form of an HTML document is a requirement for many Prolog programs. Just using format/2 is not satisfactory as it leads to poorly readable programs generating poor HTML. This library is based on using DCG rules.

The library(http/html_write) structures the generation of HTML from a program. It is an extensible library, providing a DCG framework for generating legal HTML under (Prolog) program control. It is especially useful for the generation of structured pages (e.g. tables) from Prolog data structures.

The normal way to use this library is through the DCG html//1. This non-terminal provides the central translation from a structured term with embedded calls to additional translation rules to a list of atoms that can then be printed using print_html/[1,2].

html(:Spec)//
The DCG non-terminal html//1 is the main predicate of this library. It translates the specification for an HTML page into a list of atoms that can be written to a stream using print_html/[1,2]. The expansion rules of this predicate may be extended by defining the multifile DCG html_write:expand//1. Spec is either a single specification or a list of single specifications. Using nested lists is not allowed to avoid ambiguity caused by the atom []

  • Atomic data
    Atomic data is quoted using html_quoted//1.

  • Fmt - Args
    Fmt and Args are used as format-specification and argument list to format/3. The result is quoted and added to the output list.

  • \List
    Escape sequence to add atoms directly to the output list. This can be used to embed external HTML code or emit script output. List is a list of the following terms:

    • Fmt - Args
      Fmt and Args are used as format-specification and argument list to format/3. The result is added to the output list.
    • Atomic
      Atomic values are added directly to the output list.

  • \Term
    Invoke the non-terminal Term in the calling module. This is the common mechanism to realise abstraction and modularisation in generating HTML.

  • Module:Term
    Invoke the non-terminal <Module>:<Term>. This is similar to \Term but allows for invoking grammar rules in external packages.

  • &(Entity)
    Emit &<Entity>; or &#<Entity>; if Entity is an integer. SWI-Prolog atoms and strings are represented as Unicode. Explicit use of this construct is rarely needed because code-points that are not supported by the output encoding are automatically converted into character-entities.

  • Tag(Content)
    Emit HTML element Tag using Content and no attributes. Content is handed to html//1. See section 3.21.4 for details on the automatically generated layout.

  • Tag(Attributes, Content)
    Emit HTML element Tag using Attributes and Content. Attributes is either a single attribute of a list of attributes. Each attributes is of the format Name(Value) or Name=Value. Value is the atomic attribute value but allows for a limited functional notation:

    • A + B
      Concatenation of A and B
    • Format-Arguments
      Use format/3 and emit the result as quoted value.
    • encode(Atom)
      Use uri_encoded/3 to create a valid URL query component.
    • location_by_id(ID)
      HTTP location of the HTTP handler with given ID. See http_location_by_id/2.
    • #(ID)
      Abbreviated for for location_by_id(ID).
    • A + List
      List is handled as a URL‘search’component. The list members are terms of the format Name = Value or Name(Value). Values are encoded as in the encode option described above.
    • List
      Emit SGML multi-valued attributes (e.g., NAMES). Each value in list is separated by a space. This is particularly useful for setting multiple class attributes on an element. For example: 
              ...
        span(class([c1,c2]), ...),

    The example below generates a URL that references the predicate set_lang/1 in the application with given parameters. The http_handler/3 declaration binds /setlang to the predicate set_lang/1 for which we provide a very simple implementation. The code between ... is part of an HTML page showing the english flag which, when pressed, calls set_lang(Request) where Request contains the search parameter lang = en. Note that the HTTP location (path) /setlang can be moved without affecting this code. 

    :- http_handler('/setlang', set_lang, []).

set_lang(Request) :-
        http_parameters(Request,
                        [ lang(Lang, [])
                        ]),
        http_session_retractall(lang(_)),
        http_session_assert(lang(Lang)),
        reply_html_page(title('Switched language'),
                        p(['Switch language to ', Lang])).


        ...
        html(a(href(location_by_id(set_lang) + [lang(en)]),
               img(src('/www/images/flags/en.png')))),
        ...

page(:HeadContent, :BodyContent)//
The DCG non-terminal page//2 generated a complete page, including the SGML DOCTYPE declaration. HeadContent are elements to be placed in the head element and BodyContent are elements to be placed in the body element.

