Service Discovery Patterns with Netflix Eureka
Modern distributed systems assign network locations dynamically, so you don’t know what IP address or port each service instance has. Consequently, client applications need a mechanism to discover the IP address and port of the services they are using.
As we explore in this article, service discovery tools like Netflix Eureka enable service discovery and registration in microservice-based applications.
Service Discovery Patterns
The Problem
Modern distributed systems typically run in virtualized/containerized environments where the number of instances of a particular service and their network locations change dynamically. As a result, you can’t statically configure a client application with the IP and port of the service.
Service Registry
Service discovery includes a Service Registry mechanism, which is a special service that stores the network locations of each service instance.
A service updates the service registry when instances are created or destroyed. When a client application queries the service registry, it gets a list of available instances and routes.
Self Registration Pattern
A service instance is responsible for registering itself with the service registry. On startup the service instance invokes the service registry's registration API to register its network location.
It is also convenient that services supply a health check, which is an endpoint that is invoked periodically to verify that the service instance is healthy and available to handle requests.
Client-Side Discovery Pattern
When a client wants to invoke a service, it queries the service registry to obtain a list of available instances.
Clients may use load-balancing algorithms to determine which service instance to use for each request (load-balanced requests).
Netflix Eureka
Project Structure
As shown below, our Maven project is divided into three modules: api-gateway, order-service and service-registry.
├───api-gateway
│ ├───src
│ │ └───main
│ │ ├───java
│ │ └───resources
│ └───pom.xml
├───order-service
│ ├───src
│ │ └───main
│ │ ├───java
│ │ └───resources
│ └───pom.xml
├────service-registry
│ ├───src
│ │ └───main
│ │ ├───java
│ │ └───resources
│ └───pom.xml
└───pom.xml
Service Registry, the Eureka Server
To include Eureka Server in our project, let’s add the cloud-starter-netflix-eureka-server dependency to the service-registry/pom.xml file.
<dependency>
<artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
<groupId>org.springframework.cloud</groupId>
</dependency>
The example below shows a minimal Eureka Server application.
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@EnableEurekaServer
@SpringBootApplication
public class RegistryApplication {
public static void main(String[] args) {
SpringApplication.run(RegistryApplication.class, args);
}
}
Next, we add the service registry configuration to service-registry/src/main/resources/application.yml.
server.port: 8761
spring:
cloud.config.enabled: false
# Standalone Eureka Server
eureka:
instance:
hostname: localhost
client:
register-with-eureka: false
fetch-registry: false
serviceUrl:
defaultZone: https://${eureka.instance.hostname}:${server.port}/eureka/
We configure the Eureka Server in standalone mode by setting eureka.client.register-with-eureka=false and eureka.client.fetch-registry=false. This switches off the client side behavior so that it does not keep trying and failing to reach its peers.
Once you run the Eureka Server, you can open https://localhost:8761/ in your browser.
As you can see, no instances have yet been registered.
Order Service, the Eureka Client
When a client registers with Eureka, it provides meta-data such as host, port, health indicator URL and other details.
To include the Eureka Client in our project, let’s add spring-cloud-starter-netflix-eureka-client and spring-boot-starter-actuator to order-service/pom.xml.
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
The example below shows a minimal Spring Boot application.
@SpringBootApplication
public class OrdersApplication {
public static void main(String[] args) {
SpringApplication.run(OrdersApplication.class, args);
}
@Bean
RouterFunction<ServerResponse> orders() {
return route()
.POST("/orders", request -> ServerResponse.accepted().build())
.build();
}
}
Here, we expose the /orders endpoint which always returns 202 Accepted as a response.
Next, we add the self-registration configuration to order-service/src/main/resources/application.yml.
server.port: 8585
spring:
application.name: order-service
eureka:
server:
host: localhost:8761
client:
serviceUrl:
defaultZone: https://${eureka.server.host}/eureka
register-with-eureka: true
fetch-registry: false
# actuator
management:
endpoints.web.exposure.include: health,info
info.env.enabled: true
info:
application.name: ${spring.application.name}
The default status page and status indicators of a Eureka instance are /info and /health respectively, which are the default locations of useful endpoints in a Spring Boot Actuator application.
API Gateway, the Discovery Client
Once you have an application instance in the service registry, you can use the discovery client to look for service instances on the Eureka Server.
First, let’s add spring-cloud-starter-netflix-eureka-client and spring-boot-starter-webflux to api-gateway/pom.xml.
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-webflux</artifactId>
</dependency>
Next, we add the client-side discovery configuration to api-gateway/src/main/resources/application.yml.
server.port: 9090
eureka:
server:
host: localhost:8761
client:
serviceUrl:
defaultZone: https://${eureka.server.host}/eureka
register-with-eureka: false
fetch-registry: true
In the example below, we create a @Configuration class where we set up a load-balanced instance of WebClient.
@Configuration
class WebClientConfiguration {
@Bean
@LoadBalanced
WebClient webClient() {
return WebClient.builder().build();
}
}
We use the native com.netflix.discovery.EurekaClient as the service discovery client to get available service instances, as shown below.
@RestController
public class OrdersServiceProxy {
private static final String ORDER_SERVICE = "order-service";
private final WebClient webClient;
private final EurekaClient discoveryClient;
public OrdersServiceProxy(WebClient webClient, EurekaClient discoveryClient) {
this.webClient = webClient;
this.discoveryClient = discoveryClient;
}
@PostMapping("v1/orders")
public Mono<ResponseEntity<Void>> ordersV1(Order order) {
String baseUrl =
discoveryClient.getNextServerFromEureka(ORDER_SERVICE, false)
.getHomePageUrl();
return webClient.post()
.uri(baseUrl + "/orders")
.body(Mono.just(order), Order.class)
.retrieve()
.toBodilessEntity();
}
}
Thanks for reading. I hope this was helpful!
The example code is available on GitHub.