Spring Tips: Spring Cloud Loadbalancer

Engineering | Josh Long | March 25, 2020 | ...

speaker: Josh Long (@starbuxman)

Hi, Spring fans! Welcome to another installment of Spring Tips! In this installment, we're going to look at a new feature in Spring Cloud, Spring Cloud Loadbalancer. Spring Cloud Loadbalancer is a generic abstraction that can do the work that we used to do with Netflix's Ribbon project. Spring Cloud still supports Netflix Ribbon, but Netflix Ribbons days are numbered, like so much else of the Netflix microservices stack, so we've provided an abstraction to support an alternative.

The Service Registry

For us to use the Spring Cloud Load Balancer, we need to have a service registry up and running. A service registry makes it trivial to programmatically query for the location of a given service in a system. There are several popular implementations, including Apache Zookeeper, Netflix's Eureka, Hashicorp Consul, and others. You can even use Kubernetes and Cloud Foundry as service registries. Spring Cloud provides an abstraction, DiscoveryClient, that you can use to talk to these service registries generically. There are several patterns that a service registry enables that just arent possible with good 'ol DNS. One thing I love to do is client-side load-balancing. Client-side load-balancing requires the client code to decide which node receives the request. There is any number of instances of the service out there, and their suitability to handle a particular request is something each client can decide. It's even better if it can make the decision before launching a request that might otherwise be doomed to failure. It saves time, unburdens the services with tedious flow control requirements, and makes our system more dynamic since we can query its topology.

You can run any service registry you like. I like to use Netflix Eureka for these sorts of things because it is simpler to setup. Lets set up a new instance. You could download and run a stock-standard image if you want, but I want to use the pre-configured instance provided as part of Spring Cloud.

Go to the Spring Initializer, choose Eureka Server and Lombok. I named mine eureka-service. Hit Generate.

Most of the work of using the built-in Eureka Service is in the configuration, which I've reprinted here.


Then you'll need to customize the Java class. Add the @EnableEurekaServer annotation to your class.

package com.example.eurekaservice;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

public class EurekaServiceApplication {

    public static void main(String[] args) {
        SpringApplication.run(EurekaServiceApplication.class, args);


You can run that now. It'll be available on port 8761 and other clients will connect to that port by default.

A Simple API

Let's now turn to the API. Our API is as trivial as these things come. We just want an endpoint to which our client can issue requests.

Go to the Spring Initializr, generate a new project with Reactive Web and Lombok and the Eureka Discovery Client. That last bit is the critical part! You're not going to see it used in the following Java code. It's all autoconfiguration, which we also covered way back in 2016, that runs at application startup. The autoconfiguration will automatically register the application with the specified registry (in this case, we're using the DiscoveryClient implementation for Netflix's Eureka) using the spring.application.name property.

Specify the following properties.


Our HTTP endpoint is a "Hello, world!" handler that uses the functional reactive HTTP style that we introduced in another Spring Tips video way, way back in 2017.

package com.example.api;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.web.reactive.function.server.HandlerFunction;
import org.springframework.web.reactive.function.server.RouterFunction;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;
import reactor.core.publisher.Mono;

import java.util.Map;

import static org.springframework.web.reactive.function.server.RouterFunctions.route;
import static org.springframework.web.reactive.function.server.ServerResponse.*;

public class ApiApplication {
    RouterFunction<ServerResponse> routes() {
        return route()
            .GET("/greetings", r -> ok().bodyValue(Map.of("greetings", "Hello, world!")))

    public static void main(String[] args) {
        SpringApplication.run(ApiApplication.class, args);

Run the application, and you'll see it reflected in the Netlfix Eureka instance. You can change the server.port value to 0 in application.properties. If you run multiple instances, you'll see them reflected in the console.

The Load-Balancing Client

All right, now we're ready to demonstrate laod balancing in action. We'll need a new spring boot application. Go to the Spring Intiialzir and generate a new project using the Eureka Discovery Client, Lombok, Cloud Loadbalancer, and Reactive Web. Click Generate and open the project in your favorite IDE.

Add the Caffeine Cache to the classpath. It's not on the Spring Initializr, so I added it manually. It's Maven coordinates are com.github.ben-manes.caffeine:caffeine:${caffeine.version}. If this dependency is present, then the load balancer will use it to cache resolved instances.

Let's review what we want to happen. We want to make a call to our service, api. We know that there could be more than one instance of the service in the load balancer. We could put the API behind a load balancer and just call it done. But what we want to do is to use the information available to us about the state of each application to make smarter load balancing decisions. There are a lot of reasons we might use the client-side load balancer instead of DNS. First, Java DNS clients tend to cache the resolved IP information, which means that subsequent calls to the same resolved IP would end up subsequently dogpiling on top of one service. You can disable that, but you're working against the grain of DNS, a caching-centric system. DNS only tells you where something is, not if it is. Put another way; you don't know if there is going to be anything waiting for your request on the other side of that DNS based load balancer. Wouldnt you like to be able to know before making the call, sparing your client the tedious timeout period before the call fails? Additionally, some patterns like service hedging - also the topic of another Spring Tips video - is only possible with a service registry.

