SWI-Prolog OpenAPI (Swagger) interface

Design

There are two options to support OpenAPI:

1. Use an OpenAPI file as a specification for the interface, only providing the body implementation in Prolog.
2. Specify the API in Prolog and generate an OpenAPI document for it.

This library implements the first option. Future versions may reuse a lot of the infrastructure to implement the second option.

Components

The single library(openapi) implements the following components:

1. A compiler that creates a Prolog friendly representation of the operations described in the OpenAPI file.
2. An HTTP request dispatcher that uses the above to
   • Find the operation
   • Extract the parameters (from headers, path, query and request body)
   • Check the argument types
   • Call the (user defined) implementation
   • Check the response type
   • Return the reply document
3. A compiler that creates the client predicates as wrappers around http_open/3. The wrapper does
   • Type-check the parameters
   • Use the parameters to populate the request header, formulate the URL and optional request body for http_open/3.
   • Run the request
   • Type-check and return the returned answer.

In addition, there is a SWI-Prolog app script openapi that wraps the above library to create the skeleton server and client, including PlDoc comments for the operations.

Using this package

First of all, obtain an OpenAPI file using OpenAPI version 3. Now, to generate a server, do

swipl openapi --server=server.pl spec.yaml

This creates a Prolog file server.pl with documented predicate skeletons that must be filled by you. We advice to write the actual implementation of the server in a separate module such that the implementation you have to add to this file is short. This makes upgrading and deploying multiple versions of the server API much easier. There are additional options:

• `--httpd` includes an HTTP server in server.pl. For more complicated projects you probably want something more advanced. This quickly gets you started though.
• `--ui` includes the swagger ui, so you can interactively explore the API in your browser.

To generate a client run

swipl openapi --client=client.pl spec.yaml

This creates documented predicates that call the API. The server address is extracted from spec.yaml. Using the option `--module`, the client file is created as a Prolog module that exports the API.

The predicate mapping

The operationId from the OpenAPI file is used as the name of the predicate, both for the server and client. The arguments are extracted from the parameters specification in the OpenAPI file. Required arguments use a normal Prolog argument. Optional parameters are passed using a Prolog option list. If there is a return value, this is positioned after the required argument and before the option list. On the client side, normal responses are returned as data. The default is mapped to an exception of the form error(rest_error(Code, Data),_), where Code is the HTTP status code and Data the data returned by the server. If an operation only defines code 204 (no content) and a default, the parameter is missing in the client and the predicate succeeds if the server replies 204 or throws an exception as above otherwise.

Server reply

The implementation skeleton for each server operation has a variable Response. The implementation must succeed and bind Response to one of the terms below.

• status(Code)
• status(Code, Data)

Currently, Data must be a term suitable for json_write_dict/3. Future versions will support a other replies and a hook to extend the reply types.

Examples

See the examples directory for two examples from the Swagger site.

Prerequisites

SWI-Prolog 9.2/9.3.