moodle-external-api-development

•

24.4k23983 months ago

Create custom external web service APIs for Moodle LMS. Use when implementing web services for course management, user tracking, quiz operations, or custom plugin functionality. Covers parameter validation, database operations, error handling, service registration, and Moodle coding standards.

Quick Install

npxskills add davila7/claude-code-templates--skill moodle-external-api-development

Instructions

Moodle External API Development

This skill guides you through creating custom external web service APIs for Moodle LMS, following Moodle's external API framework and coding standards.

When to Use This Skill

Creating custom web services for Moodle plugins
Implementing REST/AJAX endpoints for course management
Building APIs for quiz operations, user tracking, or reporting
Exposing Moodle functionality to external applications
Developing mobile app backends using Moodle

Core Architecture Pattern

Moodle external APIs follow a strict three-method pattern:

execute_parameters() - Defines input parameter structure
execute() - Contains business logic
execute_returns() - Defines return structure

Step-by-Step Implementation

Step 1: Create the External API Class File

Location: /local/yourplugin/classes/external/your_api_name.php

<?php
namespace local_yourplugin\external;

defined('MOODLE_INTERNAL') || die();
require_once("$CFG->libdir/externallib.php");

use external_api;
use external_function_parameters;
use external_single_structure;
use external_value;

class your_api_name extends external_api {
    
    // Three required methods will go here
    
}

Key Points:

Class must extend external_api
Namespace follows: local_pluginname\external or mod_modname\external
Include the security check: defined('MOODLE_INTERNAL') || die();
Require externallib.php for base classes

Step 2: Define Input Parameters

public static function execute_parameters() {
    return new external_function_parameters([
        'userid' => new external_value(PARAM_INT, 'User ID', VALUE_REQUIRED),
        'courseid' => new external_value(PARAM_INT, 'Course ID', VALUE_REQUIRED),
        'options' => new external_single_structure([
            'includedetails' => new external_value(PARAM_BOOL, 'Include details', VALUE_DEFAULT, false),
            'limit' => new external_value(PARAM_INT, 'Result limit', VALUE_DEFAULT, 10)
        ], 'Options', VALUE_OPTIONAL)
    ]);
}

Common Parameter Types:

PARAM_INT - Integers
PARAM_TEXT - Plain text (HTML stripped)
PARAM_RAW - Raw text (no cleaning)
PARAM_BOOL - Boolean values
PARAM_FLOAT - Floating point numbers
PARAM_ALPHANUMEXT - Alphanumeric with extended chars

Structures:

external_value - Single value
external_single_structure - Object with named fields
external_multiple_structure - Array of items

Value Flags:

VALUE_REQUIRED - Parameter must be provided
VALUE_OPTIONAL - Parameter is optional
VALUE_DEFAULT, defaultvalue - Optional with default

Step 3: Implement Business Logic

public static function execute($userid, $courseid, $options = []) {
    global $DB, $USER;

    // 1. Validate parameters
    $params = self::validate_parameters(self::execute_parameters(), [
        'userid' => $userid,
        'courseid' => $courseid,
        'options' => $options
    ]);

    // 2. Check permissions/capabilities
    $context = \context_course::instance($params['courseid']);
    self::validate_context($context);
    require_capability('moodle/course:view', $context);

    // 3. Verify user access
    if ($params['userid'] != $USER->id) {
        require_capability('moodle/course:viewhiddenactivities', $context);
    }

    // 4. Database operations
    $sql = "SELECT id, name, timecreated
            FROM {your_table}
            WHERE userid = :userid
              AND courseid = :courseid
            LIMIT :limit";
    
    $records = $DB->get_records_sql($sql, [
        'userid' => $params['userid'],
        'courseid' => $params['courseid'],
        'limit' => $params['options']['limit']
    ]);

    // 5. Process and return data
    $results = [];
    foreach ($records as $record) {
        $results[] = [
            'id' => $record->id,
            'name' => $record->name,
            'timestamp' => $record->timecreated
        ];
    }

    return [
        'items' => $results,
        'count' => count($results)
    ];
}

