2 The TIPC libraries: library(tipc/...)

2.1 library(tipc/tipc): TIPC Sockets

author

Jeffrey Rosenwald (JeffRose@acm.org)

See also

http://tipc.sf.net, http://www.erlang.org

Compatibility

Linux only

Transparent Inter-Process Communication (TIPC) provides a flexible, reliable, fault-tolerant, high-speed, and low-overhead framework for inter-process communication between federations of trusted peers, operating as a unit. It was developed by Ericsson AB, as a means to provide for communications between Common Control Systems processes and Network Element peers in telephone switching systems, sometimes operating at arm's length on different line cards or mainframes. Delegation of responsibility in this way is one of the fundamental precepts of the Erlang programming system, also developed at Ericsson. TIPC represents a more generalized version of the same behavioral design pattern. For an overview, please see: tipc_overview.md.

Errors

The TIPC module uses the error handling functions from library(socket) and therefore all the functions below may throw error(socket_error(Code, Message)) where Code is the lowercase version of the C-macro error code and Message is an atom describing the error in a human friendly format, depending on the current locale. See the socket library for details.

[det]tipc_socket(-SocketId, +SocketType)

Creates a TIPC-domain socket of the type specified by SocketType, and unifies it to an identifier, SocketId.

SocketType

is one of the following atoms: rdm - unnumbered, reliable datagram service, dgram - unnumbered, unreliable datagram service, seqpacket - numbered, reliable datagram service, and stream - reliable, connection-oriented byte-stream service

[det]tipc_close_socket(+SocketId)

Closes the indicated socket, making SocketId invalid. In stream applications, sockets are closed by closing both stream handles returned by tipc_open_socket/3. There are two cases where tipc_close_socket/1 is used because there are no stream-handles:

• After tipc_accept/3, the server does a fork/1 to handle the client in a sub-process. In this case the accepted socket is not longer needed from the main server and must be discarded using tipc_close_socket/1.
• If, after discovering the connecting client with tipc_accept/3, the server does not want to accept the connection, it should discard the accepted socket immediately using tipc_close_socket/1.

SocketId

the socket identifier returned by tipc_socket/2 or tipc_accept/3.

[det]tipc_open_socket(+SocketId, -InStream, -OutStream)

Opens two SWI-Prolog I/O-streams, one to deal with input from the socket and one with output to the socket. If tipc_bind/3 has been called on the socket, OutStream is useless and will not be created. After closing both InStream and OutStream, the socket itself is discarded.

[det]tipc_bind(+Socket, +Address, +ScopingOption)

Associates/disassociates a socket with the name/3 or name_seq/3 address specified in Address. It also registers/unregisters it in the topology server name table. This makes the address visible/invisible to the rest of the network according to the scope specified in ScopingOption. ScopingOption is a grounded term that is one of:

scope(Scope)

where Scope is one of: zone, cluster, or node. Servers may bind to more than one address by making successive calls to tipc_bind/3, one for each address that it wishes to advertise. The server will receive traffic for all of them. A server may, for example, register one address with node scope, another with cluster scope, and a third with zone scope. A client may then limit the scope of its transmission by specifying the appropriate address.

no_scope(Scope)

where Scope is as defined above. An application may target a specific address for removal from its collection of addresses by specifying the address and its scope. The scoping option, no_scope(all), may be used to unbind the socket from all of its registered addresses. This feature allows an application to gracefully exit from service. Because the socket remains open, the application may continue to service current transactions to completion. TIPC however, will not schedule any new work for the server instance. If no other servers are available, the work will be rejected or dropped according to the socket options specified by the client.

Connection-oriented, byte-stream services are implemented with this predicate combined with tipc_listen/2 and tipc_accept/3. Connectionless, datagram services may be implemented using tipc_receive/4.

Note that clients do not need to bind to any address. Its port-id is sufficient for this role. And server sockets (e.g. those that are bound to name/3 or name_seq/3, addresses) may not act as clients. That is, they may not originate connections from the socket using tipc_connect/2. Servers however, may originate datagrams from bound sockets using tipc_send/4. Please see the TIPC programmers's guide for other restrictions.

