I'm working on an application that uses SLF4J and ...

# coroutines

[Klitos Kyriacou]

I'm working on an application that uses SLF4J and MDC. In one place, there is some code like this:

MDC.put(some context...)
withContext(<http://Dispatchers.IO|Dispatchers.IO> + MDCContext()) {
   ...some asynchronous processing...
}
// At this point MDC is clear, the original context has disappeared!

After the withContext call, the entries in the MDC have all been removed and the context map is empty. Does anyone have any clues how I can go about figuring out how/why/where this context is getting zapped?

[Sam]

Since withContext changes dispatchers, there's no guarantee that line 5 is running on the same thread as line 1. You'd need the MDCContext installed higher up if you want to rely on its contents outside the withContext block.

[Klitos Kyriacou]

Ah, I see, thank you Sam

[Klitos Kyriacou]

Looking at the logs from before and after the withContext call, it seems to be the same thread (at least same ID) on lines 1 and 5. Is there any other way the context can disappear? (I'll change this anyway, to make sure the context is saved higher up.)

[Sam]

Perhaps MDCContext is already installed in the context higher up? Since withContext is a suspension point, it would trigger the MDCContext to restore its original saved state when withContext returns.

[Sam]

There's a comparable example in the docs showing how the context disappears after calling delay().

[Klitos Kyriacou]

Thanks Sam, it is indeed being done higher up the call tree.

[Sam]

I wish there was an easier way to integrate coroutines with thread locals 😞 this stuff hurts my brain every single time I have to think about it