To achieve common style (background, page header and footer), it is possible to define DCG non-terminals head//1 and/or body//1. Non-terminal page//1 checks for the definition of these non-terminals in the module it is called from as well as in the user module. If no definition is found, it creates a head with only the HeadContent (note that the title is obligatory) and a body with bgcolor set to white and the provided BodyContent.

Note that further customisation is easily achieved using html//1 directly as page//2 is (besides handling the hooks) defined as: 

page(Head, Body) -->
        html([ \['<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 4.0//EN">\n'],
               html([ head(Head),
                      body(bgcolor(white), Body)
                    ])
             ]).
page(:Contents)//
This version of the page/[1,2] only gives you the SGML DOCTYPE and the HTML element. Contents is used to generate both the head and body of the page.
html_begin(+Begin)//
Just open the given element. Begin is either an atom or a compound term, In the latter case the arguments are used as arguments to the begin-tag. Some examples: 
        html_begin(table)
        html_begin(table(border(2), align(center)))

This predicate provides an alternative to using the \Command syntax in the html//1 specification. The following two fragments are the same. The preferred solution depends on your preferences as well as whether the specification is generated or entered by the programmer. 

table(Rows) -->
        html(table([border(1), align(center), width('80%')],
                   [ \table_header,
                     \table_rows(Rows)
                   ])).

% or

table(Rows) -->
        html_begin(table(border(1), align(center), width('80%'))),
        table_header,
        table_rows,
        html_end(table).
html_end(+End)//
End an element. See html_begin/1 for details.

3.21.1 Emitting HTML documents

The non-terminal html//1 translates a specification into a list of atoms and layout instructions. Currently the layout instructions are terms of the format nl(N), requesting at least N newlines. Multiple consecutive nl(1) terms are combined to an atom containing the maximum of the requested number of newline characters.

To simplify handing the data to a client or storing it into a file, the following predicates are available from this library:

reply_html_page(:Head, :Body)
Same as reply_html_page(default, Head, Body).
reply_html_page(+Style, :Head, :Body)
Writes an HTML page preceded by an HTTP header as required by library(http_wrapper) (CGI-style). Here is a simple typical example: 
reply(Request) :-
        reply_html_page(title('Welcome'),
                        [ h1('Welcome'),
                          p('Welcome to our ...')
                        ]).

The header and footer of the page can be hooked using the grammar-rules user:head//2 and user:body//2. The first argument passed to these hooks is the Style argument of reply_html_page/3 and the second is the 2nd (for head//2) or 3rd (for body//2) argument of reply_html_page/3. These hooks can be used to restyle the page, typically by embedding the real body content in a div. E.g., the following code provides a menu on top of each page of that is identified using the style myapp. 

:- multifile
        user:body//2.

user:body(myapp, Body) -->
        html(body([ div(id(top), \application_menu),
                    div(id(content), Body)
                  ])).

Redefining the head can be used to pull in scripts, but typically html_requires//1 provides a more modular approach for pulling scripts and CSS-files.

reply_html_partial(+HTML)
Reply with partial HTML document. The reply only contains the element from HTML, i.e., this predicate does not add a DOCTYPE header, html, head or body. It is intended for JavaScript handlers that request a partial document and insert that somewhere into the existing page DOM. See reply_html_page/3 to reply with a complete (valid) HTML page.
print_html(+List)
Print the token list to the Prolog current output stream.
print_html(+Stream, +List)
Print the token list to the specified output stream
html_print_length(+List, -Length)
When calling html_print/[1,2] on List, Length characters will be produced. Knowing the length is needed to provide the Content-length field of an HTTP reply-header.

3.21.2 Repositioning HTML for CSS and javascript links

Modern HTML commonly uses CSS and Javascript. This requires <link> elements in the HTML <head> element or <script> elements in the <body>. Unfortunately this seriously harms re-using HTML DCG rules as components as each of these components may rely on their own style sheets or JavaScript code. We added a‘mailing’system to reposition and collect fragments of HTML. This is implemented by html_post//2, html_receive//1 and html_receive//2.

[det]html_post(+Id, :HTML)//
Reposition HTML to the receiving Id. The html_post//2 call processes HTML using html//1. Embedded \-commands are executed by mailman/1 from print_html/1 or html_print_length/2. These commands are called in the calling context of the html_post//2 call.

A typical usage scenario is to get required CSS links in the document head in a reusable fashion. First, we define css//1 as: 

css(URL) -->
        html_post(css,
                  link([ type('text/css'),
                         rel('stylesheet'),
                         href(URL)
                       ])).