[det]tipc_listen(+Socket, +Backlog)

Listens for incoming requests for connections. Backlog indicates how many pending connection requests are allowed. Pending requests are requests that are not yet acknowledged using tipc_accept/3. If the indicated number is exceeded, the requesting client will be signalled that the service is currently not available. A suggested default value is 5.

[det]tipc_accept(+Socket, -Slave, -Peer)

Blocks on a server socket and waits for connection requests from clients. On success, it creates a new socket for the client and binds the identifier to Slave. Peer is bound to the TIPC address, port_id/2, of the client.

[det]tipc_connect(+Socket, +TIPC_address)

Provides a connection-oriented, client-interface to connect a socket to a given TIPC_address. After successful completion, tipc_open_socket/3 may be used to create I/O-Streams to the remote socket.

[det]tipc_get_name(+Socket, -TIPC_address)

Unifies TIPC_address with the port-id assigned to the socket.

[det]tipc_get_peer_name(+Socket, -TIPC_address)

Unifies TIPC_address with the port-id assigned to the socket that this socket is connected to.

[det]tipc_setopt(+Socket, +Option)

Sets options on the socket. Defined options are:

importance(+Priority)

Allow sockets to assign a priority to their traffic. Priority is one of : low (default), medium, high, or critical.

src_droppable(+Boolean)

Allow TIPC to silently discard packets in congested situations, rather than queuing them for later transmission.

dest_droppable(+Boolean)

Allow TIPC to silently discard packets in congested situations, rather than returning them to the sender as undeliverable.

conn_timeout(+Seconds)

Specifies the time interval that tipc_connect/2 will use before abandoning a connection attempt. Default: 8.000 sec.

[det]tipc_receive(+Socket, -Data, -From, +OptionList)

Waits for, and returns the next datagram. Like its UDP counterpart, the data are returned as a Prolog string object (see string_codes/2). From is an address structure of the form port_id/2, indicating the sender of the message.

Defined options are:

as(+Type)

Defines the returned term-type. Type is one of atom, codes or string (default).

nonblock

Poll the socket and return immediately. If a message is present, it is returned. If not, then an exception, error(socket_error(eagain, Message), _), will be thrown. Users are cautioned not to "spin" unnecessarily on non-blocking receives as they may prevent the system from servicing other background activities such as XPCE event dispatching.

The typical sequence to receive a connectionless TIPC datagram is:

receive :-
        tipc_socket(S, dgram),
        tipc_bind(S, name(18888, 10, 0), scope(zone)),
        repeat,
            tipc_receive(S, Data, From, [as(atom)]),
            format('Got ~q from ~q~n', [Data, From]),
            Data == quit,
        !, tipc_close_socket(S).

[det]tipc_send(+Socket, +Data, +To, +Options)

sends a TIPC datagram to one or more destinations. Like its UDP counterpart, Data is a string, atom or code-list providing the data to be sent. To is a name/3, name_seq/3, or port_id/2 address structure. See tipc_overview.txt, for more information on TIPC Address Structures. Options is currently unused.

A simple example to send a connectionless TIPC datagram is:

send(Message) :-
        tipc_socket(S, dgram),
        tipc_send(S, Message, name(18888, 10,0), []),
        tipc_close_socket(S).

Messages are delivered silently unless some form of congestion was encountered and the dest_droppable(false) option was issued on the sender's socket. In this case, the send succeeds but a notification in the form of an empty message is returned to the sender from the receiver, indicating some kind of delivery failure. The port-id of the receiver is returned in congestion conditions. A port_id(0,0), is returned if the destination address was invalid. Senders and receivers should beware of this possibility.

[det]tipc_canonical_address(-CanonicalAddress, +PortId)

Translates a port_id/2 address into canonical TIPC form:

tipc_address(Zone, Cluster, Node, Reference)

It is provided for debugging an printing purposes only. The canonical address is not used for any other purpose.

