Which session handler minimizes latency for concurrent AJAX requests in Moodle?

Direct Answer

The \core\session\redis handler minimizes latency for concurrent AJAX requests. Unlike file or database handlers, Redis utilizes atomic operations to manage session state, significantly reducing the lock contention that typically causes simultaneous requests from the same user to queue sequentially.

Performance Comparison

Database Handler (\core\session\database)

• Locking Mechanism: Employs row-level locking (typically SELECT ... FOR UPDATE) on the sessions table.
• Behavior: Concurrent requests from a single user session are forced into a sequential queue, increasing perceived latency for dashboard blocks or simultaneous AJAX calls.
• Overhead: Each session read/write requires a database round-trip, contributing to overall request time.

Redis Handler (\core\session\redis)

• Locking Mechanism: Uses atomic Redis commands (such as SET NX EX) for lock acquisition with a defined Time-To-Live (TTL).
• Behavior: Supports highly efficient lock acquisition and release. It allows for faster state transitions and reduces the time a session remains locked compared to disk or RDBMS.
• Scalability: Ideal for clustered environments, providing a shared session state across multiple web nodes without overloading the primary database.

Early Lock Release in AJAX Endpoints

Moodle does not provide a native, global mechanism to implement session_write_close() across all standard AJAX endpoints. While Moodle manages session closure via \core\session\manager::write_close() during the request shutdown phase, the lock is held for the entire duration of the execution.

For custom AJAX endpoints, you can manually release the lock to allow other concurrent requests to proceed once session data is no longer needed:

require_once(__DIR__ . '/../../config.php');
require_login();

// Read necessary session data
$userdata = \core\session\manager::instance()->get_userdata();

// Release the lock early
session_write_close();

// Perform expensive processing or API calls
$result = perform_heavy_task();
echo json_encode($result);

Configuration and Verification

To implement the Redis handler, update your config.php:

$CFG->session_handler_class = '\\core\\session\\redis';
$CFG->session_redis_config = [
    'host' => '127.0.0.1',
    'port' => 6379,
    'prefix' => 'moodle_sess_',
    'database' => 0,
    'acquire_lock_timeout' => 5.0,
    'lock_expire' => 7200,
];

Verification:

1. Confirm the active handler via /admin/phpinfo.php by searching for session.handler.
2. Use a tool like Apache Bench (ab) to simulate concurrent requests with the same session cookie and compare the p95 response times between the database and Redis handlers.

One Diagnostic Needed

What is the average number of concurrent AJAX requests triggered per user page load? If the count is consistently above 3-5, the transition to Redis is critical; otherwise, the database handler may be sufficient.