Load Balancing with Spring Cloud LoadBalancer
Utilize Spring Cloud LoadBalancer for client-side load balancing when routing requests to multiple instances of a service.
Load Balancing with Spring Cloud LoadBalancer is a free API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What is Client-Side Load Balancing?
In a microservices architecture, you often have multiple instances of the same service running. Client-side load balancing means the client (the service consumer) is responsible for choosing which instance to send a request to.
Unlike server-side load balancing (like Nginx), where a central proxy distributes requests, here the client actively participates in the decision. This can reduce network hops and improve responsiveness.
Introducing Spring Cloud LoadBalancer
Spring Cloud LoadBalancer is a client-side load balancer that works seamlessly with Spring Cloud applications. It's the modern successor to Netflix Ribbon in the Spring ecosystem.
It integrates with service discovery mechanisms (like Eureka or Consul) to get a list of available service instances and then applies a load balancing algorithm to select one for each request.
Adding LoadBalancer to Your Project
To use Spring Cloud LoadBalancer, you typically add the spring-cloud-starter-loadbalancer dependency to your Spring Boot project. This starter brings in all necessary components.
It's often used alongside a service discovery client (e.g., Eureka Client) so LoadBalancer knows which service instances are available.
Load Balancing with RestTemplate
You can easily enable client-side load balancing for RestTemplate by adding the @LoadBalanced annotation to its bean definition. This tells Spring Cloud to intercept calls and use LoadBalancer.
Notice how we call the service using its logical name (MY-SERVICE) instead of a specific IP:port.
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;
@SpringBootApplication
public class RestTemplateClient {
@LoadBalanced
@Bean
public RestTemplate restTemplate() {
return new RestTemplate();
}
public static void main(String[] args) {
SpringApplication.run(RestTemplateClient.class, args);
// Example usage (in a real app, this would be in a service)
// RestTemplate restTemplate = applicationContext.getBean(RestTemplate.class);
// String response = restTemplate.getForObject("http://MY-SERVICE/hello", String.class);
// System.out.println("Response from MY-SERVICE: " + response);
}
}Load Balancing with WebClient
For reactive applications, WebClient is the preferred choice. You can enable load balancing for it by applying @LoadBalanced to a WebClient.Builder bean.
Similar to RestTemplate, you use the service's logical name in the URL, and LoadBalancer handles the instance resolution.
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.context.annotation.Bean;
import org.springframework.web.reactive.function.client.WebClient;
@SpringBootApplication
public class WebClientClient {
@LoadBalanced
@Bean
public WebClient.Builder webClientBuilder() {
return WebClient.builder();
}
public static void main(String[] args) {
SpringApplication.run(WebClientClient.class, args);
// Example usage (in a real app, this would be in a service)
// WebClient webClient = applicationContext.getBean(WebClient.Builder.class).build();
// webClient.get().uri("http://MY-SERVICE/greet")
// .retrieve().bodyToMono(String.class)
// .subscribe(response -> System.out.println("Response: " + response));
}
}How Service Discovery Powers It
Spring Cloud LoadBalancer doesn't magically know about service instances. It relies on service discovery, like Eureka or Consul, to provide a list of healthy, available instances for a given service ID.
When a client makes a request using a logical service name (e.g., MY-SERVICE), LoadBalancer queries the service registry, gets the list of instances, and then picks one.
Default Strategy: Round Robin
By default, Spring Cloud LoadBalancer uses the Round Robin algorithm. This means it cycles through the available service instances in order, distributing requests evenly.
For example, if you have three instances (A, B, C), the first request goes to A, the second to B, the third to C, the fourth back to A, and so on.
Gateway Routing with LoadBalancer
Spring Cloud Gateway automatically integrates with Spring Cloud LoadBalancer. When you define a route using a service ID (e.g., lb://MY-SERVICE), the Gateway uses LoadBalancer to resolve the actual instance address.
This means your gateway can route requests to multiple instances of a backend service without you needing to manually manage their IPs and ports.
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.gateway.route.RouteLocator;
import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
import org.springframework.context.annotation.Bean;
@SpringBootApplication
@EnableDiscoveryClient // Enables service discovery client (e.g., Eureka)
public class GatewayApplication {
public static void main(String[] args) {
SpringApplication.run(GatewayApplication.class, args);
}
@Bean
public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
return builder.routes()
.route("my_service_route", r -> r.path("/api/**")
.uri("lb://MY-SERVICE")) // 'lb://' indicates load-balanced service
.build();
}
}Customizing Load Balancing Strategies
While Round Robin is the default, Spring Cloud LoadBalancer is extensible. You can configure custom load balancing algorithms if your needs are more complex.
For example, you might want a 'Least Connections' strategy, which sends requests to the instance with the fewest active connections, or a 'Sticky Session' approach to route a user's requests consistently to the same server.
Quick Check: LoadBalancer Purpose
Which of the following are primary benefits or characteristics of using Spring Cloud LoadBalancer?
Recap: Client-Side Load Balancing
We've explored Spring Cloud LoadBalancer, a key component for building resilient microservices. It enables clients to intelligently distribute requests across multiple instances of a service.
- It's a client-side solution, distinct from server-side proxies.
- It works with service discovery (e.g., Eureka) to find service instances.
- You can use it with
@LoadBalanced RestTemplateorWebClient.Builder. - It's automatically integrated into Spring Cloud Gateway for routing.
- The default load balancing strategy is Round Robin.
This ensures your applications can scale horizontally and remain available even if some service instances fail.
Frequently asked questions
Is the “Load Balancing with Spring Cloud LoadBalancer” lesson free?
Yes — the full text of “Load Balancing with Spring Cloud LoadBalancer” is free to read here on the web, and the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) course, upgrade to CoddyKit PRO.
What will I learn in “Load Balancing with Spring Cloud LoadBalancer”?
Utilize Spring Cloud LoadBalancer for client-side load balancing when routing requests to multiple instances of a service. You practise API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway)?
No prior experience is required. API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Load Balancing with Spring Cloud LoadBalancer” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson?
Yes. Every API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Integrating with Eureka/Consul
- Dynamic Routing with Service Discovery
- Load Balancing with Spring Cloud LoadBalancer
- The lb:// URI & Discovery Locator