How to execute blocking calls within a Spring Webflux / Reactor Netty web application

Question

In my use case where I have a Spring Webflux microservice with Reactor Netty, I have the following dependencies:

• org.springframework.boot.spring-boot-starter-webflux (2.0.1.RELEASE)
• org.springframework.boot.spring-boot-starter-data-mongodb-reactive (2.0.1.RELEASE)
• org.projectreactor.reactor-spring (1.0.1.RELEASE)

For a very specific case I need to retrieve some information from my Mongo database, and process this into query parameters send with my reactive WebClient. As the WebClient nor the UriComponentsBuilder accepts a Publisher (Mono / Flux) I used a #block() call to receive the results.

Since reactor-core (version 0.7.6.RELEASE) which has been included in the latest spring-boot-dependencies (version 2.0.1.RELEASE) it is not possible anymore to use: block()/blockFirst()/blockLast() are blocking, which is not supported in thread xxx, see -> https://github.com/reactor/reactor-netty/issues/312

My code snippet:

public Mono<FooBar> getFooBar(Foo foo) {
    MultiValueMap<String, String> parameters = new LinkedMultiValueMap<>();
    parameters.add("size", foo.getSize());
    parameters.addAll("bars", barReactiveCrudRepository.findAllByIdentifierIn(foo.getBarIdentifiers()) // This obviously returns a Flux
        .map(Bar::toString)
        .collectList()
        .block());

    String url = UriComponentsBuilder.fromHttpUrl("https://base-url/")
        .port(8081)
        .path("/foo-bar")
        .queryParams(parameters)
        .build()
        .toString();

    return webClient.get()
        .uri(url)
        .retrieve()
        .bodyToMono(FooBar.class);
}

This worked with spring-boot version 2.0.0.RELEASE, but since the upgrade to version 2.0.1.RELEASE and hence the upgrade from reactor-core to version 0.7.6.RELEASE it is not allowed anymore.

The only real solution I see is to include a block (non-reactive) repository / mongo client as well, but I'm not sure if that is encouraged. Any suggestions?

Answer 1

The WebClient does not accept a Publisher type for its request URL, but nothing prevents you from doing the following:

public Mono<FooBar> getFooBar(Foo foo) {

    Mono<List<String>> bars = barReactiveCrudRepository
        .findAllByIdentifierIn(foo.getBarIdentifiers())
        .map(Bar::toString)
        .collectList();

    Mono<FooBar> foobar = bars.flatMap(b -> {

        MultiValueMap<String, String> parameters = new LinkedMultiValueMap<>();
        parameters.add("size", foo.getSize());
        parameters.addAll("bars", b);

        String url = UriComponentsBuilder.fromHttpUrl("https://base-url/")
            .port(8081)
            .path("/foo-bar")
            .queryParams(parameters)
            .build()
            .toString();

        return webClient.get()
            .uri(url)
            .retrieve()
            .bodyToMono(FooBar.class);
    });
    return foobar;         
}

If anything, this new reactor-core inspection saved you from crashing your whole application with this blocking call in the middle of a WebFlux handler.