Event loops and Jenga; or 24 Advent Calendar Events in One Go

There are many event loops systems in Perl. Do they play together?

I was thinking about this recently, at my LPW2010 talk about IO::Async. In the hackathon the following day, I managed to write IO::Async::Loop::POE; a way to run IO::Async atop POE.

So I started thinking further; if you can run one event loop system on top of another, how high can we stack them? Can we build a tower, putting each atop the previous, growing taller. Each new layer we try to add would start to get harder, more difficult, increasing the chances the whole thing came crashing down. Sortof like a Jenga tower.

So what would a Perl event loop Jenga tower look like?

My attempt looks like this: (326 lines, jenga.pl)

The output looks something like this:

$ perl jenga.pl
AnyEvent resolved 127.0.0.1:80
Glib reads Hello world!
POE reads Hello world!
POE resolved 127.0.0.1:80
IO::Async reads Hello world!
AnyEvent reads Hello world!
AnyEvent listener accepted
POE listener accepted
IO::Async resolved 127.0.0.1:80
AnyEvent connected received
POE connected received
IO::Async listener accepted
IO::Async connected received
Glib child exited 0
POE child exited 0
IO::Async child exited 0
AnyEvent child exited 0
Glib timer
POE timer
IO::Async timer
AnyEvent timer
^CIO::Async SIGINT
AnyEvent SIGINT
POE SIGINT
Stopping...

That's 24 events. Count them. It combines Glib, POE, IO::Async and AnyEvent. It performs a basic filehandle read, a child process watch, and a timed wait in each of these four systems. Because Glib lacks signal watching, only the other three perform this. The other three are also used to perform name resolution, socket listening, and socket connecting.

Everyone seems to be doing Advent Calendar blogs this year. 24 daily posts, each showing one small thing. Someone suggested I should write a Perl Event systems advent calendar. So perhaps here, consider this to be one. Except it has 24 windows all in one go.

As it turns out, it's possible to make this tower a little higher. There's a module to run Event beneath Glib; that is, it replaces the core polling function of Glib to use Event instead. And I suspect it may just about be possible to run Tk on Glib, and the POE on Tk.

At some point in the new year, I have some plans to turn this one-program script into a more useful resource of examples and translations. The Rosetta Stone for Unix provides a cross-reference for looking up Unix concepts between different systems. I feel that a similar attempt at Perl event loops could be quite useful too.