Creating a web service and a web service function
Moodle 2.0
In this document you will learn how to create a web service function.
This document expect that you first read the administration manual Setting_up_a_web_service.
To explain how to write a new web service function, we will write a web service function create_groups($groups). This function create a group into a Moodle course. - actually the function already exists into Moodle. You'll be able to browse CVS to have a look to the actual code.
This function will take a list of group object as parameters and it will return the same groups with their newly created id. If ever one group creation fails, the function will throw an exception, and no creation will happen.
We'll finally add the web service function to a core web service.
Preparation
We need to identify:
- the core functions: for our case we will use groups_create_group() from /group/lib.php. Of course not every core function exists and in some other cases you could have to write them.
- the parameter types: for our case we'll send a list of object. This object are groups, with id/name/courseid.
- the returned value types: for our case we want to send back a list of group object with their id.
- the user capabilities: for our case the user will need the moodle/course:managegroups capability.
Declare the web service function
Web service functions are declared into db/services.php of each component. In our case create_groups() is a core external function, so we will add the function to /lib/db/services.php.
$functions = array(
'moodle_group_create_groups' => array( //web service function name
'classname' => 'moodle_group_external', //class containing the external function
'methodname' => 'create_groups', //external function name
'classpath' => 'group/externallib.php', //file containing the class/external function
'description' => 'Creates new groups.', //human readable description of the web service function
'type' => 'write', //database rights of the web service function (read, write)
),
);
Here you should ask yourself what is the difference between a web service function and an external function. Web service function could be considered as a redirection to the external function. The web service function name is the name that is served by web service server to the client. By this way the client doesn't know about the external functions.
The web service function name is arbitrary, but it must be globally unique so we highly recommend using the component name as prefix (and "moodle" for core functions).
Define the web service description
Some web service protocol need a full description of the web service function parameters and return value. For example we need to generate a full WSDL for SOAP if we expect a JAVA/.Net client to call our function. This WSDL need a complete and fully detailed description of the parameters.
Moreover we implemented a web service parameters validation process that also require a full description of these parameters.
In conclusion, you must write these descriptions. These descriptions are not applied on the web service function but the external function, remember that the web service function is just a kind of redirection.
Descriptions are usually located into a class into an externallib.php in the component folder. You probably remember that we referenced this moodle_group_external class into /lib/db/services.php. Let's take a minuteto talk about the /group/externallib.php that we need to create.
This file will contain all external functions. These functions are all related together. In our case the externallib.php file will contain all group related functions. For each external function, we will have:
- the external function, calling a core function.
- a function describing the parameters of the external function
- a function describing the return values of the external function
Now we'll have look to the description of the create_group($groups) parameters. All parameter descriptions are included into a function called externalfunctionname_parameters(). In our case it will be create_groups_parameters() :
require_once("$CFG->libdir/externallib.php");
class moodle_group_external extends external_api {
/**
* Returns description of method parameters
* @return external_function_parameters
*/
public static function create_groups_parameters() {
return new external_function_parameters(
array(
'groups' => new external_multiple_structure(
new external_single_structure(
array(
'courseid' => new external_value(PARAM_INT, 'id of course'),
'name' => new external_value(PARAM_TEXT, 'multilang compatible name, course unique'),
'description' => new external_value(PARAM_RAW, 'group description text'),
'enrolmentkey' => new external_value(PARAM_RAW, 'group enrol secret phrase'),
)
)
)
)
);
}
Ok you are probably already here :) We have to understand this function very well. There is different approaches to understand this function, here is my personal one.
The function xxx_parameters() always return an external_function_parameters object. This object is initialized with an associative array. This array contains the parameter descriptions. If the external function have three parameters, this array will have three elements, the keys being the paramters name.
Each of its element is a parameter description. A parameter can be a list (external_multiple_structure), an object (external_single_structure) or a primary type (external_value).
Our create_groups() function expect one parameter named groups, so we will first write:
/**
* Returns description of method parameters
* @return external_function_parameters
*/
public static function create_groups_parameters() {
return new external_function_parameters(
array(
'groups' => ...
)
);
}
This groups parameter is a list of group. So we will write :
'groups' => new external_multiple_structure(
...