Critical Steps:

Always validate parameters using validate_parameters()
Check context using validate_context()
Verify capabilities using require_capability()
Use parameterized queries to prevent SQL injection
Return structured data matching return definition

Step 4: Define Return Structure

public static function execute_returns() {
    return new external_single_structure([
        'items' => new external_multiple_structure(
            new external_single_structure([
                'id' => new external_value(PARAM_INT, 'Item ID'),
                'name' => new external_value(PARAM_TEXT, 'Item name'),
                'timestamp' => new external_value(PARAM_INT, 'Creation time')
            ])
        ),
        'count' => new external_value(PARAM_INT, 'Total items')
    ]);
}

Return Structure Rules:

Must match exactly what execute() returns
Use appropriate parameter types
Document each field with description
Nested structures allowed

Step 5: Register the Service

Location: /local/yourplugin/db/services.php

<?php
defined('MOODLE_INTERNAL') || die();

$functions = [
    'local_yourplugin_your_api_name' => [
        'classname'   => 'local_yourplugin\external\your_api_name',
        'methodname'  => 'execute',
        'classpath'   => 'local/yourplugin/classes/external/your_api_name.php',
        'description' => 'Brief description of what this API does',
        'type'        => 'read',  // or 'write'
        'ajax'        => true,
        'capabilities'=> 'moodle/course:view', // comma-separated if multiple
        'services'    => [MOODLE_OFFICIAL_MOBILE_SERVICE] // Optional
    ],
];

$services = [
    'Your Plugin Web Service' => [
        'functions' => [
            'local_yourplugin_your_api_name'
        ],
        'restrictedusers' => 0,
        'enabled' => 1
    ]
];

Service Registration Keys:

classname - Full namespaced class name
methodname - Always 'execute'
type - 'read' (SELECT) or 'write' (INSERT/UPDATE/DELETE)
ajax - Set true for AJAX/REST access
capabilities - Required Moodle capabilities
services - Optional service bundles

Step 6: Implement Error Handling & Logging

private static function log_debug($message) {
    global $CFG;
    $logdir = $CFG->dataroot . '/local_yourplugin';
    if (!file_exists($logdir)) {
        mkdir($logdir, 0777, true);
    }
    $debuglog = $logdir . '/api_debug.log';
    $timestamp = date('Y-m-d H:i:s');
    file_put_contents($debuglog, "[$timestamp] $message\n", FILE_APPEND | LOCK_EX);
}

public static function execute($userid, $courseid) {
    global $DB;

    try {
        self::log_debug("API called: userid=$userid, courseid=$courseid");
        
        // Validate parameters
        $params = self::validate_parameters(self::execute_parameters(), [
            'userid' => $userid,
            'courseid' => $courseid
        ]);

        // Your logic here
        
        self::log_debug("API completed successfully");
        return $result;

    } catch (\invalid_parameter_exception $e) {
        self::log_debug("Parameter validation failed: " . $e->getMessage());
        throw $e;
    } catch (\moodle_exception $e) {
        self::log_debug("Moodle exception: " . $e->getMessage());
        throw $e;
    } catch (\Exception $e) {
        // Log detailed error info
        $lastsql = method_exists($DB, 'get_last_sql') ? $DB->get_last_sql() : '[N/A]';
        self::log_debug("Fatal error: " . $e->getMessage());
        self::log_debug("Last SQL: " . $lastsql);
        self::log_debug("Stack trace: " . $e->getTraceAsString());
        throw $e;
    }
}

Error Handling Best Practices:

Wrap logic in try-catch blocks
Log errors with timestamps and context
Capture SQL queries on database errors
Preserve stack traces for debugging
Re-throw exceptions after logging

Advanced Patterns

Complex Database Operations

// Transaction example
$transaction = $DB->start_delegated_transaction();

try {
    // Insert record
    $recordid = $DB->insert_record('your_table', $dataobject);
    
    // Update related records
    $DB->set_field('another_table', 'status', 1, ['recordid' => $recordid]);
    