[semidet]tipc_service_exists(+Address, +Timeout)

[semidet]tipc_service_exists(+Address)

Interrogates the TIPC topology server to see if a service is available at an advertised Address.

Address	is one of: name(Type, Instance, Domain) or name_seq(Type, Lower, Upper). A name/3, address is translated to a name_seq/3, following, where Lower and Upper are assigned the value of Instance. Domain is unused and must be zero. A name_seq(Type, Lower, Upper) is a multi-cast address. This predicate succeeds if there is at least one service that would answer according to multi-cast addressing rules.
Timeout	is optional. It is a non-negative real number that specifies the amount of time in seconds to block and wait for a service to become available. Fractions of a second are also permissible.

[nondet]tipc_service_probe(?Address)

[nondet]tipc_service_probe(?Address, ?PortId)

Allows a user to discover the instance ranges and/or port-ids for a particular service.

Address	is a name_seq/3 address. The address type must be grounded.
PortId	is unified with the port-id for a specific name_sequence address.

[det]tipc_service_port_monitor(+Addresses, :Goal)

[det]tipc_service_port_monitor(+Addresses, :Goal, ?Timeout)

Monitors a collection of worker threads that are bound to a list of Addresses. A single port monitor may be used to provide surveillance over workers that are providing a number of different services. For a given address type, discontiguous port ranges may be specified, but overlapping port ranges may not. Goal for example, may simply choose to broadcast the notification, thus delegating the notification event handling to others.

Addresses	is a list of name/3 or name_seq/3 addresses for the services to be monitored.
Goal	is a predicate that will be called when a worker's publication status changes. The Goal is called exactly once per event with its the last argument unified with the structure: published(-NameSeq, -PortId) when the worker binds its socket to the address. withdrawn(-NameSeq, -PortId) when the worker unbinds its socket from the address.
Timeout	is optional. It is one of: Timeout a non-negative real number that specifies the number of seconds that surveillance is to be continued. infinite causes the monitor to run forever in the current thread (e.g. never returns). detached(-ThreadId) causes the monitor to run forever as a separate thread. ThreadId is unified with the thread identifier of the monitor thread. This is useful when the monitor is required to provide continuous surveillance, while operating in the background.

[semidet]tipc_initialize

causes the TIPC service and the TIPC stack to be initialized and made ready for service. An application must call this predicate as part of its initialization prior to any use of TIPC predicates. Please note the change of the API.

2.2 library(tipc/tipc_broadcast): A TIPC Broadcast Bridge

author

Jeffrey Rosenwald (JeffRose@acm.org)

See also

tipc.pl

Compatibility

Linux only

license

LGPL

SWI-Prolog's broadcast library provides a means that may be used to facilitate publish and subscribe communication regimes between anonymous members of a community of interest. The members of the community are however, necessarily limited to a single instance of Prolog. The TIPC broadcast library removes that restriction. With this library loaded, any member of a TIPC network that also has this library loaded may hear and respond to your broadcasts. Using TIPC Broadcast, it becomes a nearly trivial matter to build an instance of supercomputer that researchers within the High Performance Computer community refer to as "Beowulf Class Cluster Computers."

This module has no public predicates. When this module is initialized, it does three things:

• It starts a listener daemon thread that listens for broadcasts from others, received as TIPC datagrams, and
• It registers three listeners: tipc_node/1, tipc_cluster/1, and tipc_zone/1, and
• It registers three listeners: tipc_node/2, tipc_cluster/2, and tipc_zone/2.