Next we insert the unique CSS links, in the pagehead using the following call to reply_html_page/2: 

        reply_html_page([ title(...),
                          \html_receive(css)
                        ],
                        ...)
[det]html_receive(+Id)//
Receive posted HTML tokens. Unique sequences of tokens posted with html_post//2 are inserted at the location where html_receive//1 appears.
See also
- The local predicate sorted_html//1 handles the output of html_receive//1.
- html_receive//2 allows for post-processing the posted material.
[det]html_receive(+Id, :Handler)//
This extended version of html_receive//1 causes Handler to be called to process all messages posted to the channal at the time output is generated. Handler is called as below, where PostedTerms is a list of Module:Term created from calls to html_post//2. Module is the context module of html_post and Term is the unmodified term. Members in PostedTerms are in the order posted and may contain duplicates. 
  phrase(Handler, PostedTerms, HtmlTerms, Rest)

Typically, Handler collects the posted terms, creating a term suitable for html//1 and finally calls html//1.

The library predefines the receiver channel head at the end of the head element for all pages that write the html head through this library. The following code can be used anywhere inside an HTML generating rule to demand a javascript in the header: 

js_script(URL) -->
        html_post(head, script([ src(URL),
                                 type('text/javascript')
                               ], [])).

This mechanism is also exploited to add XML namespace (xmlns) declarations to the (outer) html element using xhml_ns//2:

xhtml_ns(+Id, +Value)//
Demand an xmlns:id=Value in the outer html tag. This uses the html_post/2 mechanism to post to the xmlns channel. Rdfa (http://www.w3.org/2006/07/SWD/RDFa/syntax/), embedding RDF in (x)html provides a typical usage scenario where we want to publish the required namespaces in the header. We can define: 
rdf_ns(Id) -->
        { rdf_global_id(Id:'', Value) },
        xhtml_ns(Id, Value).

After which we can use rdf_ns//1 as a normal rule in html//1 to publish namespaces from library(semweb/rdf_db). Note that this macro only has effect if the dialect is set to xhtml. In html mode it is silently ignored.

The required xmlns receiver is installed by html_begin//1 using the html tag and thus is present in any document that opens the outer html environment through this library.

3.21.3 Adding rules for html//1

In some cases it is practical to extend the translations imposed by html//1. We used this technique to define translation rules for the output of the SWI-Prolog library(sgml) package.

The html//1 non-terminal first calls the multifile ruleset html_write:expand//1.

html_write:expand(+Spec)//
Hook to add additional translation rules for html//1.
html_quoted(+Atom)//
Emit the text in Atom, inserting entity-references for the SGML special characters <&>.
html_quoted_attribute(+Atom)//
Emit the text in Atom suitable for use as an SGML attribute, inserting entity-references for the SGML special characters <&>".

3.21.4 Generating layout

Though not strictly necessary, the library attempts to generate reasonable layout in SGML output. It does this only by inserting newlines before and after tags. It does this on the basis of the multifile predicate html_write:layout/3

html_write:layout(+Tag, -Open, -Close)
Specify the layout conventions for the element Tag, which is a lowercase atom. Open is a term Pre-Post. It defines that the element should have at least Pre newline characters before and Post after the tag. The Close specification is similar, but in addition allows for the atom -, requesting the output generator to omit the close-tag altogether or empty, telling the library that the element has declared empty content. In this case the close-tag is not emitted either, but in addition html//1 interprets Arg in Tag(Arg) as a list of attributes rather than the content.

A tag that does not appear in this table is emitted without additional layout. See also print_html/[1,2]. Please consult the library source for examples.

3.21.5 Examples for using the HTML write library

In the following example we will generate a table of Prolog predicates we find from the SWI-Prolog help system based on a keyword. The primary database is defined by the predicate predicate/5 We will make hyperlinks for the predicates pointing to their documentation. 

html_apropos(Kwd) :-
        findall(Pred, apropos_predicate(Kwd, Pred), Matches),
        phrase(apropos_page(Kwd, Matches), Tokens),
        print_html(Tokens).

%       emit page with title, header and table of matches

apropos_page(Kwd, Matches) -->
        page([ title(['Predicates for ', Kwd])
             ],
             [ h2(align(center),
                  ['Predicates for ', Kwd]),
               table([ align(center),
                       border(1),
                       width('80%')
                     ],
                     [ tr([ th('Predicate'),
                            th('Summary')
                          ])
                     | \apropos_rows(Matches)
                     ])
             ]).

