Eventhandling idempotency possibly flawed about fun-cqrs HOT 5 CLOSED

I don't get your point.

Events are applied after persisting and internal state is only update after that.

Replay only happens when aggregate is re-instantiated in which case they are not applied twice because whether we come from 0 or from a snapshot. There is no replay happening on a previous state that stays in-memory.

from fun-cqrs.

Please correct me if I'm wrong, but this is how I understand it:

1. AggregateActor receives a Command
2. The applyCommand of the behaviour/interpreter gets called. This executes asynchronously (future)
3. Upon success of the future, you have both the resulting events and updated AggregateState, which you send to yourself in a Successful
4. You receive the Successful, persist the events, and update your state to that updated AggregateState upon a successful persist

Now you want to protect against the events being applied multiple times (not the same as protecting against the same events happening because you received the command twice). This can only happen (very unlikely though) if the Successful you send in 3. arrives twice in 4.

What happens in your code is, you save the events first, and keep the state of that Successful message. If this message happens 2 times, the state will indeed be correct (events only applied once). However, your events are persisted twice. If you now replay the events when "waking up" the Aggregate, the events will be applied twice to the state.

If you want to avoid this, you have to make sure the events aren't persisted either, and have idempotent semantics in your Successful.

from fun-cqrs.

But under which circumstances can the Successful message arrive twice in 4?

from fun-cqrs.

This is impossible, but I understood this was what you were trying to protect against. Most likely my mistake, and misread / misunderstood. Can close. Apologies.

from fun-cqrs.

My intention was to prevent other commands to be processed while we are still persisting the events. Classical become/unbecome thing.

I'm closing this one.

from fun-cqrs.