A broadcast/1 or broadcast_request/1 that is not directed to one of the six listeners above, behaves as usual and is confined to the instance of Prolog that originated it. But when so directed, the broadcast will be sent to all participating systems, including itself, by way of TIPC's multicast addressing facility. A TIPC broadcast or broadcast request takes the typical form: broadcast(tipc_node(+Term, +Timeout)). The principal functors tipc_node, tipc_cluster, and tipc_zone, specify the scope of the broadcast. The functor tipc_node, specifies that the broadcast is to be confined to members of a present TIPC node. Likewise, tipc_cluster and tipc_zone, specify that the traffic should be confined to members of a present TIPC cluster and zone, respectively. To prevent the potential for feedback loops, the scope qualifier is stripped from the message before transmission. The timeout is optional. It specifies the amount to time to wait for replies to arrive in response to a broadcast_request. The default period is 0.250 seconds. The timeout is ignored for broadcasts.

An example of three separate processes cooperating on the same Node:

Process A:

   ?- listen(number(X), between(1, 5, X)).
   true.

   ?-

Process B:

   ?- listen(number(X), between(7, 9, X)).
   true.

   ?-

Process C:

   ?- findall(X, broadcast_request(tipc_node(number(X))), Xs).
   Xs = [1, 2, 3, 4, 5, 7, 8, 9].

   ?-

It is also possible to carry on a private dialog with a single responder. To do this, you supply a compound of the form, Term:PortId, to a TIPC scoped broadcast/1 or broadcast_request/1, where PortId is the port-id of the intended listener. If you supply an unbound variable, PortId, to broadcast_request, it will be unified with the address of the listener that responds to Term. You may send a directed broadcast to a specific member by simply providing this address in a similarly structured compound to a TIPC scoped broadcast/1. The message is sent via unicast to that member only by way of the member's broadcast listener. It is received by the listener just as any other broadcast would be. The listener does not know the difference.

Although this capability is needed under some circumstances, it has a tendency to compromise the resilience of the broadcast model. You should not rely on it too heavily, or fault tolerance will suffer.

For example, in order to discover who responded with a particular value:

Process A:

   ?- listen(number(X), between(1, 3, X)).
   true.

   ?-

Process B:

   ?- listen(number(X), between(7, 9, X)).
   true.

   ?-

Process C:

   ?- broadcast_request(tipc_node(number(X):From)).
   X = 7,
   From = port_id('<1.1.1:3971170279>') ;
   X = 8,
   From = port_id('<1.1.1:3971170279>') ;
   X = 9,
   From = port_id('<1.1.1:3971170279>') ;
   X = 1,
   From = port_id('<1.1.1:3971170280>') ;
   X = 2,
   From = port_id('<1.1.1:3971170280>') ;
   X = 3,
   From = port_id('<1.1.1:3971170280>') ;
   false.

?-

2.2.1 Caveats

While the implementation is mostly transparent, there are some important and subtle differences that must be taken into consideration:

• TIPC broadcast now requires an initialization step in order to launch the broadcast listener daemon. See tipc_initialize/0.
• Prolog's broadcast_request/1 is nondet. It sends the request, then evaluates the replies synchronously, backtracking as needed until a satisfactory reply is received. The remaining potential replies are not evaluated. This is not so when TIPC is involved.
• A TIPC broadcast/1 is completely asynchronous.
• A TIPC broadcast_request/1 is partially synchronous. A broadcast_request/1 is sent, then the sender balks for a period of time (default: 250 ms) while the replies are collected. Any reply that is received after this period is silently discarded. An optional second argument is provided so that a sender may specify more (or less) time for replies.
• Replies are no longer collected using findall/3. Replies are presented to the user as a choice point on arrival, until the broadcast request timer finally expires. This change allows traffic to propagate through the system faster and provides the requestor with the opportunity to terminate a broadcast request early if desired, by simply cutting choice points.
• Please beware that broadcast request transactions will now remain active and resources consumed until broadcast_request finally fails on backtracking, an uncaught exception occurs, or until choice points are cut. Failure to properly manage this will likely result in chronic exhaustion of TIPC sockets.
• If a listener is connected to a generator that always succeeds (e.g. a random number generator), then the broadcast request will never terminate and trouble is bound to ensue.
• broadcast_request/1 with TIPC scope is not reentrant (at least, not now anyway). If a listener performs a broadcast_request/1 with TIPC scope recursively, then disaster looms certain. This caveat does not apply to a TIPC scoped broadcast/1, which can safely be performed from a listener context.
• TIPC's capacity is not infinite. While TIPC can tolerate substantial bursts of activity, it is designed for short bursts of small messages. It can tolerate several thousand replies in response to a broadcast_request/1 without trouble, but it will begin to encounter congestion beyond that. And in congested conditions, things will start to become unreliable as TIPC begins prioritizing and/or discarding traffic.
• A TIPC broadcast_request/1 term that is grounded is considered to be a broadcast only. No replies are collected unless the there is at least one unbound variable to unify.
• A TIPC broadcast/1 always succeeds, even if there are no listeners.
• A TIPC broadcast_request/1 that receives no replies will fail.
• Replies may be coming from many different places in the network (or none at all). No ordering of replies is implied.
• Prolog terms are sent to others after first converting them to atoms using term_to_atom/2. Passing real numbers this way may result in a substantial truncation of precision. See prolog flag option,’float_format’, of current_prolog_flag/2.

