Software architecture

Limnoria abstracts its internal architecture away from plugins through its plugin API, which is enough most of the time. However, you may need want to understand its internal architecture, either to debug complex problems, provide advanced features that hook into the internals, contribute to the core, or simply out of curiosity.

This guide will try to walk you through this, assuming you are already familiar with using the bot and writing plugins (if not, see the Capabilities documentation, the Writing Your First Limnoria Plugin, and Special methods and catching events,

You should also be somewhat familiar with the IRC protocol <>.


This document is a work in progress and is still incomplete. As usual, feel free to ask any questions in #limnoria @ freenode.

Main loop and drivers

The main event loop is a very classic synchronous loop. It is defined in scripts/supybot, and essentially just this:

while world.ircs:
    except KeyboardInterrupt:
        # Handle Ctrl-C (trigger shutdown)
        # Handle other unhandled errors

Where does this:

for (name, driver) in _drivers.items():
     if name not in _deadDrivers:
for name in _deadDrivers:
     # Remove the driver
while _newDrivers:
     (name, driver) = _newDrivers.pop()
     # add the new driver

Drivers are the sources of events in the main thread. In a normal Limnoria setup, there are two types of drivers: the Socket driver (which connects synchronously to IRC) and the schedule driver (which runs functions periodically, like cron). Historically, there was an alternative driver to connect to IRC, based on Twisted. It was deprecated, then removed in 2019, because Python’s socket module became as powerful as Twisted as it gained support for select() and TLS.

Network drivers have a reference to a irclib.Irc object, and do three things in their run() method:

  1. check the connection is still alive (and schedule a reconnect if not)
  2. get new messages from their irclib.Irc instance (using irclib.Irc.takeMsg()) and send them to the network
  3. get new messages from the network and pass them to their irclib.Irc (using irclib.Irc.feedMsg())

The actual implementation of the current Socket driver is actually a little more complex than this, as all Socket driver instances cooperate to use select() together, but this is the rough idea. See src/drivers/ for details.


As we saw above, network drivers pass their messages to a class defined in irclib, which is where most of the IRC protocol implementation is.

Unlike most event-driven software (especially IRC implementation), Limnoria does not have hooks that are registered to call a function when a specific event/IRC command is received. Instead, event listeners receive all events, and inherit on supybot.irclib.IrcCommandDispatcher, which calls a specific method based on the IRC command name. For example, it calls the doTopic method when receiving a TOPIC message.

This dispatching is used both in the main IRC handling (supybot.irclib.Irc) and plugins (via supybot.callbacks.PluginMixin, which inherits supybot.irclib.IrcCommandDispatcher).

We saw above that the supybot.irclib.Irc object receives messages directly from the driver. It’s also in charge of keeping track of other callbacks (ie. plugins) via supybot.irclib.Irc.addCallback() and passing every message to their __call__ method (which then does the dispatching on its own again, as it inherits supybot.irclib.IrcCommandDispatcher).

As there are few callbacks (under a hundred plugins), this simple architecture is efficient enough.

Additionally, when receiving a message and before sending one, it iterates through the list of plugins and calls their inFilter and outFilter methods (respectively), if any.

If you look at the code of supybot.irclib.Irc and supybot.irclib.IrcState, you see they are mostly made of doXxx methods, which exhaustively implement every known IRC command, update some state, and optionally react to it by queuing messages.


Next is the callbacks system, mostly implemented in :mod:supybot.callbacks. This is where all the magic happens to make plugins so easy to write; it’s also the most complex part of Limnoria and the hardest to understand, because everything is tightly interleaved.