Question

I have couple of php scripts which are called by cron. The cron job runs every 20 mins. Sometimes script finishes the job in few mins and sometimes it takes more than 20 mins.

I want to prevent the race condition here. One way I have in my mind is to use the database set the flag when page is running and remove flag when the job is done. But I really don’t want to use database.

Is there any other solution?

Answer 1

The best method (that I’ve used) involves saving the process id in a “lock file”.

function getLock() {
    //First, check to see if the script is already running
    $data = @file_get_contents('path/to/process.pid');
    if ($data && posix_kill($data, 0)) {
        throw new RuntimeException(
            'Unable To Attain Lock, Another Process Is Still Running'
        );
    }
    file_put_contents('path/to/process.pid', posix_getpid());
}

Basically, it reads the file, and then tries to posix_kill the pid contained inside. Note, that when you use a signal of 0 to posix_kill, it merely tells you if the call will succeed (meaning that the process is running).

Doing it this way has the advantage that if the process dies without cleaning up the lock file, it’ll be detected and will run again.