    // Commit transaction
    $transaction->allow_commit();
} catch (\Exception $e) {
    $transaction->rollback($e);
    throw $e;
}

Working with Course Modules

// Create course module
$moduleid = $DB->get_field('modules', 'id', ['name' => 'quiz'], MUST_EXIST);

$cm = new \stdClass();
$cm->course = $courseid;
$cm->module = $moduleid;
$cm->instance = 0; // Will be updated after activity creation
$cm->visible = 1;
$cm->groupmode = 0;
$cmid = add_course_module($cm);

// Create activity instance (e.g., quiz)
$quiz = new \stdClass();
$quiz->course = $courseid;
$quiz->name = 'My Quiz';
$quiz->coursemodule = $cmid;
// ... other quiz fields ...

$quizid = quiz_add_instance($quiz, null);

// Update course module with instance ID
$DB->set_field('course_modules', 'instance', $quizid, ['id' => $cmid]);
course_add_cm_to_section($courseid, $cmid, 0);

Access Restrictions (Groups/Availability)

// Restrict activity to specific user via group
$groupname = 'activity_' . $activityid . '_user_' . $userid;

// Create or get group
if (!$groupid = $DB->get_field('groups', 'id', ['courseid' => $courseid, 'name' => $groupname])) {
    $groupdata = (object)[
        'courseid' => $courseid,
        'name' => $groupname,
        'timecreated' => time(),
        'timemodified' => time()
    ];
    $groupid = $DB->insert_record('groups', $groupdata);
}

// Add user to group
if (!$DB->record_exists('groups_members', ['groupid' => $groupid, 'userid' => $userid])) {
    $DB->insert_record('groups_members', (object)[
        'groupid' => $groupid,
        'userid' => $userid,
        'timeadded' => time()
    ]);
}

// Set availability condition
$restriction = [
    'op' => '&',
    'show' => false,
    'c' => [
        [
            'type' => 'group',
            'id' => $groupid
        ]
    ],
    'showc' => [false]
];

$DB->set_field('course_modules', 'availability', json_encode($restriction), ['id' => $cmid]);

Random Question Selection with Tags

private static function get_random_questions($categoryid, $tagname, $limit) {
    global $DB;
    
    $sql = "SELECT q.id
            FROM {question} q
            INNER JOIN {question_versions} qv ON qv.questionid = q.id
            INNER JOIN {question_bank_entries} qbe ON qbe.id = qv.questionbankentryid
            INNER JOIN {question_categories} qc ON qc.id = qbe.questioncategoryid
            JOIN {tag_instance} ti ON ti.itemid = q.id
            JOIN {tag} t ON t.id = ti.tagid
            WHERE LOWER(t.name) = :tagname
              AND qc.id = :categoryid
              AND ti.itemtype = 'question'
              AND q.qtype = 'multichoice'";
    
    $qids = $DB->get_fieldset_sql($sql, [
        'categoryid' => $categoryid,
        'tagname' => strtolower($tagname)
    ]);
    
    shuffle($qids);
    return array_slice($qids, 0, $limit);
}

Testing Your API

1. Via Moodle Web Services Test Client

Enable web services: Site administration > Advanced features
Enable REST protocol: Site administration > Plugins > Web services > Manage protocols
Create service: Site administration > Server > Web services > External services
Test function: Site administration > Development > Web service test client

2. Via curl

# Get token first
curl -X POST "https://yourmoodle.com/login/token.php" \
  -d "username=admin" \
  -d "password=yourpassword" \
  -d "service=moodle_mobile_app"

# Call your API
curl -X POST "https://yourmoodle.com/webservice/rest/server.php" \
  -d "wstoken=YOUR_TOKEN" \
  -d "wsfunction=local_yourplugin_your_api_name" \
  -d "moodlewsrestformat=json" \
  -d "userid=2" \
  -d "courseid=3"

3. Via JavaScript (AJAX)

require(['core/ajax'], function(ajax) {
    var promises = ajax.call([{
        methodname: 'local_yourplugin_your_api_name',
        args: {
            userid: 2,
            courseid: 3
        }
    }]);

    promises[0].done(function(response) {
        console.log('Success:', response);
    }).fail(function(error) {
        console.error('Error:', error);
    });
});

