multiple writer services with Idempotency guarntees?

EventStoreDB
1

If I have process stream for some workflow, like application-{app-guid} and i have multiple microservices watching for certain events that then write new events to that same application-{app-guid}, say, an event comes in and sends two services off to query two APIS and then write back to the stream with the data, doesn’t this necessarily create race conditions if I want to guarantee idempotency? Each stream will check the version and write +1.

Do I need to maintain some sort of threadsafe global stream version state or is there a fundamental design problem with this situation?

2

set expectedversion and use a deterministic uuid. idempotency is assured then.

3

I have dterministic uuids. Do I understand that the expected version will always be current version + 1? If that’s the case and I have two incoming writes, what happens if one writes after the other reads?

4

Then you get a concurrency issue as you would expect with optimistic concurrency.

5

Right, so is the answer to only have one writing service per stream? Or is this just ignored is an unlikely event (assuming mean event frequency) in practice?

6

If they both generate deterministic uuids they are idempotent.

7

Ok, thanks.