Context propagation isn't trivial on a multi-threaded async runtime. There are several ways to do it, but JVM agents that instrument bytecode are popular because they work transparently.
While that’s true, if you’ve already solved punching correlation-IDs and A/B testing (feature flags per request) through then you can use the same solution for all three. In fact you really should.
Ours was old so based on domain <dry heaving sounds>, but by the time I left the project there were just a few places left where anyone touched raw domains directly and you could switch to AsyncLocalStorage in a reasonable amount of time.
The simplest thing that could work is to pass the original request or response context everywhere but that… has its own struggles. It’s hell on your function signatures (so I sympathize with my predecessors not doing that but goddamn) and you really don’t want an entire sequence diagram being able to fire the response. That’s equivalent to having a function with 100 return statements in it.