Common Pitfalls & Solutions

1. "Function not found" Error

Solution:

Purge caches: Site administration > Development > Purge all caches
Verify function name in services.php matches exactly
Check namespace and class name are correct

2. "Invalid parameter value detected"

Solution:

Ensure parameter types match between definition and usage
Check required vs optional parameters
Validate nested structure definitions

3. SQL Injection Vulnerabilities

Solution:

Always use placeholder parameters (:paramname)
Never concatenate user input into SQL strings
Use Moodle's database methods: get_record(), get_records(), etc.

4. Permission Denied Errors

Solution:

Call self::validate_context($context) early in execute()
Check required capabilities match user's permissions
Verify user has role assignments in the context

5. Transaction Deadlocks

Solution:

Keep transactions short
Always commit or rollback in finally blocks
Avoid nested transactions

Debugging Checklist

Check Moodle debug mode: Site administration > Development > Debugging
Review web services logs: Site administration > Reports > Logs
Check custom log files in $CFG->dataroot/local_yourplugin/
Verify database queries using $DB->set_debug(true)
Test with admin user to rule out permission issues
Clear browser cache and Moodle caches
Check PHP error logs on server

Plugin Structure Checklist

local/yourplugin/
├── version.php                 # Plugin version and metadata
├── db/
│   ├── services.php           # External service definitions
│   └── access.php             # Capability definitions (optional)
├── classes/
│   └── external/
│       ├── your_api_name.php  # External API implementation
│       └── another_api.php    # Additional APIs
├── lang/
│   └── en/
│       └── local_yourplugin.php  # Language strings
└── tests/
    └── external_test.php      # Unit tests (optional but recommended)

Examples from Real Implementation

Simple Read API (Get Quiz Attempts)

<?php
namespace local_userlog\external;

defined('MOODLE_INTERNAL') || die();
require_once("$CFG->libdir/externallib.php");

use external_api;
use external_function_parameters;
use external_single_structure;
use external_value;

class get_quiz_attempts extends external_api {
    public static function execute_parameters() {
        return new external_function_parameters([
            'userid' => new external_value(PARAM_INT, 'User ID'),
            'courseid' => new external_value(PARAM_INT, 'Course ID')
        ]);
    }

    public static function execute($userid, $courseid) {
        global $DB;

        self::validate_parameters(self::execute_parameters(), [
            'userid' => $userid,
            'courseid' => $courseid
        ]);

        $sql = "SELECT COUNT(*) AS quiz_attempts
                FROM {quiz_attempts} qa
                JOIN {quiz} q ON qa.quiz = q.id
                WHERE qa.userid = :userid AND q.course = :courseid";

        $attempts = $DB->get_field_sql($sql, [
            'userid' => $userid,
            'courseid' => $courseid
        ]);

        return ['quiz_attempts' => (int)$attempts];
    }

    public static function execute_returns() {
        return new external_single_structure([
            'quiz_attempts' => new external_value(PARAM_INT, 'Total number of quiz attempts')
        ]);
    }
}

Complex Write API (Create Quiz from Categories)

See attached create_quiz_from_categories.php for a comprehensive example including:

Multiple database insertions
Course module creation
Quiz instance configuration
Random question selection with tags
Group-based access restrictions
Extensive error logging
Transaction management

Quick Reference: Common Moodle Tables

Table	Purpose
`{user}`	User accounts
`{course}`	Courses
`{course_modules}`	Activity instances in courses
`{modules}`	Available activity types (quiz, forum, etc.)
`{quiz}`	Quiz configurations
`{quiz_attempts}`	Quiz attempt records
`{question}`

Additional Resources

Guidelines

Always validate input parameters using validate_parameters()
Check user context and capabilities before operations
Use parameterized SQL queries (never string concatenation)
Implement comprehensive error handling and logging
Follow Moodle naming conventions (lowercase, underscores)
Document all parameters and return values clearly
Test with different user roles and permissions
Consider transaction safety for write operations
Purge caches after service registration changes
Keep API methods focused and single-purpose

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Tags & Topics

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Moodle External API Development

This skill guides you through creating custom external web service APIs for Moodle LMS, following Moodle's external API framework and coding standards.

