Introduce a variation of contextCapture that captures the context immediately

[dstepanov]

Currently, contextCapture is:

a convenience shortcut to capture thread local values during the subscription phase and put them in the Context that is visible upstream of this operator

Personally, I don't see it much useful as the actual subscription might be way outside of the user code.

It would be nice to have something like currentContextCapture as that would capture the current thread-local context and add it to the reactor context, something like:

var newCtx = capturedContextFromThreadLocal();
mono.contextWrite(ctx -> ctx + newCtx);

One of the use cases would be:

// update MDC context
mono.currentContextCapture()
// delete MDC context
// The MDC context is propagate to all the operators

[OlegDokuka]

Hello, @dstepanov!

Can you please provide a certain usecase where contextCapture does not work (test or any reproduceable example with explicit expectations)

Thanks,
Oleh

[dstepanov]

In Micronaut integration I need to capture the context manually https://github.com/micronaut-projects/micronaut-reactor/pull/306/files#diff-6bfb100b108ce8193896caf0431107f8a207233bd1545b7ac94e692a001ea901R38

[OlegDokuka]

@dstepanov, I meant a reproducer that shows a specific scenario where contextCapture does not work as expected (that will help us much better than a link to a pr for an external framework). In your case, it is just a different scenario, which goes beyond the defined capabilities. Also, subscription time happens usually within the same thread as pipeline assembly, hence, all the thread locals should be available (unless you have a certain reproducer that demonstrates different)

Thanks,
Oleh

P.S. As it is demonstrated in the linked PR, contextCapture is the same as contextWrite with minimal integration with the context propagation library, thus technically you can use contextWrite and manually insert required keys. Combining both can do what you need if I get your problem right (or just the pure usage of contextWrite can do it)

[dstepanov]

This is an enhancement request, not a bug report.

The contextCapture works as the JavaDoc describes. I think it should be named contextCaptureOnSubscribe as it is confusing that the capture is postponed and nothing is captured on the method invocation.

My use case differs. I have a try-resources block that modifies the thread-local. This state I want to capture into the Reactor context immediately on some new method invocation. There might be better naming than capture here as I wish to extend it with any possible new thread-local state.

[chemicL]

I agree with the sentiment and I think we could consider contextCaptureNow() as a new operator that captures during assembly time, on the spot, when the method is invoked. It feels more intuitive to capture at the time of assembling. Subscription-time capture is out of the control of the person assembling the pipeline and can be useful when an assembled pipeline is reused for more subscribers, but the regular case would be to assume the ThreadLocal values are present when assembling the pipe. @OlegDokuka WDYT?