Automatic context propagation not working with subscribed sink mono

[mrodal-onlab]

Expected Behavior

Found the issue using spring-statemachine. Somewhere in the statemachine the logging context was getting lost. Debugging I found the part where this was happening, and could extract a small code sample that reproduces the error. The error happens when using Hooks.enableAutomaticContextPropagation(); and it works as expected when not using it.

Actual Behavior

Context is not being propagated when using Hooks.enableAutomaticContextPropagation();

Steps to Reproduce

void reproCase() {
  log.info(MDC.getCopyOfContextMap().toString()); // Logging context found
  var sink = Sinks.one();

  sink.asMono().flatMap(s->{
      log.info(MDC.getCopyOfContextMap().toString()); // Logging context missing
      return Mono.just(1);
  }).subscribe();

  sink.emitValue("", Sinks.EmitFailureHandler.FAIL_FAST);
}

Your Environment

• Reactor version(s) used: 3.6.9
• Other relevant libraries versions (eg. netty, ...): spring boot 3.3.3, micrometer-core 1.13.3, micrometer-tracing 1.3.3
• JVM version (java -version): java 17
• OS and version (eg uname -a): Windows 10

[chemicL]

Can you share how you are populating the MDC and what you expect would propagate the MDC?

[mrodal]

Im not populating the MDC, it is being populated by micrometer I believe. Heres a sample application: https://github.com/mrodal/issue-context-propagation

Its just a controller with the code above and the application starter with the Hooks.enableAutomaticContextPropagation();

[chemicL]

Ok. I'm not sure what exactly you are trying to achieve, but you are creating a disconnected pipeline when you call .subscribe(). Essentially, all that is present in the MDC is coming from Context associated with the incoming request. The gist is - you return some Flux/Mono in your controllers to which the framework subscribes. A Subscriber carries a Context that represents an Observation associated with that request processing. Micrometer-Tracing takes care of populating the MDC with trace-id/span-id metadata taken from the Observation that is present in the Context. Once you create another Flux/Mono which does not form a reactive chain with the existing Subscriber then it contains an empty Context that does not have that Observation. This fire-and-forget pattern can lead to such issues. Consider asking a question on Gitter or StackOverflow how to best structure your code for the actual use case that you have and make sure you consult our reference documentation on this.

[mrodal]

The example code is a simplification of the spring-statemachine library code. I'll check over there if they can find a workaround Thank you