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Re^3: PSGI, Plack, Twiggy, AnyEvent and SockJS... I need help

by andal (Friar)
on Apr 09, 2014 at 07:40 UTC ( #1081615=note: print w/ replies, xml ) Need Help??


in reply to Re^2: PSGI, Plack, Twiggy, AnyEvent and SockJS... I need help
in thread PSGI, Plack, Twiggy, AnyEvent and SockJS... I need help

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.


Comment on Re^3: PSGI, Plack, Twiggy, AnyEvent and SockJS... I need help
Re^4: PSGI, Plack, Twiggy, AnyEvent and SockJS... I need help
by Corion (Pope) on Apr 09, 2014 at 07:53 UTC

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

Re^4: PSGI, Plack, Twiggy, AnyEvent and SockJS... I need help
by xtpu2 (Acolyte) on Apr 09, 2014 at 14:39 UTC

    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(); } ); }; ); };

      You need to mention the timer variable $w inside the subroutine to keep it alive.

      Either move it into a global variable or just mention it in the subroutine so it closes over the value. Even better would be to clean up the timer based on some condition.

        So this is the final version of the timer code that works:

        ... 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 if (0); $session->write('5 seconds have passed'); $w_cond->send(); } ); ...

        I changed undef $w; to undef $w if (0);. Now the big question is why does this work, but simply commenting out undef $w; causes the timer to not work? Is there a logical reason here, or is it voodoo? By "voodoo" I mean, code that has to be typed a certain way "just because".

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