How ServiceInstanceListSuppliers Connects to Eureka

🔗 Overview

In a Spring Cloud microservices architecture using Eureka and Spring Cloud LoadBalancer, service discovery and load balancing involve the following flow:

1. The Eureka Client registers services with the Eureka Server.
2. The client keeps a local cache of the registry information from the server.
3. Requests made via RestTemplate, WebClient, or Feign (annotated with @LoadBalanced) trigger Spring Cloud LoadBalancer.
4. Spring Cloud LoadBalancer calls a ServiceInstanceListSupplier to get the list of available service instances.

🧩 Role of ServiceInstanceListSuppliers

ServiceInstanceListSuppliers provides the list of service instances to Spring Cloud LoadBalancer. When Eureka is used, the implementation is:

org.springframework.cloud.netflix.eureka.EurekaServiceInstanceListSupplier

This class connects LoadBalancer to the Eureka registry using the DiscoveryClient.

🔄 Flow of Interaction

1. Request is made to a service.
2. LoadBalancer intercepts and asks for service instances.
3. ServiceInstanceListSupplier.get() is invoked.
4. EurekaServiceInstanceListSupplier uses Eureka’s DiscoveryClient to fetch instances.
5. Instances are returned from the local cache.

📦 Code Example

@Override
public Flux<List<ServiceInstance>> get() {
    return Flux.defer(() -> {
        List<ServiceInstance> instances = discoveryClient.getInstances(serviceId);
        return Flux.just(instances);
    });
}

⚙️ Auto-Configuration

• If spring-cloud-starter-netflix-eureka-client is on the classpath,
• And @EnableDiscoveryClient is used,
• Then EurekaServiceInstanceListSupplier is auto-configured as the default implementation.

✅ Summary