%       emit the rows for the body of the table.

apropos_rows([]) -->
        [].
apropos_rows([pred(Name, Arity, Summary)|T]) -->
        html([ tr([ td(\predref(Name/Arity)),
                    td(em(Summary))
                  ])
             ]),
        apropos_rows(T).

%       predref(Name/Arity)
%
%       Emit Name/Arity as a hyperlink to
%
%               /cgi-bin/plman?name=Name&arity=Arity
%
%       we must do form-encoding for the name as it may contain illegal
%       characters.  www_form_encode/2 is defined in library(url).

predref(Name/Arity) -->
        { www_form_encode(Name, Encoded),
          sformat(Href, '/cgi-bin/plman?name=~w&arity=~w',
                  [Encoded, Arity])
        },
        html(a(href(Href), [Name, /, Arity])).

%       Find predicates from a keyword. '$apropos_match' is an internal
%       undocumented predicate.

apropos_predicate(Pattern, pred(Name, Arity, Summary)) :-
        predicate(Name, Arity, Summary, _, _),
        (   '$apropos_match'(Pattern, Name)
        ->  true
        ;   '$apropos_match'(Pattern, Summary)
        ).

3.21.6 Remarks on the library(http/html_write) library

This library is the result of various attempts to reach at a more satisfactory and Prolog-minded way to produce HTML text from a program. We have been using Prolog for the generation of web pages in a number of projects. Just using format/2 never was not a real option, generating error-prone HTML from clumsy syntax. We started with a layer on top of format/2, keeping track of the current nesting and thus always capable of properly closing the environment.

DCG based translation however, naturally exploits Prolog's term-rewriting primitives. If generation fails for whatever reason it is easy to produce an alternative document (for example holding an error message).

In a future version we will probably define a goal_expansion/2 to do compile-time optimisation of the library. Quotation of known text and invocation of sub-rules using the \RuleSet and <Module>:<RuleSet> operators are costly operations in the analysis that can be done at compile-time.

3.22 library(http/js_write): Utilities for including JavaScript

This library is a supplement to library(http/html_write) for producing JavaScript fragments. Its main role is to be able to call JavaScript functions with valid arguments constructed from Prolog data. For example, suppose you want to call a JavaScript functions to process a list of names represented as Prolog atoms. This can be done using the call below, while without this library you would have to be careful to properly escape special characters. 

numbers_script(Names) -->
    html(script(type('text/javascript'),
         [ \js_call('ProcessNumbers'(Names)
         ]),

The accepted arguments are described with js_expression//1.

[det]js_script(+Content)//
Generate a JavaScript script element with the given content.
[det]javascript(+Content, +Vars, +VarDict, -DOM)
Quasi quotation parser for JavaScript that allows for embedding Prolog variables to substitude identifiers in the JavaScript snippet. Parameterizing a JavaScript string is achieved using the JavaScript + operator, which results in concatenation at the client side. 
    ...,
    js_script({|javascript(Id, Config)||
                $(document).ready(function() {
                   $("#"+Id).tagit(Config);
                 });
               |}),
    ...

The current implementation tokenizes the JavaScript input and yields syntax errors on unterminated comments, strings, etc. No further parsing is implemented, which makes it possible to produce syntactically incorrect and partial JavaScript. Future versions are likely to include a full parser, generating syntax errors.

The parser produces a term \List, which is suitable for js_script//1 and html//1. Embedded variables are mapped to \js_expression(Var), while the remaining text is mapped to atoms.

