Making a Perl daemon that runs 24/7 and reads from named pipes

Question

I\'m trying to make a log analyser using perl. The analyser would run 24/7 in the background on an AIX server and read from pipes that syslog directs logs to (from the entire ne

Answer 1

I'll cover part of your question: how to write a long-running Perl program that deals with IO.

The most efficient way to write a Perl program that handles many simultaneous IO operations is to use an event loop. This will allow us to write handlers for events, like "a line appeared on the named pipe" or "the email was sent successfully" or "we received SIGINT". Crucially, it will allow us to compose an arbitrary number of these event handlers in one program. This means that you can "multitask" but still easily share state between the tasks.

We'll use the AnyEvent framework. It lets us write event handlers, called watchers, that will work with any event loop that Perl supports. You probably don't care which event loop you use, so this abstraction probably doesn't matter to your application. But it will let us reuse pre-written event handlers available on CPAN; AnyEvent::SMTP to handle email, AnyEvent::Subprocess to interact with child processes, AnyEvent::Handle to deal with the pipes, and so on.

The basic structure of an AnyEvent-based daemon is very simple. You create some watchers, enter the event loop, and ... that's it; the event system does everything else. To get started, let's write a program that will print "Hello" every five seconds.

We start by loading modules:

use strict;
use warnings;
use 5.010;
use AnyEvent;

Then, we'll create a time watcher, or a "timer":

my $t = AnyEvent->timer( after => 0, interval => 5, cb => sub {
    say "Hello";
});

Note that we assign the timer to a variable. This keeps the timer alive as long as $t is in scope. If we said undef $t, then the timer would be cancelled and the callback would never be called.

About callbacks, that's the sub { ... } after cb =>, and that's how we handle events. When an event happens, the callback is invoked. We do our thing, return, and the event loop continues calling other callbacks as necessary. You can do anything you want in callbacks, including cancelling and creating other watchers. Just don't make a blocking call, like system("/bin/sh long running process") or my $line = <$fh> or sleep 10. Anything that blocks must be done by a watcher; otherwise, the event loop won't be able to run other handlers while waiting for that task to complete.

Now that we have a timer, we just need to enter the event loop. Typically, you'll choose an event loop that you want to use, and enter it in the specific way that the event loop's documentation describes. EV is a good one, and you enter it by calling EV::loop(). But, we'll let AnyEvent make the decision about what event loop to use, by writing AnyEvent->condvar->recv. Don't worry what this does; it's an idiom that means "enter the event loop and never return". (You'll see a lot about condition variables, or condvars, as you read about AnyEvent. They are nice for examples in the documentation and in unit tests, but you really don't want to ever use them in your program. If you're using them inside a .pm file, you're doing something very wrong. So just pretend they don't exist for now, and you'll write extremely clean code right from the start. And that'll put you ahead of many CPAN authors!)

So, just for completeness:

AnyEvent->condvar->recv;

If you run that program, it will print "Hello" every five seconds until the universe ends, or, more likely, you kill it with control c. What's neat about this is that you can do other things in those five seconds between printing "Hello", and you do it just by adding more watchers.

So, now onto reading from pipes. AnyEvent makes this very easy with its AnyEvent::Handle module. AnyEvent::Handle can connect to sockets or pipes and will call a callback whenever data is available to read from them. (It can also do non-blocking writes, TLS, and other stuff. But we don't care about that right now.)

First, we need to open a pipe:

use autodie 'open';
open my $fh, '<', '/path/to/pipe';

Then, we wrap it with an AnyEvent::Handle. After creating the Handle object, we'll use it for all operations on this pipe. You can completely forget about $fh, AnyEvent::Handle will handle touching it directly.

my $h = AnyEvent::Handle->new( fh => $fh );

Now we can use $h to read lines from the pipe when they become available:

$h->push_read( line => sub {
    my ($h, $line, $eol) = @_;
    say "Got a line: $line";
});

This will call the callback that prints "Got a line" when the next line becomes available. If you want to continue reading lines, then you need to make the function push itself back onto the read queue, like:

my $handle_line; $handle_line = sub {
    my ($h, $line, $eol) = @_;
    say "Got a line: $line";
    $h->push_read( line => $handle_line );
};
$h->push_read( line => $handle_line );

This will read lines and call $handle_line->() for each line until the file is closed. If you want to stop reading early, that's easy... just don't push_read again in that case. (You don't have to read at the line level; you can ask that your callback be called whenever any bytes become available. But that's more complicated and left as an exercise to the reader.)

So now we can tie this all together into a daemon that handles reading the pipes. What we want to do is: create a handler for lines, open the pipes and handle the lines, and finally set up a signal handler to cleanly exit the program. I recommend taking an OO approach to this problem; make each action ("handle lines from the access log file") a class with a start and stop method, instantiate a bunch of actions, setup a signal handler to cleanly stop the actions, start all the actions, and then enter the event loop. That's a lot of code that's not really related to this problem, so we'll do something simpler. But keep that in mind as you design your program.

#!/usr/bin/env perl
use strict;
use warnings;
use AnyEvent;
use AnyEvent::Handle;
use EV;

use autodie 'open';
use 5.010;

my @handles;

my $abort; $abort = AnyEvent->signal( signal => 'INT', cb => sub {
    say "Exiting.";
    $_->destroy for @handles;
    undef $abort; 
    # all watchers destroyed, event loop will return
});

my $handler; $handler = sub {
    my ($h, $line, $eol) = @_;
    my $name = $h->{name};
    say "$name: $line";
    $h->push_read( line => $handler );
};

for my $file (@ARGV) {
    open my $fh, '<', $file;
    my $h = AnyEvent::Handle->new( fh => $fh );
    $h->{name} = $file;
    $h->push_read( line => $handler );
}

EV::loop;

Now you have a program that reads a line from an arbitrary number of pipes, prints each line received on any pipe (prefixed with the path to the pipe), and exits cleanly when you press Control-C!