brmd: A Case for POE
In brmlab, we want to track who is unlocking the space, whether someone is inside, have some good visual indicator that live stream is on air, and so on. In other words, we have an Arduino with some further hardware, and we want to show whatever is reported by the Arduino on IRC and web, and provide some web-based control (open/closed status override) in the opposite direction too.
What to use for a service (we call it brmd) that will bind all these interfaces together? It just needs a lot of boring frontends and simple state maintenance. It turns out that Perl’s POE framework is ideal for this – most of the code for IRC, HTTP and device read/write is already there, so you just grab the modules, slam them together and you have exactly what you need with minimal effort. Right?
It turns out that there are caveats – basically, the idea is correct, aside of getting stuck on a single stupidity of mine, I’d have the whole thing put together in something like half an hour. Unfortunately, the moment you want robustness too, things are getting a lot more complex; to handle the device disappearing, IRC disconnections, not having HTTP socket fds leak away, etc., you suddenly need to either magically know what further modules to hook up or start exeting some manual effort. Still, I like how POE is making it so easy to give a simple state machine many input/output interfaces and when you get used to the idiosyncracies, you can even make it somewhat reliable.
Example POE code
While this task seems to be ideal fit for POE, I’ve found surprisingly few examples of more complex POE component interaction on the web. Therefore, I’m going to lay out at least tersed up version of brmd below to help fellow future googlers. Nothing of this is anything ground-breaking, but it should help a lot to have a template to go by. Our real version is somewhat more verbose and includes some more interfaces: brmdoor.git:brmd/brmd.pl
I assume that you already know what “POE kernel” and “POE session” is. Beware that I’m a POE beginner myself, and I haven’t gone through much effort to clean the code up the way I would if I were to work together with someone else on this. Some things surely aren’t solved optimally and you might even pick up a bad habit or two.
In order to have some neat separation, we will divide brmd to several components where each will take care of a single interface; the main package will only spawn them up and do some most basic coordination. If we were to grow much larger, it would be worth the effort to even set up some kind of message bus (I wish POE would provide that implicitly), here we just directly signal components needing the info.
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