Question

Part of the PHP web app I’m developing needs to do the following:

1. Make an AJAX request to a PHP script, which could potentially take from one second to one hour, and display the output on the page when finished.
2. Periodically update a loading bar on the web page, defined by a status variable in the long running PHP script.
3. Allow the long running PHP script to detect if the AJAX request is cancelled, so it can shut down properly and in a timely fashion.

My current solution:

• client.php: Creates an AJAX request to request.php, followed by one request per second to status.php until the initial request is complete. Generates and passes along a unique identifier (uid) in case multiple instances of the app are running.
• request.php: Each time progress is made, saves the current progress percentage to $_SESSION["progressBar"][uid]. (It must run session_start() and session_write_close() each time.) When finished, returns the data that client.php needs.
• status.php: Runs session_start(), returns $_SESSION["progressBar"][uid], and runs session_write_close().

Where it falls short:

My solution fulfills my first two requirements. For the third, I would like to use connection_aborted() in request.php to know if the request is cancelled. BUT, the docs say:

PHP will not detect that the user has aborted the connection until an attempt is made to send information to the client. Simply using an echo statement does not guarantee that information is sent, see flush().

I could simply give meaningless output, but PHP must send a cookie every time I call session_start(). I want to use the same session, BUT the docs say:

When using session cookies, specifying an id for session_id() will always send a new cookie when session_start() is called, regardless of if the current session id is identical to the one being set.

My ideas for solutions, none of which I’m happy with:

• A status database, or writing to temp files, or a task management system. This just seems more complicated than what I need!
• A custom session handler. This is basically the same as the above solution.
• Stream both progress data and result data in one request. This solves everything, but I would essentially be re-implementing AJAX. That can’t be right.

Please tell me I’m missing something! Why doesn’t PHP know immediately when a connection terminates? Why must PHP resend the cookie, even when it is exactly the same? An answer to any of these questions will be a big help!

My sincere thanks.

Answer 1

Why not set a second session variable, consisting of the unique request identifier and an access timestamp, from status.php.

If the client is closed it stops getting updates from status.php and the session variable stops being updated, which triggers a clean close in request.php if the variable isn’t updated in a certain amount of time.