Why Do My AJAX Requests Block Each Other in PHP?

Simultaneous AJAX Requests Blocking Each Other

In certain situations, AJAX requests may not run simultaneously, causing an unexpected behavior. This issue was raised when a user encountered a problem with two concurrent AJAX requests that were designed to show progress while exporting data to an XSLX file. The first request triggers a lengthy PHP script, and the second request checks for progress updates at regular intervals.

Upon investigation, it was discovered that the second AJAX request was waiting for the first one to complete before executing. This behavior is attributed to session blocking in PHP.

Session Blocking in PHP

By default, PHP uses files to store session data. When a session is started (using the session_start() function), PHP opens the session file for writing and locks it to ensure exclusive access. This locking mechanism prevents concurrent requests from accessing the file simultaneously. As a result, the second AJAX request had to wait for the first one to complete and unlock the session file before it could execute.

Solution

The solution to this issue is to disable PHP's default file-based session storage or to explicitly close the session file after writing to it, releasing the lock.

To disable file-based session storage, add the following line to your php.ini file:

session.save_path =

To explicitly close the session file, use the session_write_close() function:

<code class="php"><?php
    session_start();

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

    session_write_close(); // Close the session file

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

By implementing these solutions, you can ensure that the second AJAX request will execute concurrently with the first one, allowing you to track progress without delays.

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