Pass a context from one flux/mono to another

Gie Spaepen :

Interesting things happen inside the webflux package. However, my journey in the source didn't solve the following question.

Let's say I have the following mono (or flux):

Mono hello = Mono.empty()
            .subscriberContext(ctx -> ctx.put("message", "hello"));

I use similar construct within a webfilter to enrich the pipeline with tenant and user data. Then in a controller a construct like this is used:

Mono world = Mono.subscriberContext()
            .map(ctx -> (String)ctx.get("message"));

The context of the hello mono is filled in the world mono. I tried to figure out how this is done, also for unit testing purposes.

In the end it remains a riddle. I tried to do this with the regular methods available on both mono/flux objects but I don't succeed in making the hello context available to the world mono. How can you fuse fluxes and monos and passing the context along the way to the upstream operators?

Simon Baslé :

WebFlux takes your world Mono and builds a reactive chain on top of it, with an HTTP request within reactor-netty as the ultimate source. The WebFilter are part of the chain construction, so they can enrich the Context of the whole chain.

IIRC Mono.subscriberContext() will be used within a flatMap, which makes the main sequence Context available to its inners, so it can see the Context of hello.