Development:Web services

Note: This article is a work in progress. Please use the page comments or an appropriate moodle.org forum for any recommendations/suggestions for improvement.

Introduction

This page described the Web Services module implemented for Moodle 2.0
The tracker issue is here: MDL-12886
This module is been implemented by the DFWS Team and Moodle.

Implementation

Web Services module has been conceived in a purpose to be ported on different Moodle version, and also on different project. It has also for purpose to support multiple web service protocols (REST, SOAP, AMF, ...). Adding a new protocol support should be relatively easy.

How it works

1. The client sends a username and password to the web service protocol server script.
2. The protocol server returns a session token for that user account (how this is sent depends on the protocol).
3. The client calls a particular web service function (module name, function name, and function parameters), including the session token.
4. The protocol server uses the token to check that the session is still active.
5. The protocol server call the matching external function, located in a external.php file inside the relevant module.
6. The external function checks that the current user has_capability to do this operation.
7. The external function calls the matching Moodle core function (in lib.php usually).
8. The core function can return a result to the external function.
9. The external function will return a result to the protocol server.
10. The protocol server returns the result to the client.

Code example

Following some bits of code in order to explain the concept

REST_server.php

///REST params conversion

   $functionname = 'delete_user' //retrieve function name from the REST parameters
   $modulename = 'user' //retrieve module name from the REST parameters
   $classname = $modulename.'_external';

/// Authentication process

   $token = ... //retrieve token from the REST parameters
   if (empty($token)) {
       if ($functionname != 'generate_token') {
           throw new moodle_exception('mustidentifyfirst');
       } else {
           ///authentication + token generation
           $username = ... //retrieve username from the REST parameters
           $password = ... //retrieve password from the REST parameters
           if ($token = generate_token($username,$password)) {
               return $token;
           } else {
               throw new moodle_exception('wrongusernamepassword');
           }
       }
   } else {
       ///the client is already identified
       $user = verify_token($token);
       if (empty($user)) {
           throw new moodle_exception('wrongidentification');
       }
       else {
           ///Load the user in order to get its context
           $USER = $user;
       }
   }

/// load the external class

   require_once($CFG->dirroot.$apipath.'external.php');
   $wsapi = new $classname();
   $description = $wsapi->get_function_webservice_description($functionname); //retrieve the web service description for this function

/// retrieve all rest params in an array

   $params = retrieve_rest_params ($description); //retrieve the REST params

/// call the web service function

   $result = call_user_func_array  ( $classname.'::'.$functionname, array($params));
   

/// Return the result to the client

   echo convert_into_rest_format(result);

external.php

final class user_external extends moodle_external {

   /**
    * Constructor - We set the description of this API in order to be access by Web service
    */
   function __construct () {
         $this->descriptions = array();
      ///The desciption of the web service
      ///
      ///'params', 'optionalparams' and 'return' are used to described the web services to the end user 
      ///(can build WSDL file from these information)
         $this->descriptions['tmp_delete_user']   = array( 'params' => array('username'=> PARAM_ALPHANUM),
                                                           'optionalparams' => array( ),
                                                           'return' => array('result' => PARAM_BOOL));

   /**
    * Delete a user
    * @global object $DB
    * @param array $params
    *  ->username      string
    * @return boolean true if success
    */
   static function tmp_delete_user($params) {
       global $DB,$USER;

       $user = $DB->get_record('user', array('username'=>$params['username']));
       if (has_capability('moodle/user:delete', get_context_instance(CONTEXT_SYSTEM))) {
           return delete_user($user); //this core function is in moodlelib.php
       }
       else {
           throw new moodle_exception('couldnotdeleteuser');
       }
   }

}

?>

List of functions

All callable functions are declared into each external.php. A "description" array contains all function names, parameter names and types, and return type.