home / 2018.03.26 19:01 /java /cloud /microservices /spring boot /spring cloud netflix

Java Cloud Apps Workshop - Connecting them all

Now that we have some microservices, we need to look at a simple way to make them interact with each other and give rise to the functionality we expect from our system. We want our system to have a few features to make our work easier:

• services should be able to find each other easily
• system should allow us to start and stop multiple instances of the same service
• system should load-balance communication between services automatically
• system should have tools that allow us to inspect the running services, their state and communication between them

The supervisor - Eureka

To bring all the above mentioned functionality into our project we’ll rely on the Spring Cloud Netflix project. The first part of this project we’ll use is the service registration and discovery tool Eureka. Once we have an Eureka server set up, our services will be able to communicate with it it, to register themselves with it and to obtain URLs for services they need to work with from Eureka.

First, run over to the Spring Initializr and generate a project named com.msdm.eureka with Eureka Server dependency. Download it and copy it into your workspace. Then, open the project and add the @EnableEurekaServer annotation to the main application class EurekaApplication. Also, add the following to your application.properties file:

eureka.client.register-with-eureka=false
eureka.client.fetch-registry=false
server.port=8761

Now you can start your service and navigate to localhost:8761 in your browser to see that you have a running Eureka server.

Next, we need to make a few changes to the pom.xml file in one of your other microservices, let’s say the users service:

[...]
<properties>
	[...]
    <spring-cloud.version>Dalston.SR4</spring-cloud.version>
</properties>
<dependencies>
    [...]
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-eureka</artifactId>
    </dependency>
</dependencies>
<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>${spring-cloud.version}</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>
[...]

We need to ensure that the Eureka client dependency is downloaded for a specific spring cloud version for the required @EnableDiscoveryClient annotation to be available in your project. You need to add that annotation to you microservice application class. You also need to add a line with the name of the microservice in your application.properties file. For the users service, this line would be: spring.application.name=users-service. Now you should start your users service, then navigate to localhost:8761. Give it a minute and your new service should now show up as running. You will need to make these changes for all the microservices we have created in the previous sections of this workshop and start them up to see them connect to the Eureka server:

Injecting services

Now we need to refactor our executer service, the one service that communicates with other services in the system. We want our executer service to be able to automatically find and use other services in the system automatically, by looking at what services are registered in Eureka. We will be referring to services by name, from now on, instead of by IP and port.

We will start by adding a few dependencies to the executer pom.xml, the xml file will look like this in the end (the versions your spring cloud and spring boot will have may be different depending on when you used Spring Initializr to generate your projects):

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>

	<groupId>com.msdm</groupId>
	<artifactId>executer</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>jar</packaging>

	<name>executer</name>
	<description>Demo project for Spring Boot</description>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>1.5.8.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>

	<properties>
		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
		<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
		<java.version>1.8</java.version>
		<spring-cloud.version>Dalston.SR4</spring-cloud.version>
	</properties>

	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-actuator</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.cloud</groupId>
			<artifactId>spring-cloud-starter-eureka</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.cloud</groupId>
			<artifactId>spring-cloud-starter-ribbon</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

	<dependencyManagement>
		<dependencies>
			<dependency>
				<groupId>org.springframework.cloud</groupId>
				<artifactId>spring-cloud-dependencies</artifactId>
				<version>${spring-cloud.version}</version>
				<type>pom</type>
				<scope>import</scope>
			</dependency>
		</dependencies>
	</dependencyManagement>

	<build>
		<plugins>
			<plugin>
				<groupId>org.springframework.boot</groupId>
				<artifactId>spring-boot-maven-plugin</artifactId>
			</plugin>
		</plugins>
	</build>

</project>

Some new appearances are actuator and ribbon. The one we are interested in eight now is Ribbon, a client-side load balancer, fully integrated with RestTemplate. All we need to do to enable RestTemplate to find services by name, and load-balance calls to a service in case we have multiple instances of that services, is to add a @LoadBalanced annotation in our code, where RestTemplate is created, in ExecuterApplication

@LoadBalanced
@Bean
public RestTemplate restTemplate() {
	return new RestTemplate();
}

We also need to change the values used to refer to our services from IP and port to the service name. To make our properties more manageable, we will first change the extension of our properties file, renaming it from application.properties to application.yml. Once we have done that, we can rewrite its contents in YAML format:

server:
  port: 8092

analysis:
  url: http://analysis-service
  endpoint:
    analyses: analyses
    scheduler: scheduler
    failure: result/failed
    success: result/success

file:
  url: http://files-service
  endpoint:
    download: download

spring:
  application:
    name: executer-service

eureka:
  instance:
    leaseRenewalIntervalInSeconds: 1
    leaseExpirationDurationInSeconds: 2
  client:
    serviceUrl:
      defaultZone: http://127.0.0.1:8761/eureka/
    healthcheck:
      enabled: true
    lease:
      duration: 5

management:
  port: 8093
  context-path: /details
  security:
    enabled: false

As you can observe, we no longer refer to the analysis service URL as http://localhost:8091, we have replaced it with http://analysis-service, and we have done the same with the file service. If we now run our system and submit a new analysis to the analysis service, we will see that the executer service is able to retrieve and execute the analysis, communicating with the analysis and file service without any problems.

And that actuator dependency we also added in the POM file? It’s Spring Boot’s way of offering us a set of diagnosis tools for our microservice. It will make available a set of URLs that retrieve operation data about your microservice, like:

• http://localhost:8093/details/health for a general health overview
• http://localhost:8093/health/dump for a memory dump
• http://localhost:8093/health/beans to see a list of instantiated beans

And others. It has its own property entries in the YAML file, and in our example we are running without any security, on a different port than our main server and at the /details context path.