Functional Hystrix using Spring Cloud HystrixCommands

Spring's WebClient provides a non-blocking client for making service to service calls. Hystrix, though now in a maintenance mode, has been used for protecting service to service calls by preventing cascading failures, providing circuit breakers for calls to slow or faulty upstream services.

In this post, I will be exploring how Spring Cloud provides a newer functional approach to wrapping a remote call with Hystrix.


Consider a simple service that returns a list of entities, say a list of cities, modeled using the excellent Wiremock tool:

WIREMOCK_SERVER.stubFor(WireMock.get(WireMock.urlMatching("/cities"))
                .withHeader("Accept", WireMock.equalTo("application/json"))
                .willReturn(WireMock.aResponse()
                        .withStatus(HttpStatus.OK.value())
                        .withFixedDelay(5000)
                        .withHeader("Content-Type", "application/json")))

When called with a uri of the type "/cities" this Wiremock endpoint responds with a json of the following type:

[
  {
"country": "USA",
"id": 1,
"name": "Portland",
"pop": 1600000
  },
  {
"country": "USA",
"id": 2,
"name": "Seattle",
"pop": 3200000
  },
  {
"country": "USA",
"id": 3,
"name": "SFO",
"pop": 6400000
  }
]

after a delay of 5 seconds.

Traditional approach

There are many approaches to using Hystrix, I have traditionally preferred an approach where an explicit Hystrix Command protects the remote call, along these lines:

import com.netflix.hystrix.HystrixCommandGroupKey
import com.netflix.hystrix.HystrixCommandKey
import com.netflix.hystrix.HystrixCommandProperties
import com.netflix.hystrix.HystrixObservableCommand
import org.bk.samples.model.City
import org.slf4j.Logger
import org.slf4j.LoggerFactory
import org.springframework.http.MediaType
import org.springframework.web.reactive.function.client.WebClient
import org.springframework.web.reactive.function.client.bodyToFlux
import org.springframework.web.util.UriComponentsBuilder
import reactor.core.publisher.Flux
import rx.Observable
import rx.RxReactiveStreams
import rx.schedulers.Schedulers
import java.net.URI


class CitiesHystrixCommand(
        private val webClientBuilder: WebClient.Builder,
        private val citiesBaseUrl: String
) : HystrixObservableCommand<City>(
        HystrixObservableCommand.Setter
                .withGroupKey(HystrixCommandGroupKey.Factory.asKey("cities-service"))
                .andCommandKey(HystrixCommandKey.Factory.asKey("cities-service"))
                .andCommandPropertiesDefaults(HystrixCommandProperties.Setter()
                        .withExecutionTimeoutInMilliseconds(4000))) {
    override fun construct(): Observable<City> {
        val buildUri: URI = UriComponentsBuilder
                .fromUriString(citiesBaseUrl)
                .path("/cities")
                .build()
                .encode()
                .toUri()

        val webClient: WebClient = this.webClientBuilder.build()

        val result: Flux<City> = webClient.get()
                .uri(buildUri)
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .flatMapMany { clientResponse ->
                    clientResponse.bodyToFlux<City>()
                }

        return RxReactiveStreams.toObservable(result)
    }

    override fun resumeWithFallback(): Observable<City> {
        LOGGER.error("Falling back on cities call", executionException)
        return Observable.empty()
    }

    companion object {
        private val LOGGER: Logger = LoggerFactory.getLogger(CitiesHystrixCommand::class.java)
    }
}

This code can now be used to make a remote call the following way:

import org.springframework.http.MediaType
import org.springframework.web.reactive.function.client.WebClient


class CitiesHystrixCommandBasedClient(
        private val webClientBuilder: WebClient.Builder,
        private val citiesBaseUrl: String
) {
    fun getCities(): Flux<City> {
        val citiesObservable: Observable<City> = CitiesHystrixCommand(webClientBuilder, citiesBaseUrl)
                .observe()
                .subscribeOn(Schedulers.io())

        return Flux
                .from(RxReactiveStreams
                        .toPublisher(citiesObservable))
    }
}

Two things to note here,
1. WebClient returns a Project Reactor "Flux" type representing a list of cities, however Hystrix is Rx-Java 1 based, so Flux is being transformed to Rx-Java Observable using "RxReactiveStreams.toObservable()" call, provided by the RxJavaReactiveStreams library here.

2. I still want Project Reactor "Flux" type to be used in the rest of the application, so there is another adapter that converts the Rx-Java Observable back to a Flux - "Flux.from(RxReactiveStreams.toPublisher(citiesObservable))" once the call wrapped in Hystrix returns.


If I were to try this client with the wiremock sample with the 5 second delay, it correctly handles the delay and returns after a second.

Functional approach

There is a lot of boiler-plate with the previous approach which is avoided with the new functional approach of using HystrixCommands, a utility class which comes with Spring Cloud which provides a functional approach to making the remote call wrapped with Hystrix.

The entirety of the call using HystrixCommands looks like this:

import com.netflix.hystrix.HystrixCommandProperties
import org.bk.samples.model.City
import org.slf4j.Logger
import org.slf4j.LoggerFactory
import org.springframework.cloud.netflix.hystrix.HystrixCommands
import org.springframework.http.MediaType
import org.springframework.web.reactive.function.client.WebClient
import org.springframework.web.reactive.function.client.bodyToFlux
import org.springframework.web.util.UriComponentsBuilder
import reactor.core.publisher.Flux
import rx.schedulers.Schedulers
import java.net.URI

class CitiesFunctionalHystrixClient(
        private val webClientBuilder: WebClient.Builder,
        private val citiesBaseUrl: String
) {
    fun getCities(): Flux<City> {
        return HystrixCommands
                .from(callCitiesService())
                .commandName("cities-service")
                .groupName("cities-service")
                .commandProperties(
                        HystrixCommandProperties.Setter()
                                .withExecutionTimeoutInMilliseconds(1000)
                )
                .toObservable { obs ->
                    obs.observe()
                            .subscribeOn(Schedulers.io())
                }
                .fallback { t: Throwable ->
                    LOGGER.error(t.message, t)
                    Flux.empty()
                }
                .toFlux()
    }

    fun callCitiesService(): Flux<City> {
        val buildUri: URI = UriComponentsBuilder
                .fromUriString(citiesBaseUrl)
                .path("/cities")
                .build()
                .encode()
                .toUri()

        val webClient: WebClient = this.webClientBuilder.build()

        return webClient.get()
                .uri(buildUri)
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .flatMapMany { clientResponse ->
                    clientResponse.bodyToFlux<City>()
                }
    }

    companion object {
        private val LOGGER: Logger = LoggerFactory.getLogger(CitiesHystrixCommand::class.java)
    }
}

A lot of boiler-plate is avoided with this approach -
1. an explicit command is not required anymore
2. the call and the fallback are coded in a fluent manner
3. Any overrides can be explicitly specified - in this specific instance the timeout of 1 second.

Conclusion

I like the conciseness which HystrixCommands brings to the usage of Hystrix with WebClient. I have the entire sample available in my github repo - https://github.com/bijukunjummen/webclient-hystrix-sample, all the dependencies required to get the samples to work is part of this repo. If you are interested in sticking with Rx-Java 1, then an approach described here may help you avoid boiler-plate with vanilla Hystrix