When to Use This Skill

Creating custom web services for Moodle plugins
Implementing REST/AJAX endpoints for course management
Building APIs for quiz operations, user tracking, or reporting
Exposing Moodle functionality to external applications
Developing mobile app backends using Moodle

Core Architecture Pattern

Moodle external APIs follow a strict three-method pattern:

execute_parameters() - Defines input parameter structure
execute() - Contains business logic
execute_returns() - Defines return structure

Step-by-Step Implementation

Step 1: Create the External API Class File

Location: /local/yourplugin/classes/external/your_api_name.php

<?php
namespace local_yourplugin\external;

defined('MOODLE_INTERNAL') || die();
require_once("$CFG->libdir/externallib.php");

use external_api;
use external_function_parameters;
use external_single_structure;
use external_value;

class your_api_name extends external_api {
    
    // Three required methods will go here
    
}

Key Points:

Class must extend external_api
Namespace follows: local_pluginname\external or mod_modname\external
Include the security check: defined('MOODLE_INTERNAL') || die();
Require externallib.php for base classes

Step 2: Define Input Parameters

public static function execute_parameters() {
    return new external_function_parameters([
        'userid' => new external_value(PARAM_INT, 'User ID', VALUE_REQUIRED),
        'courseid' => new external_value(PARAM_INT, 'Course ID', VALUE_REQUIRED),
        'options' => new external_single_structure([
            'includedetails' => new external_value(PARAM_BOOL, 'Include details', VALUE_DEFAULT, false),
            'limit' => new external_value(PARAM_INT, 'Result limit', VALUE_DEFAULT, 10)
        ], 'Options', VALUE_OPTIONAL)
    ]);
}

Common Parameter Types:

PARAM_INT - Integers
PARAM_TEXT - Plain text (HTML stripped)
PARAM_RAW - Raw text (no cleaning)
PARAM_BOOL - Boolean values
PARAM_FLOAT - Floating point numbers
PARAM_ALPHANUMEXT - Alphanumeric with extended chars

Structures:

external_value - Single value
external_single_structure - Object with named fields
external_multiple_structure - Array of items

Value Flags:

VALUE_REQUIRED - Parameter must be provided
VALUE_OPTIONAL - Parameter is optional
VALUE_DEFAULT, defaultvalue - Optional with default

Step 3: Implement Business Logic

public static function execute($userid, $courseid, $options = []) {
    global $DB, $USER;

    // 1. Validate parameters
    $params = self::validate_parameters(self::execute_parameters(), [
        'userid' => $userid,
        'courseid' => $courseid,
        'options' => $options
    ]);

    // 2. Check permissions/capabilities
    $context = \context_course::instance($params['courseid']);
    self::validate_context($context);
    require_capability('moodle/course:view', $context);

    // 3. Verify user access
    if ($params['userid'] != $USER->id) {
        require_capability('moodle/course:viewhiddenactivities', $context);
    }

    // 4. Database operations
    $sql = "SELECT id, name, timecreated
            FROM {your_table}
            WHERE userid = :userid
              AND courseid = :courseid
            LIMIT :limit";
    
    $records = $DB->get_records_sql($sql, [
        'userid' => $params['userid'],
        'courseid' => $params['courseid'],
        'limit' => $params['options']['limit']
    ]);

    // 5. Process and return data
    $results = [];
    foreach ($records as $record) {
        $results[] = [
            'id' => $record->id,
            'name' => $record->name,
            'timestamp' => $record->timecreated
        ];
    }

    return [
        'items' => $results,
        'count' => count($results)
    ];
}

Critical Steps:

Always validate parameters using validate_parameters()
Check context using validate_context()
Verify capabilities using require_capability()
Use parameterized queries to prevent SQL injection
Return structured data matching return definition

