Why Are My Simultaneous AJAX Requests Not Running in Parallel?

Simultaneous AJAX Requests Failing to Run in Parallel

One might encounter an issue where two AJAX requests, one for exporting data to an XLSX file and the other for displaying progress updates, fail to run concurrently. The first request, which is time-consuming, seems to block the progress updates from being displayed.

Explanation:

This behavior can be attributed to session blocking. PHP stores session data in files by default. When a session is initiated with session_start(), the session file is opened for writing and locked to prevent concurrent modifications. As a result, any subsequent session-enabled PHP requests must wait for the previous request to release the lock before proceeding.

Solution:

To resolve this issue, one can either configure PHP to use an alternative session storage method (e.g., database, memcached) or explicitly close the session write after writing data to it. The latter approach can be achieved using the session_write_close() function. Here's an example:

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

    // Write data to session (if necessary)

    session_write_close(); // close session file, releasing lock

    // Read or use session data as needed

By closing the session write, one can unlock the session file and allow subsequent AJAX requests to proceed without waiting for the first request to complete.

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