concurrency: support context propagator SPI

[eclipse]

concurrency: support context propagator SPI

[njr-11]

The ContextState.install/restore API from Reactive Contexts (linked above) worries me a bit because I could see it leading to the possibility of applications leaking context, but I don't think we need to go that far in order to enable what you are wanting to do. It ought to be possible to accomplish as much with just the ThreadContextProvider SPI. What you are describing seems very useful and I agree is a good integration point to provide, but I wonder if it would better be done as separate from the mp-concurrency spec in order to avoid coupling to a specific version of RxJavaHooks that might possibly change over time?

[FroMage]

I don't think it's specifically related to install/restore because your proposal with begin/end is the exactly same (it uses different method names and intermediate types but serves exactly the same purpose and has no additional benefit nor drawback).

What this propagator SPI buys us is that:

• We can have optional plugins for each propagator type
• The mp-concurrency would load/set them up, so the user would be sure that context propagation is whole when loaded
• It'd be as easy as including them in the classpath to get contexts propagated

But the spec should not depend on RxJava, nor mandate that SPI plugins for RxJava exist. Its implementation would take care of that.

[njr-11]

Thanks for clarifying about the implementations, not the spec, providing the plugins. That makes much more sense. Can you elaborate more on the specifics, maybe by walking through an example of what mp-concurrency would define the propagator SPI as and how it would be implemented? When I looked at the RxJava link, it seems to already have a plugin point to the ServiceLoader, META-INF/services/io.reactiverse.reactivecontexts.core.ContextPropagator, that is defined by RxJava. So I assume the plugin would be an implementation of this RxJava interface (and utilizing ThreadContextProvider SPI) rather than of a propagator interface defined by mp-concurrency. I might be missing something here, but I don't see what change would be made to enable this other than calling this out/recommending it for implementations.

[FroMage]

Moved to eclipse/microprofile-context-propagation#7