Webflux: Add Subscriber Context Values from In-Stream Objects - spring-webflux

Question

I have a Webflux application and I'm using the subscriber context to populate the MDC values so that they'll propagate downstream. I've implemented the classes from this project to handle the MDC transfer between subscribers, setup a webfilter to add a request ID to incoming requests, and can see the request ID in the logs as part of the MDC.
public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
return chain.filter(exchange)
.contextWrite(Context.of("requestId", UUID.randomUUID().toString()));
}
Log output:
2021-09-30 17:15:29,963 [reactor-tcp-nio-2] INFO c.p.l.s.RepoService:33 - MDC[requestId=ec0b68cf-ba4d-4c7f-afa4-f67fc97ebcbf] - Found user
The request needs to pull a user record from the database and I need to add the user's ID and email to the context once the data has been fetched, but I can't figure out how to add values to the context from objects within the stream.
So here's what I'm trying to do:
public Mono<Response> doProcessing(String userId, Object object) {
return userRepo.findUserById(userId) //Returns UserEntity object
.flatMap(userEntity -> service.doMoreProcessing(userEntity, object))
.contextWrite(Context.of("userId", userId, "email", userEntity.getEmail()));
}
The problem is that I don't have access to the userEntity object when I call contextWrite(), so I can't add the user's email to the context using that call.
I've tried using Mono.deferredContextual()...
public Mono<Response> doProcessing(String userId, Object object) {
return userRepo.findUserById(userId) //Returns UserEntity object
.flatMap(userEntity -> Mono.deferContextual(ctx -> Mono.just(userEntity).contextWrite(Context.of(ctx).putAll(Context.of("userId", userId, "email", userEntity.getEmail()).readOnly()))))
.flatMap(userEntity -> service.doMoreProcessing(userEntity, object));
}
...and .transformDeferredContextual()...
public Mono<Response> doProcessing(String userId, Object object) {
return userRepo.findUserById(userId) //Returns UserEntity object
.transformDeferredContextual((userEntityMono, contextView) -> userEntityMono.flatMap(userEntity -> Mono.just(userEntity).contextWrite(Context.of(contextView).putAll(Context.of("userId", userId, "email", userEntity.getEmail()).readOnly()))))
.flatMap(userEntity -> service.doMoreProcessing(userEntity, object));
}
...but nothing has worked. How can I add data to the subscriber context from objects that are part of the process stream?

Answer 1

The context can store a mutable state object like a ConcurrentMap or better yet a #Data-annotated class that could be accessed and modified on the chain (iff, you can guarantee you're not modifying the same field in the chain).
To get the data you need to obtain it from the signal using doOnEach, as doOnNext does not provide the signal value where you can getContextView()
You can try something like this
public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
return chain.filter(exchange)
.contextWrite(Context.of("your-key", new ConcurrentHashMap<String, Object>()));
}
public Mono<Response> doProcessing(String userId, Object object) {
return userRepo.findUserById(userId)
.doOnEach(signal -> {
if (!signal.isOnNext()) return;
Optional<Map<String, Object>> optional = signal.getContextView().getOrEmpty("your-key");
optional.ifPresent(map -> {
map.put("userId", signal.get().getId());
map.put("email", signal.get().getEmail());
});
});
}