The Master Class of "Learn Spring Security" is out:
1. Overview
In this tutorial, we’ll introduce client-side service discovery via “Spring Cloud Netflix Eureka“.
Client-side service discovery allows services to find and communicate with each other without hardcoding hostname and port. The only ‘fixed point’ in such an architecture consists of a service registry with which each service has to register.
A drawback is that all clients must implement a certain logic to interact with this fixed point. This assumes an additional network round trip prior to the actual request.
With Netflix Eureka each client is able to simultaneously act as a server, to replicate its status to a connected peer. In other words, a client retrieves a list with all connected peers of a service registry and makes all further requests to any other services through a load-balancing algorithm.
To be informed about the presence of a client, they have to send a heartbeat signal to the registry.
To achieve the goal of this write-up, we will implement 3 microservices:
- a service registry (Eureka Server),
- a REST service which registers itself at the registry (Eureka Client) and
- a web-application, which is consuming the REST service as registry-aware client (Spring Cloud Netflix Feign Client).
2. Eureka Server
To implement a Eureka Server for using as service registry is as easy as: adding spring-cloud-starter-eureka-server to the dependencies, enable the Eureka Server in a @SpringBootApplication per annotate it with @EnableEurekaServer and configure some properties. But we’ll do it step by step.
Firstly we’ll create a new Maven project and put the dependencies into it. You have to notice that we’re importing the spring-cloud-starter-parent to all projects described in this tutorial:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-eureka-server</artifactId>
<version>1.1.5.RELEASE</version>
</dependency>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-parent</artifactId>
<version>Brixton.SR4</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
Next, we’re creating the main application class:
@SpringBootApplication
@EnableEurekaServer
public class EurekaServerApplication {
public static void main(String[] args) {
SpringApplication.run(EurekaServerApplication.class, args);
}
}
Finally, we’re configuring the properties in YAML format; so application.yml will be our configuration file:
server:
port: 8761
eureka:
client:
registerWithEureka: false
fetchRegistry: false
Here we’re configuring an application port – 8761 is the default one for Eureka servers. We are telling the built-in Eureka Client not to register with ‘itself’, because our application should be acting as a server.
Now we will point our browser to http://localhost:8761 to view the Eureka dashboard, where we will later inspecting the registered instances.
At the moment, we see basic indicators such as status and health indicators.
3. Eureka Client
For a @SpringBootApplication to be discovery-aware, we have to include some Spring Discovery Client (for example: spring-cloud-starter-eureka) into our classpath.
Than we need to annotate a @Configuration with either @EnableDiscoveryClient or @EnableEurekaClient.
The latter tells Spring Boot to explicitly use Spring Netflix Eureka for service discovery. To fill our client application with some sample-life, we’ll also include the spring-boot-starter-web package in the pom.xml and implement a REST controller.
But first, we will add the dependencies:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-eureka</artifactId>
<version>1.1.5.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>1.4.0.RELEASE</version>
</dependency>
Here we will implement the main application class:
@SpringBootApplication
@EnableEurekaClient
@RestController
public class EurekaClientApplication {
public static void main(String[] args) {
SpringApplication.run(EurekaClientApplication.class, args);
}
@RequestMapping("/greeting")
public String greeting() {
return "Hello from EurekaClient!";
}
}
Next, we have to set-up an application.yml with a configured Spring application name to uniquely identify our client in the list of registered applications.
We are able to let Spring Boot choose a random port for us, because later we are accessing this service with its name, and finally we have to tell our client, were it has to locate the registry:
spring:
application:
name: spring-cloud-eureka-client
server:
port: 0
eureka:
client:
serviceUrl:
defaultZone: ${EUREKA_URI:http://localhost:8761/eureka}
instance:
preferIpAddress: true
When we decided to setup our Eureka Client this way, we had in mind that this kind of service should later be easily scalable.