To be done
Implement a full JavaScript parser. Users should not rely on the ability to generate partial JavaScript snippets.
[det]js_call(+Term)//
Emit a call to a Javascript function. The Prolog functor is the name of the function. The arguments are converted from Prolog to JavaScript using js_arg_list//1. Please not that Prolog functors can be quoted atom and thus the following is legal: 
    ...
    html(script(type('text/javascript'),
         [ \js_call('x.y.z'(hello, 42))
         ]),
[det]js_new(+Id, +Term)//
Emit a call to a Javascript object declaration. This is the same as: 
['var ', Id, ' = new ', \js_call(Term)]
[det]js_arg_list(+Expressions:list)//
Write javascript (function) arguments. This writes "(", Arg, ..., ")". See js_expression//1 for valid argument values.
[det]js_expression(+Expression)//
Emit a single JSON argument. Expression is one of:
Variable
Emitted as Javascript null
List
Produces a Javascript list, where each element is processed by this library.
object(Attributes)
Where Attributes is a Key-Value list where each pair can be written as Key-Value, Key=Value or Key(Value), accomodating all common constructs for this used in Prolog. $ { K:V, ... } Same as object(Attributes), providing a more JavaScript-like syntax. This may be useful if the object appears literally in the source-code, but is generally less friendlyto produce as a result from a computation.
Dict
Emit a dict as a JSON object using json_write_dict/3.
json(Term)
Emits a term using json_write/3.
@(Atom)
Emits these constants without quotes. Normally used for the symbols true, false and null, but can also be use for emitting JavaScript symbols (i.e. function- or variable names).
Number
Emited literally
symbol(Atom)
Synonym for @(Atom). Deprecated.
Atom or String
Emitted as quoted JavaScript string.
[semidet]js_arg(+Expression)//
Same as js_expression//1, but fails if Expression is invalid, where js_expression//1 raises an error.
deprecated
New code should use js_expression//1.

3.23 library(http/http_path): Abstract specification of HTTP server locations

This module provides an abstract specification of HTTP server locations that is inspired on absolute_file_name/3. The specification is done by adding rules to the dynamic multifile predicate http:location/3. The speficiation is very similar to user:file_search_path/2, but takes an additional argument with options. Currently only one option is defined:

priority(+Integer)
If two rules match, take the one with highest priority. Using priorities is needed because we want to be able to overrule paths, but we do not want to become dependent on clause ordering.

The default priority is 0. Note however that notably libraries may decide to provide a fall-back using a negative priority. We suggest -100 for such cases.

This library predefines a single location at priority -100:

root
The root of the server. Default is /, but this may be overruled using the setting (see setting/2) http:prefix

To serve additional resource files such as CSS, JavaScript and icons, see library(http/http_server_files).

Here is an example that binds /login to login/1. The user can reuse this application while moving all locations using a new rule for the admin location with the option [priority(10)]. 

:- multifile http:location/3.
:- dynamic   http:location/3.

http:location(admin, /, []).

:- http_handler(admin(login), login, []).

login(Request) :-
        ...
[nondet,multifile]http:location(+Alias, -Expansion, -Options)
Multifile hook used to specify new HTTP locations. Alias is the name of the abstract path. Expansion is either a term Alias2(Relative), telling http_absolute_location/3 to translate Alias by first translating Alias2 and then applying the relative path Relative or, Expansion is an absolute location, i.e., one that starts with a /. Options currently only supports the priority of the path. If http:location/3 returns multiple solutions the one with the highest priority is selected. The default priority is 0.

This library provides a default for the abstract location root. This defaults to the setting http:prefix or, when not available to the path /. It is adviced to define all locations (ultimately) relative to root. For example, use root('home.html') rather than '/home.html'.

[det]http_absolute_uri(+Spec, -URI)
URI is the absolute (i.e., starting with http://) URI for the abstract specification Spec. Use http_absolute_location/3 to create references to locations on the same server.
[det]http_absolute_location(+Spec, -Path, +Options)
Path is the HTTP location for the abstract specification Spec. Options:
relative_to(Base)
Path is made relative to Base. Default is to generate absolute URLs.
See also
http_absolute_uri/2 to create a reference that can be used on another server.
http_clean_location_cache
HTTP locations resolved through http_absolute_location/3 are cached. This predicate wipes the cache. The cache is automatically wiped by make/0 and if the setting http:prefix is changed.

3.24 library(http/html_head): Automatic inclusion of CSS and scripts links

To be done
- Possibly we should add img//2 to include images from symbolic path notation.
- It would be nice if the HTTP file server could use our location declarations.

This library allows for abstract declaration of available CSS and Javascript resources and their dependencies using html_resource/2. Based on these declarations, html generating code can declare that it depends on specific CSS or Javascript functionality, after which this library ensures that the proper links appear in the HTML head. The implementation is based on mail system implemented by html_post/2 of library html_write.pl.

Declarations come in two forms. First of all http locations are declared using the http_path.pl library. Second, html_resource/2 specifies HTML resources to be used in the head and their dependencies. Resources are currently limited to Javascript files (.js) and style sheets (.css). It is trivial to add support for other material in the head. See html_include//1.

