Is it wise to use PHP for a daemon?

Question

I wish to create a background process and I have been told these are usually written in C or something of that sort. I have recently found out PHP can be used to create a daemon and I was hoping to get some advice if I should make use of PHP in this way.

Here are my requirements for a daemon.

• Continuously check if a row has been added to MySQL database table
• Run FFmpeg commands on what was retrieved from database
• Insert output into MySQL table

I am not sure what else I can offer to help make this decision. Just to add, I have not done C before. Only Java and PHP and basic bash scripting.

Does it even make that much of a performance difference?

Please allow for my ignorance, I am learning! :)

Thanks all

Answer 1

If you know what you are doing sure. You need to understand your operating system well. PHP generally isn't suited for most daemons because it isn't threaded and doesn't have a decent event based system for all tasks. However if it suits your needs then no problem. Modern PHP (5.3+) is really stable and doesn't have any memory leaks. As long as you enable the GC and don't implement your own memory leaks, etc you'll be fine.

Here are the stats for one daemon I am running: uptime 17 days (last restart due to PHP upgrade). bytes written: 200GB connections: hundreds connections handled, hundreds of thousands items/requests processed: millions

node.js is generally better suited although has some minor annoyances. Some attempts to improve PHP in the same areas have been made but they aren't really that great.

Answer 2

Cron job? Yes.

Daemon which runs forever? No.

PHP does not have a garbage collector (or at least, last time I checked it did not). Therefore, if you create a circular reference, it NEVER gets cleaned up - at least not until the main script execution finishes. In daemon process this is approximately never.

If they've added a GC in new versions, then yes you can.

Answer 3

You might want to consider making a mysql trigger that executes a system command (i.e. FFmpeg) instead of a daemon. If some lag isn't a problem, you could also put something in cron that executes every few minutes to check. Cron would be my choice, if it is an option.

To answer your question, php is perfectly fine to run as a daemon. It does not have to be done in C.

Answer 4

I wouldn't recommend it. PHP is not designed for longterm execution. Its designed primarily with short lived pages.

In my experience PHP can have problems with leaking memory for some of the larger tasks.

Answer 5

Go for it. I had to do it once also. Like others said, it's not ideal but it'll get-er-done. Using Windows, right? Good.

If you only need it to run occasionally (Once per hour, etc). Make a new shortcut to your firefox, place it somewhere relevant. Open up the properties for the shortcut, change "Target" to:

"C:\Program Files\Mozilla Firefox\firefox.exe" http://localhost/path/to/script.php

Go to Control Panel>Scheduled Tasks Point your new scheduled task at the shortcut.

If you need it to run constantly or pseudo-constantly, you'll need to spice the script up a bit.

Start your script with

set_time_limit(0);
ob_implicit_flush(true);

If the script uses a loop (like while) you have to clear the buffer:

$i=0;
while($i<sizeof($my_array)){
     //do stuff
     flush();           
     ob_clean();
     sleep(17);
     $i++;
}

Answer 6

As others have noted, various versions of PHP have issues with their garbage collectors. Of course, if you know that your version does not have such issues, you eliminate that problem. The point is, you don't know (for sure) until you write the daemon and run it through valgrind to see if the installed PHP leaks or not on any given machine. So on that hand, you may write it just to discover that what Zend thinks is fixed might still be buggy, or you are dealing with a slightly older version of PHP or some extension. Icky.

The other problem is somewhat buggy signals. In my experience, signal handlers are not always entered correctly with PHP, especially when the signal is queued instead of merged. That may not be an issue for you, i.e. if you just need to handle SIGINT/SIGUSR1/SIGUSR2/SIGHUP.

So, I suggest:

If the daemon is simple, go ahead and use PHP. If it looks like its going to get rather complex, or allocate lots of memory, you might consider writing it in C after prototyping it in PHP.

I am a pretty die hard C person. However, I see nothing wrong with hammering out something quick using PHP (beyond the cases that I explained). I also see nothing wrong with using PHP to prototype something that may or may not be later rewritten in C. For instance, handling database stuff is going to be much simpler if you use PHP, versus managing callbacks using other interfaces in C. So in that instance, for a 'one off', you will surely get it done much faster.