Step 4: Define Return Structure

public static function execute_returns() {
    return new external_single_structure([
        'items' => new external_multiple_structure(
            new external_single_structure([
                'id' => new external_value(PARAM_INT, 'Item ID'),
                'name' => new external_value(PARAM_TEXT, 'Item name'),
                'timestamp' => new external_value(PARAM_INT, 'Creation time')
            ])
        ),
        'count' => new external_value(PARAM_INT, 'Total items')
    ]);
}

Return Structure Rules:

Must match exactly what execute() returns
Use appropriate parameter types
Document each field with description
Nested structures allowed

Step 5: Register the Service

Location: /local/yourplugin/db/services.php

<?php
defined('MOODLE_INTERNAL') || die();

$functions = [
    'local_yourplugin_your_api_name' => [
        'classname'   => 'local_yourplugin\external\your_api_name',
        'methodname'  => 'execute',
        'classpath'   => 'local/yourplugin/classes/external/your_api_name.php',
        'description' => 'Brief description of what this API does',
        'type'        => 'read',  // or 'write'
        'ajax'        => true,
        'capabilities'=> 'moodle/course:view', // comma-separated if multiple
        'services'    => [MOODLE_OFFICIAL_MOBILE_SERVICE] // Optional
    ],
];

$services = [
    'Your Plugin Web Service' => [
        'functions' => [
            'local_yourplugin_your_api_name'
        ],
        'restrictedusers' => 0,
        'enabled' => 1
    ]
];

Service Registration Keys:

classname - Full namespaced class name
methodname - Always 'execute'
type - 'read' (SELECT) or 'write' (INSERT/UPDATE/DELETE)
ajax - Set true for AJAX/REST access
capabilities - Required Moodle capabilities
services - Optional service bundles

Step 6: Implement Error Handling & Logging

private static function log_debug($message) {
    global $CFG;
    $logdir = $CFG->dataroot . '/local_yourplugin';
    if (!file_exists($logdir)) {
        mkdir($logdir, 0777, true);
    }
    $debuglog = $logdir . '/api_debug.log';
    $timestamp = date('Y-m-d H:i:s');
    file_put_contents($debuglog, "[$timestamp] $message\n", FILE_APPEND | LOCK_EX);
}

public static function execute($userid, $courseid) {
    global $DB;

    try {
        self::log_debug("API called: userid=$userid, courseid=$courseid");
        
        // Validate parameters
        $params = self::validate_parameters(self::execute_parameters(), [
            'userid' => $userid,
            'courseid' => $courseid
        ]);

        // Your logic here
        
        self::log_debug("API completed successfully");
        return $result;

    } catch (\invalid_parameter_exception $e) {
        self::log_debug("Parameter validation failed: " . $e->getMessage());
        throw $e;
    } catch (\moodle_exception $e) {
        self::log_debug("Moodle exception: " . $e->getMessage());
        throw $e;
    } catch (\Exception $e) {
        // Log detailed error info
        $lastsql = method_exists($DB, 'get_last_sql') ? $DB->get_last_sql() : '[N/A]';
        self::log_debug("Fatal error: " . $e->getMessage());
        self::log_debug("Last SQL: " . $lastsql);
        self::log_debug("Stack trace: " . $e->getTraceAsString());
        throw $e;
    }
}

Error Handling Best Practices:

Wrap logic in try-catch blocks
Log errors with timestamps and context
Capture SQL queries on database errors
Preserve stack traces for debugging
Re-throw exceptions after logging

Advanced Patterns

Complex Database Operations

// Transaction example
$transaction = $DB->start_delegated_transaction();

try {
    // Insert record
    $recordid = $DB->insert_record('your_table', $dataobject);
    
    // Update related records
    $DB->set_field('another_table', 'status', 1, ['recordid' => $recordid]);
    
    // Commit transaction
    $transaction->allow_commit();
} catch (\Exception $e) {
    $transaction->rollback($e);
    throw $e;
}

Working with Course Modules

// Create course module
$moduleid = $DB->get_field('modules', 'id', ['name' => 'quiz'], MUST_EXIST);

$cm = new \stdClass();
$cm->course = $courseid;
$cm->module = $moduleid;
$cm->instance = 0; // Will be updated after activity creation
$cm->visible = 1;
$cm->groupmode = 0;
$cmid = add_course_module($cm);

// Create activity instance (e.g., quiz)
$quiz = new \stdClass();
$quiz->course = $courseid;
$quiz->name = 'My Quiz';
$quiz->coursemodule = $cmid;
// ... other quiz fields ...

