Spring Cloud Gateway chaining nested Mono - spring-webflux

Question

I have a web app (Spring Cloud Gateway with Project Reactor) where I have to logout (send another http request) when something goes wrong and set 401 to the main response. The problem is when I execute another request in onErrorResume block, the root response seems to ignore finishWithStatus() logic entirely and returns 200.
#Override
public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
return someFunctionWhichReturnsMono()
.flatMap(chain::filter)
.onErrorResume(e -> {
log.error("Unexpected Filter Error, logging out user", e);
// A. this doesn't set 401, seems like ignoring finishWithStatus(..)
// called inside this method in onErrorResume block
return logout(exchange);
// B. this works fine and I get 401 as a response
// return finishWithStatus(exchange, HttpStatus.UNAUTHORIZED);
});
}
protected Mono<Void> finishWithStatus(ServerWebExchange exchange, HttpStatus status) {
exchange.getResponse().setStatusCode(status);
return exchange.getResponse().setComplete();
}
protected void logout(ServerWebExchange exchange) {
webClient
.post()
.uri(....)
.retrieve()
.bodyToMono(Void.class)
.doOnSuccess(any -> {
log.info("Successfully logged out user");
})
.then(finishWithStatus(exchange, HttpStatus.UNAUTHORIZED))
.onErrorResume(e -> {
log.error("Failed to logout user", e);
//the following line has no effect when error happens
return finishWithStatus(exchange, HttpStatus.UNAUTHORIZED);
});
}
Could somebody explain why is that despite I return Mono in both cases. However, in case A I have nested onErrorResume (at onErrorResume of the "root" Mono I create another Mono with its own onErrorResume).
I feel I miss something fundamental like I need to "join" two Monos or say bubble up some Mono.error from the deepest onErrorResume to the top one?
What would be a generic approach to handle nested errors (like the case above when on an error you have to send another request which in turn might end up with an error).
I would greatly appreciate any advice or sample on this matter.

Answer 1

Fundamental rule in reactive programming: Nothing happens until you subscribe
finishWithStatus() works because you are passing the publisher value to the next filter by returning Mono<Void> type.
You are making chain of reactive operators in logout() method but never subscribing or passing the publisher to the next filter. Therefore nothing happens even if you call this method
Another point is you can't return logout() method in filter() method since the return type for filter() is Mono<Void> not void.
You should change the return type for logout() method:
protected Mono<Void> logout(ServerWebExchange exchange) {
return webClient
.post()
.uri(...)
.retrieve()
.bodyToMono(Void.class)
.doOnSuccess(any -> {
log.info("Successfully logged out user");
})
.then(finishWithStatus(exchange, HttpStatus.UNAUTHORIZED))
.onErrorResume(e -> {
log.error("Failed to logout user", e);
return finishWithStatus(exchange, HttpStatus.UNAUTHORIZED);
});
}