Prevent PHP Session Locking Using session_write_close

This article explains how the PHP function session_write_close() prevents session locking during concurrent HTTP requests. You will learn why PHP locks sessions by default, the performance issues this causes for concurrent AJAX requests or multiple browser tabs, and how releasing the session lock early resolves these bottlenecks to improve website responsiveness.

Understanding PHP Session Locking

By default, when you call session_start(), PHP opens a session file on the server and places an exclusive lock on it. This mechanism ensures data consistency. If two different scripts attempted to write to the same user session simultaneously, data corruption or loss could occur.

While this locking behavior is useful for data integrity, it causes issues when a user triggers multiple concurrent requests. For example, if a browser tab is loading a long-running script and the user opens another tab or triggers an AJAX request, the second request will hang. PHP will force the second request to wait until the first request completes and releases the file lock.

How session_write_close() Solves the Issue

The session_write_close() function addresses this bottleneck by saving the current session data and immediately releasing the lock on the session file. Once this function is called, the script can continue running, but it will no longer block other incoming requests from accessing the session.

By releasing the lock early, concurrent requests can execute in parallel rather than sequentially, significantly improving the loading speed of web applications that rely on multiple simultaneous requests.

Implementing session_write_close() in PHP

To use this function effectively, place it immediately after you have finished making changes to the session array.

<?php
// Start the session and read/write data
session_start();

$_SESSION['user_status'] = 'Active';
$_SESSION['last_activity'] = time();

// Write the data and release the lock immediately
session_write_close();

// The script continues to execute long-running tasks
// Other concurrent requests can now access the session without waiting
sleep(10); 

echo "Task completed successfully.";
?>

Important Considerations

Once session_write_close() is executed, the session data is converted to a read-only state for that specific request. You can still read values from the $_SESSION superglobal, but any subsequent modifications to $_SESSION will not be saved to the server.

If you must write to the session again later in the same script, you will need to re-initiate the session by calling session_start() again, which will re-lock the session file for the duration of the new write operation.