what business logic requires it?

# arrow

[pakoito]

what business logic requires it?

[dnowak]

I need somehow pass the Zipkin/Brave distributed logging context. It would be nice to pass Slf4J MDC too. 
KotlinX supports such ThreadLocal management - https://github.com/Kotlin/kotlinx.coroutines/blob/master/integration/kotlinx-coroutines-slf4j/src/MDCContext.kt

[dnowak]

Our project is based on SpringBoot & Arrow. Current case is that message is received from Kafka by Spring Cloud than it is send to IO based logic and finally message goes to Kafka via Spring Cloud again.

[Satyam Agarwal]

If you use suspendCancellable() and use the library you mentioned and add MDCContext() job along with you main coroutine it should work effortlessly.

[Satyam Agarwal]

@simon.vergauwen helped me some days back with the same problem. I’ll try to find the thread

[Satyam Agarwal]

https://kotlinlang.slack.com/archives/C5UPMM0A0/p1580805666269600

[dnowak]

I tried to use KotlinX dispatchers & they work fine - like IO.effect(<http://Dispatchers.IO|Dispatchers.IO> + MDCContext())

[dnowak]

But I’m not sure if it is allowed/safe to use KotlinX dispatchers with Arrow’s IO.

[Satyam Agarwal]

I think it should be safe as long as you are using arrow-fx-kotlinx-coroutines.
even if you aren’t, I don’t see any reason why it wouldn’t be. But take it with sack (not grain) of salt

[pakoito]

so it’s an SLF4J property that stores some Thread-local state in some context

[pakoito]

so the logger reports the traces and their corresponding threads

[pakoito]

or is it to make stacktraces work across thread jumps?

[Satyam Agarwal]

so it’s an SLF4J property that stores some Thread-local state in some context, so the logger reports the traces and their corresponding threads

Yes. That’s what I have understood from the implementation.

[pakoito]

@dnowak your proposal sounds reasonable. The kx.coroutines dispatchers are optimized for their own use, still they're reusable for us because at the bottom of it is just a ContinuationInterceptor that'll do the MDCContext and other thread-changing stuff

[pakoito]

you'll need the integration module if you'd like cancelation to be compatible, as @Satyam Agarwal has suggested

[dnowak]

Looks like I need the TracingContext only when IO is run on other thread then the calling one. We have only a few places like that. Otherwise everything works fine as the thread local is already initialized.

[dnowak]

@pakoito What is the advised way of running IO in current thread?

[pakoito]

jump-back-to-the-thread-that-created-io, or what in Rx was Schedulers.immediate() . I believe we don’t have it, you have to jump back to a specific scheduler.

[pakoito]

make a scheduler off a Threadpool with a single thread, that’s your Main Thread now. And run everything synchronous there.

[pakoito]

If you want to not jump to any new Scheduler, use EmptyCoroutineContext . It’ll still trampoline where needed when using suspended and unsafeRunAsync.

[dnowak]

@pakoito Thank you for all the details 🙂.

[simon.vergauwen]

@pakoito we should be able to easily add immediate, as well as single

[simon.vergauwen]

immediate would just be exposing our trampoline scheduler afaik.

[pakoito]

immediate should stay on the current thread or equivalent, not the one that the execution is at that point. That required installing something on a Thread IIRC

[simon.vergauwen]

should stay on the current thread or equivalent, not the one that the execution is at that point

🤔

[simon.vergauwen]

So the same as trampolining IO, right?

[pakoito]

so

// called from "Main"

IO(dispatchers().io()) { 1 } // IO
  .continueOn(dispatchers().immediate())
  .map { Thread.current().name } // should be "Main"
  .suspended()

if it fulfills that then yes

[simon.vergauwen]

And the reason for main is that immediate() was called there?

[simon.vergauwen]

Or because suspended() was called there?

[pakoito]

immediate

[simon.vergauwen]

Yes, we have that already. That’s function retrieves an immediate dispatcher attached to a ThreadLocal val.

[pakoito]

perfect

[pakoito]

so when people ask, what dispatcher/function should I point them to?

[simon.vergauwen]

It needs to be exposed properly.

https://github.com/arrow-kt/arrow-fx/blob/d06b2b5454c2dbe25ae60ca16cc795960524e24f/arrow-fx/src/main/kotlin/arrow/fx/internal/Utils.kt#L202