$quizid = quiz_add_instance($quiz, null);

// Update course module with instance ID
$DB->set_field('course_modules', 'instance', $quizid, ['id' => $cmid]);
course_add_cm_to_section($courseid, $cmid, 0);

Access Restrictions (Groups/Availability)

// Restrict activity to specific user via group
$groupname = 'activity_' . $activityid . '_user_' . $userid;

// Create or get group
if (!$groupid = $DB->get_field('groups', 'id', ['courseid' => $courseid, 'name' => $groupname])) {
    $groupdata = (object)[
        'courseid' => $courseid,
        'name' => $groupname,
        'timecreated' => time(),
        'timemodified' => time()
    ];
    $groupid = $DB->insert_record('groups', $groupdata);
}

// Add user to group
if (!$DB->record_exists('groups_members', ['groupid' => $groupid, 'userid' => $userid])) {
    $DB->insert_record('groups_members', (object)[
        'groupid' => $groupid,
        'userid' => $userid,
        'timeadded' => time()
    ]);
}

// Set availability condition
$restriction = [
    'op' => '&',
    'show' => false,
    'c' => [
        [
            'type' => 'group',
            'id' => $groupid
        ]
    ],
    'showc' => [false]
];

$DB->set_field('course_modules', 'availability', json_encode($restriction), ['id' => $cmid]);

Random Question Selection with Tags

private static function get_random_questions($categoryid, $tagname, $limit) {
    global $DB;
    
    $sql = "SELECT q.id
            FROM {question} q
            INNER JOIN {question_versions} qv ON qv.questionid = q.id
            INNER JOIN {question_bank_entries} qbe ON qbe.id = qv.questionbankentryid
            INNER JOIN {question_categories} qc ON qc.id = qbe.questioncategoryid
            JOIN {tag_instance} ti ON ti.itemid = q.id
            JOIN {tag} t ON t.id = ti.tagid
            WHERE LOWER(t.name) = :tagname
              AND qc.id = :categoryid
              AND ti.itemtype = 'question'
              AND q.qtype = 'multichoice'";
    
    $qids = $DB->get_fieldset_sql($sql, [
        'categoryid' => $categoryid,
        'tagname' => strtolower($tagname)
    ]);
    
    shuffle($qids);
    return array_slice($qids, 0, $limit);
}

Testing Your API

1. Via Moodle Web Services Test Client

Enable web services: Site administration > Advanced features
Enable REST protocol: Site administration > Plugins > Web services > Manage protocols
Create service: Site administration > Server > Web services > External services
Test function: Site administration > Development > Web service test client

2. Via curl

# Get token first
curl -X POST "https://yourmoodle.com/login/token.php" \
  -d "username=admin" \
  -d "password=yourpassword" \
  -d "service=moodle_mobile_app"

# Call your API
curl -X POST "https://yourmoodle.com/webservice/rest/server.php" \
  -d "wstoken=YOUR_TOKEN" \
  -d "wsfunction=local_yourplugin_your_api_name" \
  -d "moodlewsrestformat=json" \
  -d "userid=2" \
  -d "courseid=3"

3. Via JavaScript (AJAX)

require(['core/ajax'], function(ajax) {
    var promises = ajax.call([{
        methodname: 'local_yourplugin_your_api_name',
        args: {
            userid: 2,
            courseid: 3
        }
    }]);

    promises[0].done(function(response) {
        console.log('Success:', response);
    }).fail(function(error) {
        console.error('Error:', error);
    });
});

Common Pitfalls & Solutions

1. "Function not found" Error

Solution:

Purge caches: Site administration > Development > Purge all caches
Verify function name in services.php matches exactly
Check namespace and class name are correct

2. "Invalid parameter value detected"

Solution:

Ensure parameter types match between definition and usage
Check required vs optional parameters
Validate nested structure definitions

3. SQL Injection Vulnerabilities

Solution:

Always use placeholder parameters (:paramname)
Never concatenate user input into SQL strings
Use Moodle's database methods: get_record(), get_records(), etc.

