Nextcloud Flow
Nextcloud Flow is the workflow engine of Nextcloud. It offers flexible, user-defined event-based task automation. Apps can register events or triggers which can start a flow, as well as actions which get executed once a trigger is hit and the constraints are satisfied.
At 36c3 blizzz gave a talk explaining Flow and how to write actions and triggers. You can find the slides of their talk here.
Contributions to this documentation are, as always, very welcome!