Is It Wise to Use PHP for a Daemon

Is it wise to use PHP for a daemon?

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.

What is daemon? Their practical use? Usage with php?

Could someone explain me in two words, what is daemon and what use of them in php?

cli application or process

I, know that this is a process, which is runing all the time. But i can't understand what use of it in php app?

You can use it to do; job that is not visible to user or from interface, e.g. database stale data cleanup, schedule task that you you wanted to update part or something on db or page in background

Can someone please give examples of use? Can i use daemon to lessen memory usage of my app?

I think drupal or cron had cron script...perhaps checking it would help. Lessen memory? no, memory optimization is always on the application design or script coded.

As i understand, daemon can hold data and give it on request, so basically i can store most usable data there, to avoid getting it from mysql for each visitor?

No, a daemon is a script however you can create a JSON or XML data file that the daemon script can process.

php daemon possible memory leak

I got the same problem. Maybe the best idea is to report new bug at PEAR

BTW, code like that doesn't show that memleak:

#!/usr/bin/php -q

<?php
require_once "System/Daemon.php";
System_Daemon::setOption("appName", "smsd");
System_Daemon::start();
while(!System_Daemon::isDying()) {
 print ("debug: memory_get_peak_usage: ".memory_get_peak_usage()."\n");
 print ("debug: memory_get_usage: ".memory_get_usage()."\n\n");
 System_Daemon::iterate(2);

}

Look's like System_Daemon::info() is a problem.

Run php script as daemon process

You could start your php script from the command line (i.e. bash) by using

nohup php myscript.php &

the & puts your process in the background.

Edit:

Yes, there are some drawbacks, but not possible to control? That's just wrong.

A simple kill processid will stop it. And it's still the best and simplest solution.

What's the best way to keep a PHP script running as a daemon?

If you can't use the (proper) init structure to do this (you're on shared hosting, etc.), use cron to run a script (it can be written in whatever language you like) every few minutes that checks to see if they're running, and restarts them if necessary.

Starting a daemon from PHP

Try appending > /dev/null 2>&1 & to the command.

So this:

exec("sudo /etc/init.d/daemonToStart > /dev/null 2>&1 &");

Just in case you want to know what it does/why:

• > /dev/null - redirect STDOUT to /dev/null (blackhole it, in other words)
• 2>&1 - redirect STDERR to STDOUT (blackhole it as well)
• & detach process and run in the background

Running a PHP process as a daemon while safely killing it from background

I had related problems once too. Let me explain. I had a php 'daemon' that worked like a downloader. It accessed feeds periodically and downloads (laaaarge) content from the net. The daemon had to be stopped at a certain time, lets say 0500 in the morning to prevent it from using the whole bandwith during daytime. I decided to use a cronjob to send SIGTERM to the daemon at 0500.

In the daemon I had the following code:

pcntl_signal(SIGTERM, array($this, 'signal_handler'));

where signal_handler looked like this:

public function signal_handler($signal) {
    // some cleanup code 
    exit(1);
}

Unfortunately this did not work :|

It took me a time to find out what's going on. The first thing I figured out was that I'll have to call the method pcntl_signal_dispatch() on init to enable signal dispatching at all. Quote from the doc (comments):

If you are running PHP as CLI and as a "daemon" (i.e. in a loop), this function must be called in each loop to check if new signals are waiting dispatching.

Ok, so far, it seemed working. But I realized quickly that under certain conditions even this will not work as expected. Sometimes the daemon could only being stopped by kill -9 - as before. :|

So what's the problem?.. Answer: My program called wget to download the files via shell_exec. The problem is, that shell_exec() blocking waits until the child process has terminated. During this blocking wait no signal processing is done, the process can only being terminated using SIGKILL - what is hard. Also a problem was that child processes had to be terminated one by one as they became zombie processes after killing the father.

My solution to this was to execute the child process using proc_open() and the use stream_select() on it's output for non blocking IO.

Now it works like a charm. :) If you need further information don't hesitate to drop a comment.

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Note If you are working with PHP < 5.3 then you'll have to use `

declare(ticks=1);

instead of pcntl_signal_dispatch(). You can rfer to the the documentation of pcntl_signal() for that. But if possible you should upgrade to PHP >= 5.3

PHP script that works forever :)

What you're talking about is a daemon or long running program that waits for calls by client programs, performs and action, provides a response then keeps on waiting for more calls.

You might be familiar w/ these in the form of Apache & MySQL ;) Anyway PHP is generally OK in this regard, it does have the ability to function over raw sockets as well as fork sub-processes to handle multiple requests simultaneously.

Having said that PHP daemons are a tool where YMMV. Some folks will say they work great, other folks like me will say they have issues w/ interprocess communication and leaking memory even amidst plethora unset() calls.

Anyway you likely won't be able to deploy a daemon of any type on a shared hosting environment. You'll need to get a better server package or stick with a Cron based solution.

Here's a link about writing a PHP daemon.

Also, one more note. Daemons do crash from time to time and therefore you may still need to store state about whats going on, just in case someone trips over the power cord to your shared server :)

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