RSocket Load Balancing – Client Side

Overview:

In this tutorial, I would like to demo Spring RSocket Load Balancing on the client side.

If you are new to RSocket, take a look at these articles on RSocket.

• RSocket With Java – Getting Started
• RSocket With Spring Boot

RSocket Load Balancing:

RSocket is a binary message passing protocol for client server application development which supports Reactive Streams. It establishes a persistent connection between the client and the server (like gRPC). It is a good choice for inter-Microservices communication. However there is a challenge in load balancing because of the persistent connection.

Lets see how we can achieve RSocket Load Balancing when we have a pool of servers.

High Level Setup:

Lets consider a simple client-server application in which we have 3 instances of the server-app are running. The client wants to send multiple requests and distribute the load across all the server instances. It gets the server instance details periodically from a service-registry (like consul/eureka..etc) and sends the requests accordingly.

Project Setup:

Create a Spring Boot application with RSocket & Lombok dependency.

I create a multi-module maven project as shown here.

Server Side:

• The server-app exposes an endpoint as shown here. It is simple and nothing else is required for this demo.

@Controller
public class ServerController {

    @MessageMapping("square-calculator")
    public Mono<Integer> square(Mono<Integer> input){
        return input
                .doOnNext(i -> System.out.println("Received : " + i))
                .delayElement(Duration.ofMillis(500))
                .map(i -> i * i);
    }

}

• Run this command – do a package.

mvn clean package

• Run 3 instances of the app as shown here (run this command from the server-app/target ).

java -jar -Dspring.rsocket.server.port=6565 server-app-0.0.1-SNAPSHOT.jar
java -jar -Dspring.rsocket.server.port=6566 server-app-0.0.1-SNAPSHOT.jar
java -jar -Dspring.rsocket.server.port=6567 server-app-0.0.1-SNAPSHOT.jar

Client Side:

• I assume the client will be provided with the list of server instance details somehow (in real life you can get this from a service registry). For this demo I use this application.yaml.

rsocket:
  square-service:
      servers:
        - host: localhost
          port: 6565
        - host: localhost
          port: 6566
        - host: localhost
          port: 6567

• I create a simple model class to get the above details as shown here.

@Data
@ToString
public class RSocketServerInstance {
    private String host;
    private int port;
}

• Service Registry
  • Here I assume you use something like consul/eureka from where you can fetch the list of dynamic IP addresses of server instances for the given service name.
  • In our case, we will be running 3 instances of the server-app and every time I am going to exclude 1 just to simulate that instance is not healthy!

@Service
@ConfigurationProperties(prefix = "rsocket.square-service")
public class DummyServiceRegistry {

    private List<RSocketServerInstance> servers;
    private AtomicInteger atomicInteger = new AtomicInteger(0);

    public void setServers(List<RSocketServerInstance> servers) {
        this.servers = servers;
    }

    // we exclude 1 instance every time to simulate something is not available
    public List<RSocketServerInstance> getServers() {
        atomicInteger.getAndIncrement();
        return IntStream.rangeClosed(0, 2)
                        .filter(i -> atomicInteger.get() % 3 != i)
                        .mapToObj(servers::get)
                        .collect(Collectors.toList());
    }

}

• Load Balance Target
  • Above class is simulating a service registry.
  • Once we get the IP addresses of the healthy instances and create List<LoadbalanceTarget>
  • We periodically query the service registry & make it as a Flux<List<LoadbalanceTarget>> – a never ending stream.

@Configuration
public class LoadBalanceTargetConfig {

    @Autowired
    private DummyServiceRegistry serviceRegistry;

    @Bean
    public Flux<List<LoadbalanceTarget>> targets(){
        return Mono.fromSupplier(() -> serviceRegistry.getServers())
                .repeatWhen(longFlux -> longFlux.delayElements(Duration.ofSeconds(2)))
                .map(this::toLoadBalanceTarget);
    }

    private List<LoadbalanceTarget> toLoadBalanceTarget(List<RSocketServerInstance> rSocketServers){
        return rSocketServers.stream()
                .map(server -> LoadbalanceTarget.from(server.getHost() + server.getPort(), TcpClientTransport.create(server.getHost(), server.getPort())))
                .collect(Collectors.toList());
    }

}

• RSocket Configuration
  • Once we have the Flux<List<LoadbalanceTarget>>, we build the RSocketRequester with that flux as shown here.

@Configuration
public class RSocketConfig {

    @Bean
    public RSocketStrategies rSocketStrategies() {
        return RSocketStrategies.builder()
                .encoders(encoders -> encoders.add(new Jackson2CborEncoder()))
                .decoders(decoders -> decoders.add(new Jackson2CborDecoder()))
                .build();
    }

    @Bean
    public RSocketRequester rSocketClient(RSocketRequester.Builder builder, Flux<List<LoadbalanceTarget>> targetFlux){
       return builder.transports(targetFlux, new RoundRobinLoadbalanceStrategy());
    }

}

• That’s it! Now the RSocketRequester is ready to be used and balance the load across the given instances.

@SpringBootApplication
public class RSocketClientApplication implements CommandLineRunner {

    @Autowired
    private RSocketRequester rSocketRequester;

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

    @Override
    public void run(String... args) throws Exception {
        Flux.range(1, 10000)
            .delayElements(Duration.ofMillis(100))
            .flatMap(i -> rSocketRequester.route("square-calculator").data(i).retrieveMono(Integer.class).retry(1))
            .doOnNext(i -> System.out.println("Response : " + i))
            .blockLast();
    }
}

When we run the client side application, we can see that RSocket Load Balancing happens across all the server instances.

Summary:

We were able to successfully demonstrate RSocket Load Balancing from Client Side. Special thanks to RSocket maintainer Oleg for clarifying my issues on this topic!

Read more about RSocket.

• RSocket Security With Spring
• RSocket File Upload Example
• RSocket + WebSocket + Spring Boot = Real Time Application

The source code is available here.

Happy learning 🙂