)
An external_multiple_structure object (list) can be initialized with external_single_structure (object), external_value (primary type) or another external_multiple_structure (list). You will respectively obtain a list of a unique object type, a list of a unique primary type, or a list of a list. These last case being probably bad design.
In our case we want a list of group, a group being a object. Have a look on how would describe a group :
new external_single_structure(
array(
'courseid' => ...,
'name' => ...,
'description' => ...,
'enrolmentkey' => ...,
)
)
From this code you can guess that a external_single_structure is initialize with an associative array. This array containing the object attibuts.
Thus a list of group will be defined :
'groups' => new external_multiple_structure(
new external_single_structure(
array(
'courseid' => ...,
'name' => ...,
'description' => ...,
'enrolmentkey' => ...,
)
)
)
Every of the group attribut are primary type. courseid is an integer. name is a string, description is a string, and enrolmentkey is also a string. To define a primary type we use the external_value object. This external_value is initialize with a non associative array containing three elements:
- the primary type
- a human readable description
- is the parameter optional (if the parameter is mandatory we don't need this third element)
For example for the courseid and name attribut we would define:
'courseid' => new external_value(PARAM_INT, 'id of course'),
'name' => new external_value(PARAM_TEXT, 'multilang compatible name, course unique'),
So our list of object description will be :
'groups' => new external_multiple_structure(
new external_single_structure(
array(
'courseid' => new external_value(PARAM_INT, 'id of course'),
'name' => new external_value(PARAM_TEXT, 'multilang compatible name, course unique'),
'description' => new external_value(PARAM_RAW, 'group description text'),
'enrolmentkey' => new external_value(PARAM_RAW, 'group enrol secret phrase'),
)
)
)
Now you should be able to understand this parameter description function. You also should be able to deduce how to write the return value description function.
Return value function always return one primary or complex value, so we don't need to return external_function_parameters object. In our case we want to return the exact same list of groups. The only change being that we want the group to have an id attribut.
public static function create_groups_returns() {
return new external_multiple_structure(
new external_single_structure(
array(
'id' => new external_value(PARAM_INT, 'group record id'),
'courseid' => new external_value(PARAM_INT, 'id of course'),
'name' => new external_value(PARAM_TEXT, 'multilang compatible name, course unique'),
'description' => new external_value(PARAM_RAW, 'group description text'),
'enrolmentkey' => new external_value(PARAM_RAW, 'group enrol secret phrase'),
)
)
);
}
Here ends the most difficult part of the web service infrastructure implementation. Congratulation!
Mandatory, Optional or Default
An attribut can be defined MANDATORY, OPTIONAL, or DEFAULT.
'yearofstudy' => new external_value(PARAM_INT, 'year of study',VALUE_DEFAULT, 1979),
The third parameter VALUE_DEFAULT indicates that 'yearofstudy' is set to a default value if the attribut is not supply. When you set VALUE_DEFAULT in the description, you must also write a default value as fourth parameter. Here the default value of 'yearofstudy' is 1979.
The three values can be understood like it:
- VALUE_REQUIRED - if the parameter is not supplied, then the server throw an exception
- VALUE_OPTIONAL - if the parameter is not supplied, then the param has no value
- VALUE_DEFAULT - if the parameter is not supplied, then the default value is used
Define the external function
We declared our web service function and we defined the external function parameters and return values. We will now implement the external function. Let's have a full look to the external function.
/**
* Create groups
* @param array $groups array of group description arrays (with keys groupname and courseid)
* @return array of newly created groups
*/
public static function create_groups($groups) { //Don't forget to set it as static
global $CFG, $DB;
require_once("$CFG->dirroot/group/lib.php");
$params = self::validate_parameters(self::create_groups_parameters(), array('groups'=>$groups));
$transaction = $DB->start_delegated_transaction();
$groups = array();
foreach ($params['groups'] as $group) {
$group = (object)$group;
if (trim($group->name) == ) {
throw new invalid_parameter_exception('Invalid group name');
}
if ($DB->get_record('groups', array('courseid'=>$group->courseid, 'name'=>$group->name))) {
throw new invalid_parameter_exception('Group with the same name already exists in the course');
}
// now security checks
$context = get_context_instance(CONTEXT_COURSE, $group->courseid);
self::validate_context($context);
require_capability('moodle/course:managegroups', $context);
// finally create the group
$group->id = groups_create_group($group, false);
$groups[] = (array)$group;
}
$transaction->allow_commit();
return $groups;
}
First let's have a look at
$params = self::validate_parameters(self::create_groups_parameters(), array('groups'=>$groups));
This validate_parameters function validates the external function parameters against the description. It will return an exception if some required parameters are missing, if parameters are not well-formed, and check the parameters validity. The array('groups'=>$groups) take the expected parameters by the description function.
