Why are my AJAX requests blocking each other, and how can I fix it?

Request Parallelization in AJAX Requests

In certain scenarios, performing simultaneous AJAX requests can be advantageous to enhance user experience or optimize performance. However, in specific situations, these requests may erroneously obstruct each other's execution.

A common dilemma arises when attempting to concurrently execute two AJAX requests. The issue typically manifests as the second request being delayed until the first one completes, even though they should run independently. This can be puzzling and frustrating, especially if it used to work previously.

The underlying cause of this behavior often lies in session blocking. By default, PHP stores session data in files. When a session is initialized with session_start(), the file is opened for writing and locked to prevent simultaneous modifications. This locking mechanism ensures data integrity but inadvertently creates a bottleneck for concurrent sessions.

To resolve this issue and allow AJAX requests to run in parallel, you can modify PHP's session settings to avoid using files or close the session write explicitly. The latter option is demonstrated in the following code snippet:

<code class="php"><?php
    session_start(); // starting the session

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

    session_write_close(); // close the session file and release the lock

    echo $_SESSION['foo']; // You can still read from the session.</code>

Implementing this change will release the session lock, enabling concurrent AJAX requests to proceed without interference.

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