Re: PSGI, Plack, Twiggy, AnyEvent and SockJS... I need help

[kcott]

G'day xtpu2,

[Disclaimer: I haven't used AnyEvent previously. The following information was gleaned from its (and AnyEvent::Intro's) documentation.]

"It is obvious that $w->recv is the culprit because if I comment out that line, the server works and outputs 'got your message' when I send a message to it. However, in that case the timer doesn't work."

This may only be part of the story, but it might help resolve the AnyEvent issue.

You're calling recv() on a time watcher when it should be called on a condition variable. Also, you'll need a send() method in your timer callback.

Here's a minimal example intended to roughly mirror the AnyEvent part of the code you posted:

#!/usr/bin/env perl -l

use strict;
use warnings;

use AnyEvent;
use AnyEvent::Strict;

print 'Start at: ', scalar localtime;

my $w_cond = AnyEvent->condvar;

my $w = AnyEvent->timer(
    after => 5,
    cb => sub {
        print 'Callback: ', scalar localtime;
        $w_cond->send;
    }   
);  

$w_cond->recv;

Output:

Start at: Wed Apr  9 12:06:50 2014
Callback: Wed Apr  9 12:06:55 2014

[xtpu2]

Thanks for those tips! I have modified the code accordingly and modified the OP. Unfortunately, the behavior of my code is the same (while your code snippet works correctly).

[andal]

I guess, the module you create is already called from some event-driven system. In other words, your module is a "callback". The "callback" can't run it's own event distribution. You can register one more "callback" with the main system and then simply return control to the caller. The main system is then responsible for activating your new callback at appropriate time.

The AnyEvent module is event-distribution system. When you call $w->recv you activate this system, which blocks the main one, which in turn might trigger some time-out for killing your callback. Figure out how to add timers to the main framework. I don't know Plack and don't have any desire to study it, so I can't help you with this, sorry.

At the core, event-driven programming is very simple. The main part is manager looping over all possible events. When event is encountered, the manager checks if there are any "callbacks" waiting for this event. Those callbacks are activated and the looping continues. The callbacks must finish their work quickly and if necessary, they can register more callbacks for different events.

Everything else, is specific ways, how different managers keep track of callbacks and events.

[Corion]

At least from looking at the SockJS documentation, it is based on ( or compatible with) AnyEvent.

[xtpu2]

Wow, I think you've nailed it! After reading your post, I started digging into that and came across this, discussing this exact issue:

https://github.com/miyagawa/Twiggy/issues/33

I'm posting the modified code here so as not to make the OP confusing. The new code no longer closes the connection immediately, but the timer, for some reason only fires ONCE (the first time), whereas I would like it to keep going. I'm guessing this has something to do with the line undef $w; but when I get rid of it, the timer doesn't fire at all.

#!/usr/bin/perl

use strict;
use warnings;

use Plack::Builder;
use SockJS;
use AnyEvent;

builder {
    mount '/echo' => SockJS->new(
        handler => sub {
            my ($session) = @_;

            $session->on(
                'data' => sub {
                    my $session = shift;

                    $session->write('got your message');
                }
            );

            $session->write('connected');

            my $w_cond = AnyEvent->condvar;
            $w_cond->cb( sub { warn $_[0]->recv } );
            my $w; $w = AnyEvent->timer(
                after => 0,
                interval => 5,
                cb => sub {
                    undef $w; # cleanup
                    $session->write('5 seconds have passed');
                    $w_cond->send();
                }
            );
        };
    );
};