2014/03/13

Event Registration in Event-Reflexive Programming

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Continuing our recent theme of an IRC bot, the next step in the story concerns the suggestion someone once made to me that given the way most of the internals of this bot worked, it shouldn't be too hard to have these events broadcast over some kind of IPC socket or similar, between multiple processes, to allow parts of the bot to be written out-of-process. Indeed, given the right kind of serialisation, there's no reason these extra parts had to even be perl, they could be implemented in a different language.

This idea has come up twice now in two different concepts, so I decided to think about it in some more detail. In principle the idea is sound enough, but as ever the devil comes down to the details. If every event was serialised and broadcast to every listening process, the IPC overheads could get very large, because most of the time most of the processes would ignore it. The simple form of event-reflexivity we have been using up to now has relied heavily on the very cheap (virtually free) cost of introspection within the code of one process, but now we need to find a better way to implement it.

The obvious way to start this is some kind of registration system. When each process connects to the central core, it starts off telling the core which events it is interested in, perhaps in a set of strings, or regexp matches, or something. This is a good first step in cutting down plenty of unwanted noise over the serialisation links, and generally improves things. This filtering doesn't have to be perfect as each connected process can still state it isn't interested in specific events it still manages to receive, but anything we can do on the core side to cut that down will obviously help.

However, further consideration of the specific domain of interest in being an IRC bot starts to suggest we can do something more powerful. Within IRC, it's quite likely that most events of interest to plugged-in processes will concern some specific IRC channel or user. It's also quite likely that at least some plugins may be interested only in events on specific channels or users, or matching only specific text, or some other criteria. If we could get the core event distribution mechanism to filter on these as well, we can further cut down on pointless IPC overheads.

The full implications and decisions of how this might work aren't really related to event-reflexivity, but what is of interest here is that this kind of event registration system doesn't have to be only for out-of-process management. In fact, as soon as we start to consider how event-reflexive programming might be implemented in a static language like C, as compared a dynamic language like Perl, we fairly soon conclude that there must be some kind of registration call, to hook up pieces of code and help in the event dispatch process on some level or another.

This leads us on to in fact two questions this time:

How useful is it to implement event reflexivity using explicit registrations of interest in events?
Does the answer depend on whether the language is static or dynamic? Can explicit registrations provide useful abilities even in dynamic languages, or they just add unnecessary complication

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