Using server-sent events and php sessions

Question

I'm using in my project server-sent events where the JS is calling a PHP page, say eventserver.php which consists basically of an infinite loop which checks the existence of an event in a $_SESSION variable. On my first implementation this lead my website to hung up because the eventserver took the lock on the session and did not release it until the timeout expired; however, I managed to resolve this issue by locking/unlocking the session by using session_write_lock() and session_start() continuously in the loop.

This is actually causing a lot of PHP warnings (on Apache error.log) saying that "cannot send session cache limiter - headers already sent", "cannot send session cookies" and so on.

Posting some code here

session_start();

header('Cache-Control: no-cache');
header('Content-Type: text/event-stream');

class EventServer
{        

    public function WaitForEvents( $eventType )
    {   
        // ... do stuff

        while( true )
        {
            // lock the session to this instance
            session_start();

            // ...check/output the event

            ob_flush();
            flush();

            // unlock the session 
            session_write_close();
            sleep( 1 );
        }          
    }
}

Why is this happening?

Answer 1

I am doing the same thing as the OP and ran into the same issue. Some of these answers don't understand how eventSource should work. My code is identical to yours and uses a session variable to know what view the user is on which drives what data to return in the event of a server trigger. It's part of a realtime collaboration app.

I simply prepended an @ to the session_start() to suppress the warnings in the log. Not really a fix, but it keeps the log from filling up.

Alternatively, not sure how well it would work for your application, but you could use ajax to write the session variable you are monitoring to the database, then your eventSource script can monitor for a change in the DB instead of having to start sessions.