This is a daemon MQTT client which runs Lua callbacks on received messages.
It started with a backup graph that did not match the connectivity graph, so some data had to be pushed while other had to be pulled. So I needed machine-to-machine signalling to pull data from a source after it has been pushed completely.
I thought of MQTT for the signalling, and a kind of cron-but-for-MQTT-messages-instead-of-time to run actions. But then I imagined writing a parser for a crontab-like configuration file, and then writing such a configuration file, so I reconsidered my life choices.
It turns out that a Lua script is much easier for development (thanks to existing interpreters), for setup (thanks to a friendlier language), and for maintenance (thanks to basic logic not being scattered across a lot of small shell scripts).