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Service Discovery

Service Discovery allows microservices to automatically register themselves and discover other services through the Service Registry (Eureka) instead of hard-coded URLs.

Key Points

  • Register each microservice as a Discovery Client
  • Discovery Client enables microservices to locate other services dynamically via the registry

Service Discovery Client Setup (Eureka Client)

1. Add starter dependenct : Eureka Client 

<!-- https://mvnrepository.com/artifact/org.springframework.cloud/spring-cloud-starter-netflix-eureka-client -->
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
    <version>4.3.0</version>
</dependency>

2. Enable Discovery Client Annotation

In each microservice main class : add annotation @EnableDiscoveryClient

@EnableDiscoveryClient
@SpringBootApplication
public class UserServiceApplication {

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

3. Add Service Registry URL (in each microservice)

suppose service registry url : http://localhost:8761/

# Service Registry URL
eureka.client.service-url.defaultzone=http://localhost:8761

Successful Registration Log Sample

2025-11-01T16:35:55.783+06:00 INFO 46780 --- [service-registry] [nio-8761-exec-4] c.n.e.registry.AbstractInstanceRegistry : Registered instance USER-SERVICE/fedora:user-service:8081 with status UP (replication=true)

API Gateway as Discovery Client

Enable Gateway application also as Discovery Client and then

1. Enable Discovery in Gateway

spring.cloud.gateway.discovery.locator.enabled=true

2. Routing setup

1. Manual Routing Example
http://localhost:8080/user/create
http://localhost:8080/order/create
2. Dynamic Routing Format

Syntax : gatewayhostName:portNumber/<serivce-id>/<end-point>

 localhost:8080/USER-SERVICE/user/create
 localhost:8080/ORDER-SERVICE/order/create
3. Service Name Case

Eureka converts service IDs to UPPERCASE To use lowercase IDs: 

spring.cloud.gateway.discovery.locator.lower-case-service-id=true
Then

http://localhost:8080/user-service/user/create
http://localhost:8080/order-service/order/create

Dynamic Routing

Dynamic Routing allows the gateway to automatically discover and route requests to microservice instances without manual configuration.

If user-service has multiple instances (8081, 8082, 8083):

  • Without discovery client → manually configure all instances
  • With discovery client → all instances auto-detected
  • If one instance goes down → Gateway auto-routes to available instance .

No manual routing required. Eureka + Gateway handles it.

Load Balancing

Load Balancing distributes incoming traffic across multiple service instances to ensure performance, reliability, and no single server overload.

When client hits API Gateway:

  • Gateway checks service registry for available instances
  • Uses load-balancing algorithm (ex: Round-Robin)
  • Sends request to the least busy instance

Overview

Concept Description Dependency / Annotation
Service Registry Central registry that stores and manages all microservice instances spring-cloud-starter-netflix-eureka-server (Eureka Server)
Service Discovery Mechanism that allows microservices to find and communicate with each other via registry spring-cloud-starter-netflix-eureka-client (Eureka Client)
Discovery Client A microservice that registers itself with the service registry automatically @EnableDiscoveryClient (or included auto with Eureka Client)