A very important point: the parameters of your external function ($groups into public static function create_groups($groups) {) should be in the exact same order as the ones into your description function (return new external_function_parameters( array( 'groups' => ... )). I agree that our example function doesn't give us a good example :)
Then we have the following line
$transaction = $DB->start_delegated_transaction();
Our function has for goal to create multiple group. The function is calling multiple time the groups_create_group() core function. If ever one function fail and return an exception, we do not want to commit any change. It's why we declare this line that suspend any commit till we write the following line
$transaction->allow_commit();
You will also notice that our function throws exception. It is automatically handle by Moodle web service servers.
throw new invalid_parameter_exception('Group with the same name already exists in the course');
And yet again another important point: the context checking and capability checking happens into the external function. In a long term, in Moodle 2.0, all context/capability checks will disappear from the core functions and will be located into the external functions
/// now security checks
$context = get_context_instance(CONTEXT_COURSE, $group->courseid);
self::validate_context($context);
require_capability('moodle/course:managegroups', $context);
Last point, the return values will be parsed by the Moodle web service servers. Only the required values describe into the function description will be returned. An error will be returned if the external function doesn't return some required values.
Add the web service function into a service
In some specific case we could want to set a service by default, so the Moodle administrator doesn't need to set a new one. We can do that by writing the following code into /lib/db/services.php:
$services = array(
'groupservice' => array( //the name of the web service
'functions' => array ('moodle_add_groups', 'moodle_create_groups'), //web service functions of this service
'requiredcapability' => 'some/capability:specified', //if set, the web service user need this capability to access
//any function of this service
'restrictedusers' = >1, //if enabled, the Moodle administrator must link some user to this service
//into the administration
'enabled'=>0, //if enabled, the service can be reachable on a default installation
)
);
In this code we created a service called groupservice. This service contains two functions moodle_add_groups/moodle_create_groups.
This possibility to preset a service exists to facilitate the administrator life. He can easily enable a bunch of function.
However it is not possible for an administrator to add/remove any function from preset service. For security purpose, it is not a recommended to preset a service.
Test the web service function
Moodle is bundled with a test client that can be accessed into webservice/testclient.php. You are more than adviced to run it before to look at the code as the client can be used for multiple protocols, and multiple functions. You will have a better understanding.
In order to support our function we will need to add a form into webservice/testclient_forms.php.
class moodle_group_create_groups_form extends moodleform {
public function definition() {
global $CFG;
$mform = $this->_form;
$mform->addElement('header', 'wstestclienthdr', get_string('testclient', 'webservice'));
//note: these element are intentionally text aera without validation - we want users to test any rubbish as parameters
$mform->addElement('text', 'wsusername', 'wsusername');
$mform->addElement('text', 'wspassword', 'wspassword');
$mform->addElement('text', 'courseid', 'courseid');
$mform->addElement('text', 'name', 'name');
$mform->addElement('text', 'description', 'description');
$mform->addElement('text', 'enrolmentkey', 'enrolmentkey');
$mform->addElement('hidden', 'function');
$mform->setType('function', PARAM_SAFEDIR);
$mform->addElement('hidden', 'protocol');
$mform->setType('protocol', PARAM_SAFEDIR);
$mform->addElement('static', 'warning', , get_string('executewarnign', 'webservice'));
$this->add_action_buttons(true, get_string('execute', 'webservice'));
}
public function get_params() {
if (!$data = $this->get_data()) {
return null;
}
// remove unused from form data
unset($data->submitbutton);
unset($data->protocol);
unset($data->function);
unset($data->wsusername);
unset($data->wspassword);
$params = array();
$params['groups'] = array();
$params['groups'][] = (array)$data;
return $params;
}
}
That's all for our create_groups() function. We've been able to implement it and to test it.
In Creating_a_web_service_client we will talk about web service client.