For usage in HTML generation, there is the DCG rule html_requires//1 that demands named resources in the HTML head.

3.24.1 About resource ordering

All calls to html_requires//1 for the page are collected and duplicates are removed. Next, the following steps are taken:

  1. Add all dependencies to the set
  2. Replace multiple members by‘aggregate’scripts or css files. see use_agregates/4.
  3. Order all resources by demanding that their dependencies preceede the resource itself. Note that the ordering of resources in the dependency list is ignored. This implies that if the order matters the dependency list must be split and only the primary dependency must be added.

3.24.2 Debugging dependencies

Use ?- debug(html(script)). to see the requested and final set of resources. All declared resources are in html_resource/3. The edit/1 command recognises the names of HTML resources.

3.24.3 Predicates

[det]html_resource(+About, +Properties)
Register an HTML head resource. About is either an atom that specifies an HTTP location or a term Alias(Sub). This works similar to absolute_file_name/2. See http:location_path/2 for details. Recognised properties are:
requires(+Requirements)
Other required script and css files. If this is a plain file name, it is interpreted relative to the declared resource. Requirements can be a list, which is equivalent to multiple requires properties.
virtual(+Bool)
If true (default false), do not include About itself, but only its dependencies. This allows for defining an alias for one or more resources.
ordered(+Bool)
Defines that the list of requirements is ordered, which means that each requirement in the list depends on its predecessor.
aggregate(+List)
States that About is an aggregate of the resources in List. This means that if both About and one of the elements of List appears in the dependencies, About is kept and the smaller one is dropped. If there are a number of dependencies on the small members, these are replaced with dependency on the big (aggregate) one, for example, to specify that a big javascript is actually the composition of a number of smaller ones.
mime_type(-Mime)
May be specified for non-virtual resources to specify the mime-type of the resource. By default, the mime type is derived from the file name using file_mime_type/2.

Registering the same About multiple times extends the properties defined for About. In particular, this allows for adding additional dependencies to a (virtual) resource.

[nondet]html_current_resource(?About)
True when About is a currently known resource.
[det]html_requires(+ResourceOrList)//
Include ResourceOrList and all dependencies derived from it and add them to the HTML head using html_post/2. The actual dependencies are computed during the HTML output phase by html_insert_resource//1.
[det]html_insert_resource(+ResourceOrList)//
Actually include HTML head resources. Called through html_post//2 from html_requires//1 after rewrite by html_head_expansion/2. We are guaranteed we will only get one call that is passed a flat list of requested requirements. We have three jobs:

  1. Figure out all indirect requirements
  2. See whether we can use any‘aggregate’resources
  3. Put required resources before their requiree.
[semidet,multifile]mime_include(+Mime, +Path)//
Hook called to include a link to an HTML resource of type Mime into the HTML head. The Mime type is computed from Path using file_mime_type/2. If the hook fails, two built-in rules for text/css and text/javascript are tried. For example, to include a =.pl= files as a Prolog script, use: 
:- multifile
    html_head:mime_include//2.

html_head:mime_include(text/'x-prolog', Path) --> !,
    html(script([ type('text/x-prolog'),
                  src(Path)
                ],  [])).

3.25 library(http/http_pwp): Serve PWP pages through the HTTP server

To be done
- Support elements in the HTML header that allow controlling the page, such as setting the CGI-header, authorization, etc.
- Allow external styling. Pass through reply_html_page/2? Allow filtering the DOM before/after PWP?

This module provides convience predicates to include PWP (Prolog Well-formed Pages) in a Prolog web-server. It provides the following predicates:

pwp_handler / 2
This is a complete web-server aimed at serving static pages, some of which include PWP. This API is intended to allow for programming the web-server from a hierarchy of pwp files, prolog files and static web-pages.
reply_pwp_page / 3
Return a single PWP page that is executed in the context of the calling module. This API is intended for individual pages that include so much text that generating from Prolog is undesirable.
pwp_handler(+Options, +Request)
Handle PWP files. This predicate is defined to create a simple HTTP server from a hierarchy of PWP, HTML and other files. The interface is kept compatible with the library(http/http_dispatch). In the typical usage scenario, one needs to define an http location and a file-search path that is used as the root of the server. E.g., the following declarations create a self-contained web-server for files in /web/pwp/. 
user:file_search_path(pwp, '/web/pwp').