4. Permission Denied Errors

Solution:

Call self::validate_context($context) early in execute()
Check required capabilities match user's permissions
Verify user has role assignments in the context

5. Transaction Deadlocks

Solution:

Keep transactions short
Always commit or rollback in finally blocks
Avoid nested transactions

Debugging Checklist

Check Moodle debug mode: Site administration > Development > Debugging
Review web services logs: Site administration > Reports > Logs
Check custom log files in $CFG->dataroot/local_yourplugin/
Verify database queries using $DB->set_debug(true)
Test with admin user to rule out permission issues
Clear browser cache and Moodle caches
Check PHP error logs on server

Plugin Structure Checklist

local/yourplugin/
├── version.php                 # Plugin version and metadata
├── db/
│   ├── services.php           # External service definitions
│   └── access.php             # Capability definitions (optional)
├── classes/
│   └── external/
│       ├── your_api_name.php  # External API implementation
│       └── another_api.php    # Additional APIs
├── lang/
│   └── en/
│       └── local_yourplugin.php  # Language strings
└── tests/
    └── external_test.php      # Unit tests (optional but recommended)

Examples from Real Implementation

Simple Read API (Get Quiz Attempts)

<?php
namespace local_userlog\external;

defined('MOODLE_INTERNAL') || die();
require_once("$CFG->libdir/externallib.php");

use external_api;
use external_function_parameters;
use external_single_structure;
use external_value;

class get_quiz_attempts extends external_api {
    public static function execute_parameters() {
        return new external_function_parameters([
            'userid' => new external_value(PARAM_INT, 'User ID'),
            'courseid' => new external_value(PARAM_INT, 'Course ID')
        ]);
    }

    public static function execute($userid, $courseid) {
        global $DB;

        self::validate_parameters(self::execute_parameters(), [
            'userid' => $userid,
            'courseid' => $courseid
        ]);

        $sql = "SELECT COUNT(*) AS quiz_attempts
                FROM {quiz_attempts} qa
                JOIN {quiz} q ON qa.quiz = q.id
                WHERE qa.userid = :userid AND q.course = :courseid";

        $attempts = $DB->get_field_sql($sql, [
            'userid' => $userid,
            'courseid' => $courseid
        ]);

        return ['quiz_attempts' => (int)$attempts];
    }

    public static function execute_returns() {
        return new external_single_structure([
            'quiz_attempts' => new external_value(PARAM_INT, 'Total number of quiz attempts')
        ]);
    }
}

Complex Write API (Create Quiz from Categories)

See attached create_quiz_from_categories.php for a comprehensive example including:

Multiple database insertions
Course module creation
Quiz instance configuration
Random question selection with tags
Group-based access restrictions
Extensive error logging
Transaction management

Quick Reference: Common Moodle Tables

Table	Purpose
`{user}`	User accounts
`{course}`	Courses
`{course_modules}`	Activity instances in courses
`{modules}`	Available activity types (quiz, forum, etc.)
`{quiz}`	Quiz configurations
`{quiz_attempts}`	Quiz attempt records
`{question}`

Additional Resources

Guidelines

Always validate input parameters using validate_parameters()
Check user context and capabilities before operations
Use parameterized SQL queries (never string concatenation)
Implement comprehensive error handling and logging
Follow Moodle naming conventions (lowercase, underscores)
Document all parameters and return values clearly
Test with different user roles and permissions
Consider transaction safety for write operations
Purge caches after service registration changes
Keep API methods focused and single-purpose