Real-World Event Sourcing

Evolving Process Managers

It should be second nature by now to remember that process managers take in events and emit commands that move a process forward. Since commands are ephemeral, the only evidence you have of a process’s completion (or lack thereof) is the events that correspond to that process. Unprocessed or rejected commands leave no evidence behind (except maybe in logging/tracing, but that’s another topic).

Process managers rarely evolve in isolation. If a process manager adds a new step to a process, that typically produces a new command which means an aggregate needs to be updated to handle that command. This in turn might produce a new event that the aggregate needs to handle to update internal state, and a projector might also ...