[nondet]tipc_host_to_address(?Service, ?Address)

locates a TIPC service by name. Service is an atom or grounded term representing the common name of the service. Address is a TIPC address structure. A server may advertise its services by name by including the fact, tipc:host_to_address(+Service, +Address), somewhere in its source. This predicate can also be used to perform reverse searches. That is it will also resolve an Address to a Service name. The search is zone-wide. Locating a service however, does not imply that the service is actually reachable from any particular node within the zone.

[semidet]tipc_initialize

See tipc:tipc_initialize/0

2.3 library(tipc/tipc_paxos): Paxos on TIPC

This module provides compatibility for using paxos over TIPC. As of SWI-Prolog 7.7.15 the core of this module has been moved to the core library as library(paxos) and can be used with other distributed implementations of library(broadcast) such as library(udb_broadcast).

[semidet]tipc_paxos_set(?Term)

[semidet]tipc_paxos_get(?Term)

[semidet]tipc_paxos_get(?Term, +Options)

[semidet]tipc_paxos_set(?Term, +Options)

[det]tipc_paxos_on_change(?Term, :Goal)

[semidet]tipc_initialize

causes the TIPC service and the TIPC stack to be initialized and made ready for service. An application must call this predicate as part of its initialization prior to any use of TIPC predicates. Please note the change of the API.

2.4 library(tipc/tipc_linda): A Process Communication Interface

author

Jeffrey A. Rosenwald

See also

Nicholas Carriero and David Gelernter. How to Write Parallel Programs: A First Course. The MIT Press, Cambridge, MA, 1990.

Compatibility

- SWI-Prolog for Linux only
- tipc_broadcast library

Linda is a framework for building systems that are composed of programs that cooperate among themselves in order to realize a larger goal. A Linda application is composed of two or more processes acting in concert. One process acts as a server and the others act as clients. Fine-grained communications between client and server is provided by way of message passing over sockets and support networks, TIPC sockets in this case. Clients interact indirectly by way of the server. The server is in principle an eraseable blackboard that clients can use to write (out/1), read (rd/1) and remove (in/1) messages called tuples. Some predicates will fail if a requested tuple is not present on the blackboard. Others will block until a tuple instance becomes available. Tuple instances are made available to clients by writing them on the blackboard using out/1.

In TIPC Linda, there is a subtle difference between the in and the rd predicates that is worth noting. The in predicates succeed exactly once for each tuple placed in the tuple space. The tuple is provided to exactly one requesting client. Clients can contend for tuples in this way, thus enabling multi-server operations. The rd predicates succeed nondeterministically, providing all matching tuples in the tuple space at a given time to the requesting client as a choice point without disturbing them.

TIPC Linda is inspired by and adapted from the SICStus Prolog API. But unlike SICStus TCP Linda, TIPC Linda is connectionless. There is no specific session between client and server. The server receives and responds to datagrams originated by clients in an epiperiodic manner.