:- http_handler(root(.), pwp_handler([path_alias(pwp)]), [prefix]).

Options include:

path_alias(+Alias)
Search for PWP files as Alias(Path). See absolute_file_name/3.
index(+Index)
Name of the directory index (pwp) file. This option may appear multiple times. If no such option is provided, pwp_handler/2 looks for index.pwp.
view(+Boolean)
If true (default is false), allow for ?view=source to serve PWP file as source.
index_hook(:Hook)
If a directory has no index-file, pwp_handler/2 calls Hook(PhysicalDir, Options, Request). If this semidet predicate succeeds, the request is considered handled.
hide_extensions(+List)
Hide files of the given extensions. The default is to hide .pl files.
dtd(?DTD)
DTD to parse the input file with. If unbound, the generated DTD is returned
Errors
permission_error(index, http_location, Location) is raised if the handler resolves to a directory that has no index.
See also
reply_pwp_page/3
reply_pwp_page(:File, +Options, +Request)
Reply a PWP file. This interface is provided to server individual locations from PWP files. Using a PWP file rather than generating the page from Prolog may be desirable because the page contains a lot of text (which is cumbersome to generate from Prolog) or because the maintainer is not familiar with Prolog.

Options supported are:

mime_type(+Type)
Serve the file using the given mime-type. Default is text/html.
unsafe(+Boolean)
Passed to http_safe_file/2 to check for unsafe paths.
pwp_module(+Boolean)
If true, (default false), process the PWP file in a module constructed from its canonical absolute path. Otherwise, the PWP file is processed in the calling module.

Initial context:

SCRIPT_NAME
Virtual path of the script.
SCRIPT_DIRECTORY
Physical directory where the script lives
QUERY
Var=Value list representing the query-parameters
REMOTE_USER
If access has been authenticated, this is the authenticated user.
REQUEST_METHOD
One of get, post, put or head
CONTENT_TYPE
Content-type provided with HTTP POST and PUT requests
CONTENT_LENGTH
Content-length provided with HTTP POST and PUT requests

While processing the script, the file-search-path pwp includes the current location of the script. I.e., the following will find myprolog in the same directory as where the PWP file resides. 

pwp:ask="ensure_loaded(pwp(myprolog))"
See also
pwp_handler/2.
To be done
complete the initial context, as far as possible from CGI variables. See http://hoohoo.ncsa.illinois.edu/docs/cgi/env.html

3.26 library(http/htmx): Support htmx.org

Quoted from htmx.org:

htmx gives you access to AJAX, CSS Transitions, WebSockets and Server Sent Events directly in HTML, using attributes, so you can build modern user interfaces with the simplicity and power of hypertext

The idea behind htmx is to allow adding attributes to any HTML element that cause an HTTP request. The HTTP response is typically a (short) HTML fragment that extends or replaces an element on the page. This allows us to program a most functionality interactive seen in modern web applications using the powerful SWI-Prolog HTML generation framework rather than having to write a JSON backend and accompagnying JavaScript frontend that runs in the browser.

Below is a minimal, yet fully functional application 

:- use_module(library(http/http_server)).
:- use_module(library(http/htmx)).
:- use_module(library(main)).

:- initialization(main, main).

main(_Argv) :-
    http_server([port(8080)]),
    thread_get_message(quit).

http:location(htmx, root(htmx), []).

:- http_handler(root(.), home, []).

home(_Request) :-
    reply_html_page(
        [ title('HTMX demo'),
          script(src('https://unpkg.com/htmx.org'), [])
        ],
        [ button([ 'hx-post'('/htmx/clicked'),
                   'hx-swap'('outerHTML')
                 ],
                 'Click me')
        ]).

:- http_handler(htmx(clicked), reply_htmx(\clicked), []).

clicked -->
    html('Thanks for clicking me!').

HTMX requires no dedicated support from the server. This library provides reply_htmx/1,2 to reply with a single HTML element rather than an entire page. Future versions of this library may provide some additional utility predicates.

[det]reply_htmx(+HTML)
[det]reply_htmx(+HTML, +Request)
Reply a plain HTML element as opposed to a complete HTML page as created using reply_html_page/2,3. While reply_htmx/1 is to be used in a normal HTTP handler (route), reply_htmx/2 may be registered directly in the http_handler/3 declaration to deal with simple cases where we do not need the Request data.

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