How Service Discovery Works in Spring Applications

Service discovery helps your Spring applications find each other automatically at runtime. Instead of hardcoding service locations, you register each service with a central registry. When a service needs to call another, it asks the registry for the current location. This makes your system flexible and easier to scale.

The Service Registry

A service registry acts like a phone book for your services. When a service starts, it registers itself with the registry, sharing its address. Other services then use the registry to look up where to send requests.

Using Spring Cloud for Service Discovery

Spring Cloud makes service discovery simple. You can use a registry like Eureka. Here’s how it works:

• Each service includes the Spring Cloud discovery client;
• On startup, each service registers itself with Eureka;
• When a service wants to call another, it asks Eureka for the current address;
• The address can change, but the registry always knows where to find the service.

Example: Discovering a Service

Suppose you have two services: order-service and inventory-service. order-service needs to talk to inventory-service.

Step 1: Register with Eureka

Add this annotation to both services:

@EnableDiscoveryClient

Step 2: Use Discovery to Call Another Service

In order-service, use the service name instead of a fixed address:

@RestController
public class OrderController {
    @Autowired
    private RestTemplate restTemplate;

    @GetMapping("/order/{id}")
    public String createOrder(@PathVariable String id) {
        // Use the service name registered in Eureka
        String inventoryUrl = "http://inventory-service/inventory/check/" + id;
        String response = restTemplate.getForObject(inventoryUrl, String.class);
        return "Order created. Inventory status: " + response;
    }
}

Explanation:

• The URL http://inventory-service/inventory/check/{id} uses the service name, not a hardcoded address;
• Spring Cloud talks to Eureka to find the current address of inventory-service;
• If inventory-service moves or scales, order-service still finds it automatically.

This approach keeps your system flexible and resilient as services grow or change.