Example: A simple producer-consumer.

In client 1:

init_producer :-
       linda_client(global),
       producer.

producer :-
       produce(X),
       out(p(X)),
       producer.

produce(X) :- .....

In client 2:

init_consumer :-
        linda_client(global),
        consumer.

consumer :-
       in(p(A)),
       consume(A),
       consumer.

consume(A) :- .....

Example: Synchronization

       ...,
       in(ready),  %Waits here until someone does out(ready)
       ...,

Example: A critical region

       ...,
       in(region_free),  % wait for region to be free
       critical_part,
       out(region_free), % let next one in
       ...,

Example: Reading global data

       ...,
       rd(data(Data)),
       ...,

or, without blocking:

       ...,
       (rd_noblock(data(Data)) ->
             do_something(Data)
       ;     write('Data not available!'),nl
       ),
       ...,

Example: Waiting for any one of several events

       ...,
       in([e(1),e(2),...,e(n)], E),
%  Here is E instantiated to the first tuple that became available
       ...,

Example: Producers and Consumers in the same process using linda_eval threads and/or tuple predicates

  consumer1 :-
        repeat,
        in([p(_), quit], Y),
        (   Y = p(Z) -> writeln(consuming(Z)); !),
        fail.

  producer1 :-
        forall(between(1,40, X), out(p(X))).

  producer_consumer1 :-
        linda_eval(consumer1),
        call_cleanup(producer1, out(quit)), !.
%
%
  consumer2 :-
       between(1,4,_),
       in_noblock(p(X)), !,
       writeln(consuming(X)),
       consumer2.

  producer2 :-
        linda_eval(p(X), between(1,40, X)).

  producer_consumer2 :-
        producer2,
        linda_eval(consumer2), !.
%
%
  consumer3 :-
        forall(rd_noblock(p(X)), writeln(consuming(X))).

  producer3 :-
        tuple(p(X), between(1,40, X)).

  producer_consumer3 :-
        producer3,
        linda_eval(done, consumer3),
        in(done), !.

2.4.1 Servers

The server is the process running the "blackboard process". It is part of TIPC Linda. It is a collection of predicates that are registered as tipc_broadcast listeners. The server process can be run on a separate machine if necessary.

To load the package, enter the query:

?- use_module(library(tipc/tipc_linda)).

?- linda.
   TIPC Linda server now listening at: port_id('<1.1.1:3200515722>')
   true.

2.4.2 Clients

The clients are one or more Prolog processes that have connection(s) to the server.

To load the package, enter the query:

?- use_module(library(tipc/tipc_linda)).

?- linda_client(global).
   TIPC Linda server listening at: port_id('<1.1.1:3200515722>')
   true.

[det]linda

[det]linda(:Goal)

Starts a Linda-server in this process. The network address is written to current output stream as a TIPC port_id/2 reference (e.g. port_id('<1.1.1:3200515722>') ). This predicates looks to see if a server is already listening on the cluster. If so, it reports the address of the existing server. Otherwise, it registers a new server and reports its address.

?- linda.
   TIPC Linda server now listening at: port_id('<1.1.1:3200515722>')
   true.

?- linda.
   TIPC Linda server still listening at: port_id('<1.1.1:3200515722>')
   true.

The following will call my_init/0 in the current module after the server is successfully started or is found already listening. my_init/0 could start client-processes, initialize the tuple space, etc.

?- linda(my_init).

[semidet]linda_client(+Domain)

Establishes a connection to a Linda-server providing a named tuple space. Domain is an atom specifying a particular tuple-space, selected from a universe of tuple-spaces. At present however, only one tuple-space, global, is supported. A client may interact with any server reachable on the TIPC cluster. This predicate will fail if no server is reachable for that tuple space.

[det]close_client

Closes the connection to the Linda-server. Causes the server to release resources associated with this client.