Now we will run the client and point our browser to http://localhost:8761 again, to see its registration status on the Eureka Dashboard. By using the Dashboard we can do further configuration e.g. link the homepage of a registered client with the Dashboard for administrative purposes. The configuration options however, are beyond the scope of this article.
4. Feign Client
To finalize our project with 3 dependent microservices, we will now implement a REST-consuming web-application using Spring Netflix Feign Client.
Think of Feign as discovery-aware Spring RestTemplate using interfaces to communicate with endpoints. This interfaces will be automatically implemented at runtime and instead of service-urls, it is using service-names.
Without Feign we would have to autowire an instance of EurekaClient into our controller with which we could receive a service-information by service-name as Application object.
We would use this Application to get a list of all instances of this service, pick a suitable one and use this InstanceInfo to get hostname and port. With this we could do a normal request with any http client.
For example:
@Autowired
private EurekaClient eurekaClient;
public void doRequest() {
Application application = eurekaClient.getApplication("spring-cloud-eureka-client");
InstanceInfo instanceInfo = application.getInstances().get(0);
String hostname = instanceInfo.getHostName();
int port = instanceInfo.getPort();
...
}
A RestTemplate can also be used to access Eureka client-services by name, but this topic is beyond this write-up.
To setup our Feign Client project, we’re going to add the following 4 dependencies to its pom.xml:
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-feign</artifactId>
<version>1.1.5.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-eureka</artifactId>
<version>1.1.5.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>1.4.0.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-thymeleaf</artifactId>
<version>1.4.0.RELEASE</version>
</dependency>
The Feign Client is located in the spring-cloud-starter-feign package. To enable it, we have to annotate a @Configuration with @EnableFeignClients. To use it, we simply annotate an interface with @FeignClient(“service-name”) and auto-wire it into a controller.
A good method to create such Feign Clients is to create interfaces with @RequestMapping annotated methods and put them into a separate module. This way they can be shared between server and client. On server-side you can implement them as @Controller and on client-side they can be extended and annotated as @FeignClient.
Furthermore the spring-cloud-starter-eureka package needs to be included in the project and enabled by annotate the main application class with @EnableEurekaClient.
The spring-boot-starter-web and spring-boot-starter-thymeleaf dependencies are used to present a view, containing fetched data from our REST service.
This will be our Feign Client interface:
@FeignClient("spring-cloud-eureka-client")
public interface GreetingClient {
@RequestMapping("/greeting")
String greeting();
}
Here we will implement the main application class which simultaneously acts as controller:
@SpringBootApplication
@EnableEurekaClient
@EnableFeignClients
@Controller
public class FeignClientApplication {
@Autowired
private GreetingClient greetingClient;
public static void main(String[] args) {
SpringApplication.run(FeignClientApplication.class, args);
}
@RequestMapping("/get-greeting")
public String greeting(Model model) {
model.addAttribute("greeting", greetingClient.greeting());
return "greeting-view";
}
}
This will be the HTML template for our view:
<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
<title>Greeting Page</title>
</head>
<body>
<h2 th:text="${greeting}"/>
</body>
</html>
At least the application.yml configuration file is almost the same as in the previous step:
spring:
application:
name: spring-cloud-eureka-feign-client
server:
port: 8080
eureka:
client:
serviceUrl:
defaultZone: ${EUREKA_URI:http://localhost:8761/eureka}
Now we are able to build and run this service. Finally we’ll point our browser to http://localhost:8080/get-greeting and it should display something like the following:
Hello from EurekaClient!
5. Conclusion
As we’ve seen, we’re now able to implement a service registry using Spring Netflix Eureka Server and register some Eureka Clients with it.
Because our Eureka Client from step 3 listens on a randomly chosen port, it doesn’t know its location without the information from the registry. With a Feign Client and our registry, we are able to locate and consume the REST service, even when the location changes.
Finally we’ve got a big picture about using service discovery in a microservice architecture.
As usual, you’ll find the sources on GitHub, which includes also a set of Docker-related files for using with docker-compose to create containers from our project.