Hi there. I wonder if there is anything in Flow AP...

# coroutines

[Jakub Gwóźdź]

Hi there. I wonder if there is anything in Flow API that would allow passing some correlation_id in logger’s MDC along the items?
Suppose I have my fun emitter() = flow {... } , some .onEach {} calls and .collect() at the end and I’d like to have consistent correlation_id in logs in MDC for that.
Problem is, I cannot use

private fun emitter() = flow {
        (1..3).forEach {
            withContext(MDCContext(mapOf("correlation_id" to UUID.randomUUID().toString()))) {
                logger.info { "Emitting $it" }
                emit(it)
                logger.info { "Emitted $it" }
            }
        }
    }

because, well, it’s rightfully forbidden to emit in different context than the collecting. .flowOn(…) also won’t solve the problem as it is one context per flow, not per item.
Best I can do is to emit Pair<UUID, Int> instead of just Int and unpack it on each step, but maybe there is better solution in API?

[Michael Strasser]

Hi @Jakub Gwóźdź I haven’t tried it with Flows, but maybe the Klogging library might work.

Klogging works with Spring Boot and Ktor as a drop-in replacement for Logback for existing logging. After that you can use it directly in suspend functions to store context information like correlation IDs. You store context information in coroutine scope that is inherited by child scopes.

(I am the creator of Klogging.)

[taer]

We've run into the same thing. No solution yet. 🙂

[Jakub Gwóźdź]

I ended up with passing the correlation id along the item, regular way 🙂

[taer]

yeah. we kinda did the same. Increase the payload and manually set the MDC directly

[taer]

if your operations are simple enough, setting the MDC direct works. problem is if you don't then you end up w/ sometimes old stale things. Manual was the only way we could pull it off

[Jakub Gwóźdź]

Well, I don't set mdc directly, as it is lost during coroutines context switching. But there is mdccontext somewhere in coroutines api that takes care of it.

[taer]

yeah.. I think you need to be slightly careful. The withContext using the MDCContext(mapOfThingsToSet) will only apply the MDC things given on a switch. I'm not sure it takes effect immediately.