Simultaneously streaming request / response bodies with the Spring Reactive WebClient?

Question

I am seeing some strange behaviour with the following client-server interaction and I am wondering whether I am running into HTTP/1.1 semantics or my Reactive programming skills need work (or both).

I am attempting to create a client-server interaction where both the request and response bodies are long-running streams of data.

The client is a Spring Reactive WebClient that sends an infinite stream in the request body. It expects to receive (and log) an infinite stream of results.

Flux<Long> requests = Flux.interval(Duration.ofSeconds(2));

return WebClient.create()
        .post()
            .uri("/instructions")
            .contentType(MediaType.APPLICATION_STREAM_JSON)
            .body(requests, Long.class)
        .retrieve()
            .bodyToFlux(Object.class)
            .map(response -> {
                log.info("Received Response Object {}", response);
                return response;
            });

The server is a spring-boot-starter-webflux application with a route handler to log the request objects as they are received and provide an infinite stream of results:

public Mono<ServerResponse> instructions(ServerRequest request) {
    // Log the request objects as they are received
    Flux<Object> requestStream = request.bodyToFlux(Object.class)
            .map(r -> {
                log.info("Received Request Object: {}", r);
                return r;
            });

    requestStream.subscribe();

    // Infinite stream of responses
    Flux<Long> responses = Flux.interval(Duration.ofSeconds(5));

    return ServerResponse.ok()
            .contentType(MediaType.APPLICATION_STREAM_JSON)
            .body(responses, Long.class);
}

When the above code is run, the server logs the infinite stream of request objects but the client never logs any response objects.

If I bound the request stream by doing something like so: Flux<Long> requests = Flux.interval(Duration.ofSeconds(2)).take(20); then the client begins to log the responses after all requests are received.

What are the problems here? * Is there something wrong with the reactive code? * Is this part of the HTTP/1.1 specification where a response header should not be sent until the request body is completely received?

I don't think there is anything wrong with the spec, but I've tried similar things, and some (many?) HTTP servers and clients don't deal with simultaneously streaming request and response objects very well. — Frank Lee

Larik Borts Larik Borts · Accepted Answer · 2020-11-08T20:33:42

See this question: WebClient doesn't read response until request write is completed.

Apparently, Netty-based WebClient starts processing the response only after it is finished sending the request

Netty server might also behave similarly (only start sending the response after it is finished reading the request body), I'm not sure

Interestingly, Jetty-based WebClient is able to process request and response simultaneously

Simultaneously streaming request / response bodies with the Spring Reactive WebClient?

1 Answers