Ribbon load balancer with webclient differs from rest template one (not properly balanced)

Question

I've tried to use WebClient with LoadBalancerExchangeFilterFunction:

WebClient config:

@Bean
public WebClient myWebClient(final LoadBalancerExchangeFilterFunction lbFunction) {
    return WebClient.builder()
            .filter(lbFunction)
            .defaultHeader(ACCEPT, APPLICATION_JSON_VALUE)
            .defaultHeader(CONTENT_ENCODING, APPLICATION_JSON_VALUE)
            .build();
} 

Then I've noticed that calls to underlying service are not properly load balanced - there is constant difference of RPS on each instance.

Then I've tried to move back to RestTemplate. And it's working fine.

Config for RestTemplate:

private static final int CONNECT_TIMEOUT_MILLIS = 18 * DateTimeConstants.MILLIS_PER_SECOND;
private static final int READ_TIMEOUT_MILLIS = 18 * DateTimeConstants.MILLIS_PER_SECOND;

@LoadBalanced
@Bean
public RestTemplate restTemplateSearch(final RestTemplateBuilder restTemplateBuilder) {
    return restTemplateBuilder
            .errorHandler(errorHandlerSearch())
            .requestFactory(this::bufferedClientHttpRequestFactory)
            .build();
}

private ClientHttpRequestFactory bufferedClientHttpRequestFactory() {
    final SimpleClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();
    requestFactory.setConnectTimeout(CONNECT_TIMEOUT_MILLIS);
    requestFactory.setReadTimeout(READ_TIMEOUT_MILLIS);
    return new BufferingClientHttpRequestFactory(requestFactory);
}

private ResponseErrorHandler errorHandlerSearch() {
    return new DefaultResponseErrorHandler() {
        @Override
        public boolean hasError(ClientHttpResponse response) throws IOException {
            return response.getStatusCode().is5xxServerError();
        }
    };
}

Load balancing using WebClient config up to 11:25, then switching back to RestTemplate:

Is there a reason why there is such difference and how I can use WebClient to have same amount of RPS on each instance? Clue might be that older instances are getting more requests than new ones.

I've tried bit of debugging and same (defaults like ZoneAwareLoadBalancer) logic is being called.

Answer 1

I did simple POC and everything works exactly the same with web client and rest template for default configuration.

Rest server code:

@SpringBootApplication
internal class RestServerApplication

fun main(args: Array<String>) {
    runApplication<RestServerApplication>(*args)
}

class BeansInitializer : ApplicationContextInitializer<GenericApplicationContext> {
    override fun initialize(context: GenericApplicationContext) {
        serverBeans().initialize(context)
    }
}

fun serverBeans() = beans {
    bean("serverRoutes") {
        PingRoutes(ref()).router()
    }
    bean<PingHandler>()
}

internal class PingRoutes(private val pingHandler: PingHandler) {
    fun router() = router {
        GET("/api/ping", pingHandler::ping)
    }
}

class PingHandler(private val env: Environment) {
    fun ping(serverRequest: ServerRequest): Mono<ServerResponse> {
        return Mono
            .fromCallable {
                // sleap added to simulate some work
                Thread.sleep(2000)
            }
            .subscribeOn(elastic())
            .flatMap {
                ServerResponse.ok()
                    .syncBody("pong-${env["HOSTNAME"]}-${env["server.port"]}")
            }
    }
}

In application.yaml add:

context.initializer.classes: com.lbpoc.server.BeansInitializer

Dependencies in gradle:

implementation('org.springframework.boot:spring-boot-starter-webflux')

Rest client code:

@SpringBootApplication
internal class RestClientApplication {
    @Bean
    @LoadBalanced
    fun webClientBuilder(): WebClient.Builder {
        return WebClient.builder()
    }

    @Bean
    @LoadBalanced
    fun restTemplate() = RestTemplateBuilder().build()
}

fun main(args: Array<String>) {
    runApplication<RestClientApplication>(*args)
}

class BeansInitializer : ApplicationContextInitializer<GenericApplicationContext> {
    override fun initialize(context: GenericApplicationContext) {
        clientBeans().initialize(context)
    }
}

fun clientBeans() = beans {
    bean("clientRoutes") {
        PingRoutes(ref()).router()
    }
    bean<PingHandlerWithWebClient>()
    bean<PingHandlerWithRestTemplate>()
}

internal class PingRoutes(private val pingHandlerWithWebClient: PingHandlerWithWebClient) {
    fun router() = org.springframework.web.reactive.function.server.router {
        GET("/api/ping", pingHandlerWithWebClient::ping)
    }
}

class PingHandlerWithWebClient(private val webClientBuilder: WebClient.Builder) {
    fun ping(serverRequest: ServerRequest) = webClientBuilder.build()
        .get()
        .uri("http://rest-server-poc/api/ping")
        .retrieve()
        .bodyToMono(String::class.java)
        .onErrorReturn(TimeoutException::class.java, "Read/write timeout")
        .flatMap {
            ServerResponse.ok().syncBody(it)
        }
}

class PingHandlerWithRestTemplate(private val restTemplate: RestTemplate) {
    fun ping(serverRequest: ServerRequest) = Mono.fromCallable {
        restTemplate.getForEntity("http://rest-server-poc/api/ping", String::class.java)
    }.flatMap {
        ServerResponse.ok().syncBody(it.body!!)
    }
}

In application.yaml add:

context.initializer.classes: com.lbpoc.client.BeansInitializer
spring:
  application:
    name: rest-client-poc-for-load-balancing
logging:
  level.org.springframework.cloud: DEBUG
  level.com.netflix.loadbalancer: DEBUG
rest-server-poc:
  listOfServers: localhost:8081,localhost:8082

Dependencies in gradle:

implementation('org.springframework.boot:spring-boot-starter-webflux')
implementation('org.springframework.cloud:spring-cloud-starter-netflix-ribbon')

You can try it with two or more instances for server and it works exactly the same with web client and rest template.

Ribbon use by default zoneAwareLoadBalancer and if you have only one zone all instances for server will be registered in "unknown" zone.

You might have a problem with keeping connections by web client. Web client reuse the same connection in multiple requests, rest template do not do that. If you have some kind of proxy between your client and server then you might have a problem with reusing connections by web client. To verify it you can modify web client bean like this and run tests:

@Bean
@LoadBalanced
fun webClientBuilder(): WebClient.Builder {
    return WebClient.builder()
        .clientConnector(ReactorClientHttpConnector { options ->
            options
                .compression(true)
                .afterNettyContextInit { ctx ->
                    ctx.markPersistent(false)
                }
        })
}

Of course it's not a good solution for production but doing that you can check if you have a problem with configuration inside your client application or maybe problem is outside, something between your client and server. E.g. if you are using kubernetes and register your services in service discovery using server node IP address then every call to such service will go though kube-proxy load balancer and will be (by default round robin will be used) routed to some pod for that service.