Let's look at the usual configuration properties for the client. The properties specify the spring.applicatino.name, nothing novel about that. The second property is important. It disables the default Netflix Ribbon-backed load balancing strategy that's been in place since Spring Cloud debuted in 2015. We want to use the new Spring Cloud Load balancer, after all.


So, let's look at the use of our service registry. First thing's first, our client needs to establish a connection to the service registry with the Eureka DiscoveryClient implementation. The Spring Cloud DiscoveryClient abstraction is on the classpath, so it'll automatically start-up and register the client with the service registry.

Here are the beginnings of our application, an entry point class.

package com.example.client;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.extern.log4j.Log4j2;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.cloud.client.loadbalancer.reactive.ReactiveLoadBalancer;
import org.springframework.cloud.client.loadbalancer.reactive.ReactorLoadBalancerExchangeFilterFunction;
import org.springframework.cloud.client.loadbalancer.reactive.Response;
import org.springframework.context.annotation.Bean;
import org.springframework.stereotype.Component;
import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Flux;

import static com.example.client.ClientApplication.call;

public class ClientApplication {

    public static void main(String[] args) {
        SpringApplication.run(ClientApplication.class, args);

We'll add to this a DTO class to convey the JSON structure returned from the service to the clients. This class uses some of Lombok's convenient annoations.

class Greeting {
    private String greetings;

Now, let's look at three different approaches to load balancing, each progressively more sophisticated.

Using the Loadbalancer Abstraction Directly

This first approach is the simplest, albeit most verbose, of the three. In this approach, we'll work with the load balancing abstraction directly. The component injects a pointer to the ReactiveLoadBalancer.Factory<ServiceInstance>, which we can then use to vend a ReactiveLoadBalancer<ServiceInstance>. This ReactiveLoadBalancer is the interface with which we load-balance calls to the api service by invoking api.choose(). I then use that ServiceInstance to build up a URL to the particular host and port of that specific ServiceInstance and then make an HTTP request with WebClient, our reactive HTTP client.

class ReactiveLoadBalancerFactoryRunner {

  ReactiveLoadBalancerFactoryRunner(ReactiveLoadBalancer.Factory<ServiceInstance> serviceInstanceFactory) {
        var http = WebClient.builder().build();
        ReactiveLoadBalancer<ServiceInstance> api = serviceInstanceFactory.getInstance("api");
        Flux<Response<ServiceInstance>> chosen = Flux.from(api.choose());
            .map(responseServiceInstance -> {
                ServiceInstance server = responseServiceInstance.getServer();
                var url = "http://" + server.getHost() + ':' + server.getPort() + "/greetings";
                return url;
            .flatMap(url -> call(http, url))
            .subscribe(greeting -> log.info("manual: " + greeting.toString()));


The actual work of making the HTTP request is done by a static method, call, that I have stashed in the application class. It expects a valid WebClient reference and an HTTP URL.

   static Flux<Greeting> call(WebClient http, String url) {
       return http.get().uri(url).retrieve().bodyToFlux(Greeting.class);

This approach works, but it's a lot of code to make one HTTP call.

Using the ReactorLoadBalancerExchangeFilterFunction

This next approach hides a lot of that boilerplate load-balancing logic in a WebClient filter, of the type ExchangeFilterFunction, called ReactorLoadBalancerExchangeFilterFunction. We plug in that filter before making the request, and a lot of the previous code disappears.

class WebClientRunner {

    WebClientRunner(ReactiveLoadBalancer.Factory<ServiceInstance> serviceInstanceFactory) {

        var filter = new ReactorLoadBalancerExchangeFilterFunction(serviceInstanceFactory);

        var http = WebClient.builder()

        call(http, "http://api/greetings").subscribe(greeting -> log.info("filter: " + greeting.toString()));

Ahhhhhh. Much better! But we can do better.

The @LoadBalanced Annotation

In this final example, we'll have Spring Cloud configure the WebClient instance for us. This approach is excellent if all requests that pass through that shared WebClient instance require load balancing. Just define a provider method for the WebClient.Builder and annotate it with @LoadBalanced. You can then use that WebClient.Builder to define a WebClient that'll load balance automatically for us.

    WebClient.Builder builder() {
        return WebClient.builder();
    WebClient webClient(WebClient.Builder builder) {
        return builder.build();

With that done, our code shrinks to virtually nothing.

class ConfiguredWebClientRunner {

    ConfiguredWebClientRunner(WebClient http) {
        call(http, "http://api/greetings").subscribe(greeting -> log.info("configured: " + greeting.toString()));

Now, that is convenient.

The load balancer uses round-robin load balancing, where it randomly distributes the load across any of a number co configured instances, using the org.springframework.cloud.loadbalancer.core.RoundRobinLoadBalancer strategy. The nice thing about this is that this is pluggable. You can plugin in other heuristics if you wanted as well.

Next Steps

In this Spring Tip installment, we've only begun to scratch the surface of the load balancing abstraction, but we have already achieved immense flexibility and conciseness. If you're further interested in customizing the load balancer, you might look into the @LoadBalancedClient annotation.

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