Eureka is REST based application service, That primarily used for service register and middle-tier api load balancing.

Why Eureka required ?

There is many reason where we can consider eureka
– Service registry
– Client side load balancing
– Peer to peer connectivity between server
– Maintain self preservation state in case network collapse at certain threshold
– Scope of customisation
– Mid-tier load balancing

How Eureka work?

Eureka come up with two components. Eureka client and Eureka server. Any application who doing service discovery on Eureka server should have Eureka client enabled. actually if we complete server setup there is three application that came into picture –

• Eureka server – that hold the client details and do mid-tier load balancing
• Application client – That is Eureka client that called to other services.
• Application services – That is also Eureka client but called by other services.

Let’s understand with example – Suppose we have web-application that have two services. one web client that hold front end implementation. and other backend services that hold business logics. here both backend and frontend is kind of client for Eureka server and both add eureka client component so they can send their heart bit to eureka server. and In this scenarios eureka server maintain registry of both application(web client and backend services). also web client not required to directly called to backend services. that can call to eureka server. where eureka server redirect their call to specific instance as per availability. Here Eureka server use Round-Robin algorithm to redirect request of client.

Eureka server client communication and stats ?

Register – Eureka client registers the information about the running instance to the Eureka server.
Renew – Eureka client needs to renew the lease by sending heartbeats every 30 seconds. The renewal informs the Eureka server that the instance is still alive. If the server hasn’t seen a renewal for 90 seconds, it removes the instance out of its registry. It is advisable not to change the renewal interval since the server uses that information to determine if there is a wide spread problem with the client to server communication.
Fetch registry – Eureka clients fetches the registry information from the server and caches it locally. After that, the clients use that information to find other services
Cancel – Eureka client sends a cancel request to Eureka server on shutdown. This removes the instance from the server’s instance registry thereby effectively taking the instance out of traffic.
Time lag– All operations from Eureka client may take some time to reflect in the Eureka servers and subsequently in other Eureka clients. This is because of the caching of the payload on the eureka server which is refreshed periodically to reflect new information. Eureka clients also fetch deltas periodically. Hence, it may take up to 2 mins for changes to propagate to all Eureka clients.

Let’s go through example ,
Here i’m going create these applications.
1. Eureka server – that will register all the services.
2. Application Service – This will backend application called by client but its also registered with Eureka as client
3. Application Client – this will be client application that Called Application services via eureka server.

Eureka Server

I would suggest you to visit – Spring initialiser and generate Spring application from there and don’t forget to include Eureka server dependencies. for more details please visit this page to know more details.

Once you imported the project into your IDE. then go to resource folder and open application.properties/yml files. defined below bare minimum properties to make your server up and visible.

//YAML format
server:
    port: 8761

eureka:
    instance:
        hostname: localhost
    client:
        fetch-registry: false
        register-with-eureka: false
        serviceUrl:
            defaultZone: http://${eureka.instance.hostname}:${server.port}/eureka/
spring:
    freemarker:
        prefer-file-system-access: false
        template-loader-path: classpath:/templates/
-------------------------------------------------------------------------
OR
// properties format

eureka.instance.hostname=localhost
eureka.client.fetch-registry=false
eureka.client.register-with-eureka=false
eureka.client.serviceUrl.defaultZone=http://${eureka.instance.hostname}:${server.port}/eureka/
server.port=8761
spring.freemarker.prefer-file-system-access=false
spring.freemarker.template-loader-path=classpath:/templates/

server.port – Server port details, Here we require to define unique port no.
eureka.client.fetch-registry – mark this as false so it will not try to fetch registry details as client
eureka.client.register-with-eureka – So it will not register them self
eureka.client.serviceUrl.defaultZone – Define default zone address for the client, so client connect at this address
spring.freemarker.template-loader-path – as UI page is by default included with Eureka server. so pointed template path in class path incase if by default not detected
spring.freemarker.prefer-file-system-access – there is no need to read local file system.

Now open application.java files and enable Eureka server using @EnableEurekaServer annotation.Then start server

@EnableEurekaServer
@SpringBootApplication
public class EurekaserverApplication {

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

Once server up, now visit on this link – http://localhost:8761, you will see similar dashboard as below,

Here there is no application registered with Eureka server now. Lets create Eureka Client.

Eureka Client

Same as Eureka server we require to generate project from Spring Initialiser , Once project generated and open into IDE, we require to edit application.properties/yml file.

spring:
  application:
    name: eureka-service-client

server:
  port: 8082

eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka
---------------------------------------------------------------------
OR
eureka.client.serviceUrl.defaultZone=http://localhost:8761/eureka
server.port=8082
spring.application.name=eureka-service-client

eureka.client.serviceUrl.defaultZone – Default zone for eureka client to register
server.port – server port
spring.application.name – named the application the same will be visible to eureka server

Open application.java files and @EnableDiscoveryClient.

@SpringBootApplication
@EnableDiscoveryClient
public class EurekaclientApplication {

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

}

Now start the client server and check on Eureka if they are visible.