Springboot v2.0.0.M6 WebClient making multiple duplicate HTTP POST calls

I happend to have experienced similar problem:

When calling the same service (here marked as ExternalService) in parallel (webflux) sometimes the same request was being sent and the problem indeed seems to reside in Webclient.

The solution was in changing the way Webclient is being created.

Before:

Here ExternalCall is configured client which calls ExternalService. So the parallel execution here was of internalCall method. Note we pass WebClient.RequestBodySpec class to ExternalCall class

@Bean
ExternalCall externalCall(WebClient.Builder webClientBuilder) {

    var exchangeStrategies = getExchangeStrategies();

    var endpoint = "v1/data";
    var timeout = 10000;
    var uri = "https://externalService.com/";

    var requestBodySpec = webClientBuilder.clone()
            .clientConnector(new ReactorClientHttpConnector(HttpClient.create()))
            .exchangeStrategies(exchangeStrategies)
            .build()
            .post()
            .uri(endpoint)
            .accept(TEXT_XML)
            .contentType(TEXT_XML);

    return new ExternalCall(requestBodySpec, uri, timeout);
}

Then within ExternalCall class I had had

private Mono<String> internalCall(String rq) {
    return requestBodySpec.bodyValue(rq)
            .retrieve()
            .bodyToMono(String.class)
            .timeout(timeout, Mono.error(() -> new TimeoutException(String.format("%s - timeout after %s seconds", "ExternalService", timeout.getSeconds()))));
}

After:

We pass WebClient class to ExternalCall.

@Bean
ExternalCall externalCall(WebClient.Builder webClientBuilder) {

    var exchangeStrategies = getExchangeStrategies();

    var timeout = 10000;
    var uri = "https://externalService.com/";

    var webClient = webClientBuilder.clone()
            .clientConnector(new ReactorClientHttpConnector(HttpClient.create()))
            .exchangeStrategies(exchangeStrategies)
            .build();


    return new ExternalCall(webClient, uri, timeout);
}

Now we specify RequestBodySpec within ExternalCall class:

private Mono<String> internalCall(String rq) {
    return webClient
            .post()
            .uri(endpoint)
            .accept(TEXT_XML)
            .contentType(TEXT_XML)
            .bodyValue(rq)
            .retrieve()
            .bodyToMono(String.class)
            .timeout(timeout, Mono.error(() -> new TimeoutException(String.format("%s - timeout after %s seconds", "ExternalService", timeout.getSeconds()))));
}

Conclusions: So apparently the moment you create WebClient.RequestBodySpec instance matters. Hope that helps somebody

Adding my few observations:

webClient.get() or webClient.post() always returns new DefaultRequestBodyUriSpec on every call on which URI being called and I think it does not look like URI getting swapped.

class DefaultWebClient implements WebClient {

..
    @Override
    public RequestHeadersUriSpec<?> get() {
        return methodInternal(HttpMethod.GET);
    }
    @Override
    public RequestBodyUriSpec post() {
        return methodInternal(HttpMethod.POST);
    }


..


    @Override
    public Mono<ClientResponse> exchange() {
        ClientRequest request = (this.inserter != null ?
                initRequestBuilder().body(this.inserter).build() :
                initRequestBuilder().build());
        return exchangeFunction.exchange(request).switchIfEmpty(NO_HTTP_CLIENT_RESPONSE_ERROR);
    }

    private ClientRequest.Builder initRequestBuilder() {
        URI uri = (this.uri != null ? this.uri : uriBuilderFactory.expand(""));
        return ClientRequest.create(this.httpMethod, uri)
                .headers(headers -> headers.addAll(initHeaders()))
                .cookies(cookies -> cookies.addAll(initCookies()))
                .attributes(attributes -> attributes.putAll(this.attributes));
    }
..
}

and methodInternal method looks like below

@SuppressWarnings("unchecked")
private RequestBodyUriSpec methodInternal(HttpMethod httpMethod) {
    return new DefaultRequestBodyUriSpec(httpMethod);
}

In addition while making actual request also, new ClientRequest is created.

Class source

https://github.com/spring-projects/spring-framework/blob/master/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java