[semidet]linda_timeout(?OldTime, ?NewTime)

Controls Linda's message-passing timeout. It specifies the time window where clients will accept server replies in response to in and rd requests. Replies arriving outside of this window are silently ignored. OldTime is unified with the old timeout and then timeout is set to NewTime. NewTime is of the form Seconds:Milliseconds. A non-negative real number, seconds, is also recognized. The default is 0.250 seconds. This timeout is thread local and is not inherited from its parent. New threads are initialized to the default.

Note: The synchronous behavior afforded by in/1 and rd/1 is implemented by periodically polling the server. The poll rate is set according to this timeout. Setting the timeout too small may result in substantial network traffic that is of little value.

throws

error(feature_not_supported). SICStus Linda can disable the timeout by specifying off as NewTime. This feature does not exist for safety reasons.

[semidet]linda_timeout(+NewTime)

Temporarily sets Linda's timeout. Internally, the original timeout is saved and then the timeout is set to NewTime. NewTime is as described in linda_timeout/2. The original timeout is restored automatically on cut of choice points, failure on backtracking, or uncaught exception.

[det]out(+Tuple)

Places a Tuple in Linda's tuple-space.

[det]in(?Tuple)

Atomically removes the tuple Tuple from Linda's tuple-space if it is there. The tuple will be returned to exactly one requestor. If no tuple is available, the predicate blocks until it is available (that is, someone performs an out/1).

[semidet]in_noblock(?Tuple)

Atomically removes the tuple Tuple from Linda's tuple-space if it is there. If not, the predicate fails. This predicate can fail due to a timeout.

[det]in(+TupleList, -Tuple)

As in/1 but succeeds when any one of the tuples in TupleList is available. Tuple is unified with the fetched tuple.

[nondet]rd(?Tuple)

Succeeds nondeterministically if Tuple is available in the tuple-space, suspends otherwise until it is available. Compare this with in/1: the tuple is not removed.

[nondet]rd_noblock(?Tuple)

Succeeds nondeterministically if Tuple is available in the tuple-space, fails otherwise. This predicate can fail due to a timeout.

[nondet]rd(?TupleList, -Tuple)

As in/2 but provides a choice point that does not remove any tuples.

[nondet]bagof_in_noblock(?Template, ?Tuple, -Bag)

[nondet]bagof_rd_noblock(?Template, ?Tuple, -Bag)

Bag is the list of all instances of Template such that Tuple exists in the tuple-space. The behavior of variables in Tuple and Template is as in bagof/3. The variables could be existentially quantified with ^/2 as in bagof/3. The operation is performed as an atomic operation. This predicate can fail due to a timeout. Example: Assume that only one client is connected to the server and that the tuple-space initially is empty.

  ?- out(x(a,3)), out(x(a,4)), out(x(b,3)), out(x(c,3)).

  true.
  ?- bagof_rd_noblock(C-N, x(C,N), L).

  L = [a-3,a-4,b-3,c-3] .

  true.
  ?- bagof_rd_noblock(C, N^x(C,N), L).

  L = [a,a,b,c] .

  true.

[det]linda_eval(:Goal)

[det]linda_eval(?Head, :Goal)

[det]linda_eval_detached(:Goal)

[det]linda_eval_detached(?Head, :Goal)

Causes Goal to be evaluated in parallel with a parent predicate. The child thread is a full-fledged client, possessing the same capabilities as the parent. Upon successful completion of Goal, unbound variables are unified and the result is sent to the Linda server via out/1, where it is made available to others. linda_eval/2 evaluates Goal, then unifies the result with Head, providing a means of customizing the resulting output structure. In linda_eval/1, Head, and Goal are identical, except that the module name for Head is stripped before output. If the child fails or receives an uncaught exception, no such output occurs.

