And if you're using an immutable log (Kafka/Pulsar/Liftbridge etc.) fixing those mistakes is hard.

And given you're basically recording interim states of a state machine, how do you verify that event Z being submitted after event Y is a valid state transition?

Validating a record against a schema is trivial. But validating three consecutive events against the state machine? I'm not aware of a good way to do that cheaply.

I've done it before using apps that sat in the data pipeline, and maintained state to reject invalid transitions, but that can end up being GiB of state, then you have to persist that across restarts or horizontal scaling.

It's all doable, but it costs. Is the event streaming paradigm actually worth the cost? I haven't been convinced yet.

But, I'm a big fan of streaming DB change events for auditing purposes - but those are coming from software that already enforces valid state changes.

So maybe the answer is - every event has to be submitted via Postgres.