Here are a few title options, playing with the question format: * Why Are My AJAX Requests Not Running Concurrently in PHP? (Focuses on the problem) * How to Achieve True Concurrency with AJAX Reques

AJAX Requests Not Running Concurrently

In an attempt to optimize performance, two simultaneous AJAX requests are utilized in a web application to track the progress of a lengthy task. However, the second request stalls until the first one completes.

Root Cause: Session Blocking

PHP, by default, employs a file-based approach for session storage. When a session is initiated with session_start(), it acquires an exclusive write lock on the session file. Consequently, all other requests attempting to access the session file will be blocked until the lock is released.

Solution: Disable File-Based Sessions

One solution is to disable file-based session storage by modifying the php.ini configuration file:

session.save_handler = redis
session.save_path = tcp://localhost:6379

This change redirects session data to a Redis server, eliminating the file locking issue.

Alternatively: Close Session File Write

Alternatively, you can release the session file write lock by explicitly closing the session file after writing data:

<?php
    session_start();

    $_SESSION['foo'] = 'bar'; // Write data to the session

    session_write_close(); // Release the file write lock

This technique allows the session data to be read later, but prevents other requests from being blocked by the session write lock.

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