Joining Threads: Threads created using linda_eval/(1-2) are not allowed to linger. They are joined (blocking the parent, if necessary) under three conditions: backtracking on failure into an linda_eval/(1-2), receipt of an uncaught exception, and cut of choice-points. Goals are evaluated using forall/2. They are expected to provide nondeterministic behavior. That is they may succeed zero or more times on backtracking. They must however, eventually fail or succeed deterministically. Otherwise, the thread will hang, which will eventually hang the parent thread. Cutting choice points in the parent's body has the effect of joining all children created by the parent. This provides a barrier that guarantees that all child instances of Goal have run to completion before the parent proceeds. Detached threads behave as above, except that they operate independently and cannot be joined. They will continue to run while the host process continues to run.

Here is an example of a parallel quicksort:

qksort([], []).

qksort([X | List], Sorted) :-
      partition(@>(X), List, Less, More),
      linda_eval(qksort(More, SortedMore)),
      qksort(Less, SortedLess), !,
      in_noblock(qksort(More, SortedMore)),
      append(SortedLess, [X | SortedMore], Sorted).

[det]tuple(:Goal)

[det]tuple(?Head, :Goal)

registers Head as a virtual tuple in TIPC Linda's tuple space. On success, any client on the cluster may reference the tuple, Head, using rd/1 or rd_noblock/1. On reference, Goal is executed by a separate thread of execution in the host client's Prolog process. The result is unified with Head, which is then returned to the guest client. As in linda_eval/(1-2) above, Goal is evaluated using forall/2. The virtual tuple is unregistered on backtracking into a tuple/(1-2), receipt of uncaught exception, or cut of choice-points. In tuple/1, Head and Goal are identical, except that the module name is stripped from Head.

Note: A virtual tuple is an extension of the server. Even though it is operating in the client's Prolog environment, it is restricted in the server operations that it may perform. It is generally safe for tuple predicates to perform out/1 operations, but it is unsafe for them to perform any variant of in or rd, either directly or indirectly. This restriction is however, relaxed if the server and client are operating in separate heavyweight processes (not threads) on the node or cluster. This is most easily achieved by starting a stand-alone Linda server somewhere on the cluster. See tipc_linda_server/0, below.

[nondet]tipc_linda_server

Acts as a stand-alone Linda server. This predicate initializes the TIPC stack and then starts a Linda server in the current thread. If a client performs an out(server_quit), the server's Prolog process will exit via halt/1. It is intended for use in scripting as follows:

swipl -q -g 'use_module(library(tipc/tipc_linda)),
       tipc_linda_server' -t 'halt(1)'

See also manual section 2.10.2.1 Using PrologScript.

Note: Prolog will return a non-zero exit status if this predicate is executed on a cluster that already has an active server. An exit status of zero is returned on graceful shutdown.

throws

error(permission_error(halt,thread,2),context(halt/1,Only from thread’main’)), if this predicate is executed in a thread other than main.

[semidet]tipc_initialize

See tipc:tipc_initialize/0.

Index

?

bagof_in_noblock/3

bagof_rd_noblock/3

close_client/0

in/1

in/2

in_noblock/1

linda/0

linda/1

linda_client/1

linda_eval/1

linda_eval/2

linda_eval_detached/1

linda_eval_detached/2

linda_timeout/1

linda_timeout/2

out/1

rd/1

rd/2

rd_noblock/1

tipc_accept/3

tipc_bind/3

tipc_canonical_address/2

tipc_close_socket/1

tipc_connect/2

tipc_get_name/2

tipc_get_peer_name/2

tipc_host_to_address/2

tipc_initialize/0

tipc_linda_server/0

tipc_listen/2

tipc_open_socket/3

tipc_paxos_get/1

tipc_paxos_get/2

tipc_paxos_on_change/2

tipc_paxos_set/1

tipc_paxos_set/2

tipc_receive/4

tipc_send/4

tipc_service_exists/1

tipc_service_exists/2

tipc_service_port_monitor/2

tipc_service_port_monitor/3

tipc_service_probe/1

tipc_service_probe/2

tipc_setopt/2

tipc_socket/2

tuple/1

tuple/2