Microservices with Spring Boot - complete tutorial

In this tutorial we are going to learn how to develop microservices using spring boot with examples. Main focus of this tutorial is on learning by doing hands-on. Before hands-on we will first understand what is microservices and related terminologies like DDD, 12-Factors App, Dev Ops.

What is a Microservice

In simple terms microservice is a piece of software which has a single responsibility and can be developed, tested & deployed independently. In microservices we focus on developing independent and single fully functioning modules.
Opposite to microservice, with monolithic application it focuses on all the functionality or modules in a single application. So when any changes required to monolithic application it has to deploy and test the complete application while with microservice it has to develop and deploy only affected component which is a small service. It saves lot of development and deployment time in a large application.
It's basically an architectural style which can be followed to develop an application as a small service which is independent or may be interdependent (talking to other service to complete the job).

Designing Microservice

While designing application developers and domain experts should speak the same language so both have the same understanding. When we design a microservice we need to follow specific approach/ patterns, for example: DDD. When developing microservice then we need to make sure that our microservice implements certain attributes, for example: 12-factors. And then we can have other common design pattern/principles for  application development.

1. What is Domain driven design (DDD)

DDD is a top to bottom design approach similar to OOAD which helps to reflect the business domain into software. It helps defining the domains, elements, their relationship, boundary and communication to replicate the real business. That's why we say it focuses on the business domain and good domain knowledge is required to design the system perfectly. Having complete domain knowledge is good for any application but in my experience I have seen when developers know only the portion of application where they are working actually. But when we design system in small pieces it becomes very important to understand the whole system and how these pieces will communicate with each other where they reflects a unit of the real business. A proper understanding helps in designing the clear interfaces for behaviour and communication between components.

2. What are the Twelve-Factors

Twelve factors are defined as a must have attributes for any service to be developed as "Software as a Service". These attributes are well suited for microservices. While developing a microservice we can design and test it against these factors to verify the various aspect of a microservice.
Below are the twelve factors which I have copied from Wikipedia for your ease.

#	Factor	Description
I	Codebase	There should be exactly one codebase for a deployed service with the codebase being used for many deployments.
II	Dependencies	All dependencies should be declared, with no implicit reliance on system tools or libraries.
III	Config	Configuration that varies between deployments should be stored in the environment.
IV	Backing services	All backing services are treated as attached resources and attached and detached by the execution environment.
V	Build, release, run	The delivery pipeline should strictly consist of build, release, run.
VI	Processes	Applications should be deployed as one or more stateless processes with persisted data stored on a backing service.
VII	Port binding	Self-contained services should make themselves available to other services by specified ports.
VIII	Concurrency	Concurrency is advocated by scaling individual processes.
IX	Disposability	Fast startup and shutdown are advocated for a more robust and resilient system.
X	Dev/Prod parity	All environments should be as similar as possible.
XI	Logs	Applications should produce logs as event streams and leave the execution environment to aggregate.
XII	Admin Processes	Any needed admin tasks should be kept in source control and packaged with the application.

This twelve factors attribute specifies almost most of the thing to be taken care for microservices development but still there are several patterns which can be applied as per business requirement. For example Database per Service pattern also can be used which says each microservice should have it's own database and it must not be shared by other services.

Developing Microservice

As a part of microservice development, we will create a microservice, cloud config server, service registry or discovery server and API gateway. Below architecture depicts the implementation of all components in our example application.

Now in further sections we will see the development of all components mentioned in above architecture.

1. API Gateway

API gateway provides a single entry point to all associated microservices, either directly or through discovery server. It's very useful to access group of services from different providers. There are many other things we can do at API gateway like, auhtentication/authorization, running interceptor codes, filters, logging etc.
However in this example we are using only single microservice with API gateway but we can register multiple microservices at a time.
Please refer below post for detailed tutorial and source code.
Spring cloud API Gateway

2. Discovery Server (Service Registry)

Discovery server provides capability to microservices to register themselves and allows clients to discover these microservices.
Please refer below post for detailed tutorial and source code.
Registry/ discovery server using Spring boot and Eureka


3. User Management Service (Microservice)

User management service is a REST based microservice and have below dependencies to run the application.

• Cloud config server for configuration management
• RabbitMQ to subscribe the refresh event published by cloud config server
• Discovery server for service registration
• Mongo DB for persistence store
Below are the endpoints published by this service.
• /users: It displays a welcome message
• /users/{ID}: Provides the details for given user ID.
• /users/create: Creates new user and returns user ID.
• /users/listAll: Returns a list of all existing users.
Required code and configurations are as given below.

• Maven dependencies

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-mongodb</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-config</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <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>

• Main class (Spring boot class)

@EnableDiscoveryClient //required to register itself with eureka discovery server
@SpringBootApplication
@ComponentScan(basePackages = {"com.ttj"})
public class UserMgmtServiceApplication {

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

• Configuration (bootstrap.yml)

server.port : 8080
spring:
  application.name: UserManagementService
  cloud:
    config:
#config server details
      name: microservices-config
      uri: http://localhost:8888/
      username: config_user
      password: config_user
#rabbit mq to subscribe to refresh events published by config server
  rabbitmq:
    host: localhost
    port: 5672
    virtual-host: /
    username: guest
    password: guest
  profiles:
    active: DEV
  data:
#mongo DB configuraitons
    mongodb:
      database: microservice
      host: localhost
      port: 27017
      repositories:
        type: auto
      uri: mongodb://localhost:27017/microservice
#Eureka server configurations to register itself
eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8089/eureka

• Domain Object (User)

@Document(collection = "appusers")
public class User {
    @Id
    private String id;
    private String userId;
    private String firstName;
    private String lastName;
    private Integer age;

    public User() {}
    public User(String userId, String firstName, String lastName, Integer age) {
        super();
        this.userId = userId;
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }
    //getter methods
    //setter methods

    @Override
    public String toString() {
        return userId+", "+firstName+", "+lastName+", "+age;
    }
}

• Repository Interface

public interface UserRepository extends MongoRepository<User, String> {
    @Query("{ 'userId' : ?0 }")
    public User findByUserId(String userId);
}

• Service Implementation (Business Logic)

@Service
public class UserMgmtServiceImpl implements UserMgmtService{
    @Autowired
    UserRepository userRepo;

    @Override
    public void saveUser(User user) {
        userRepo.save(user);
    }

    @Override
    public User findByUserId(String userId) {
        return userRepo.findByUserId(userId);
    }

    @Override
    public void deleteUser(String userId) {
        User user = userRepo.findByUserId(userId);
        userRepo.delete(user);
    }

    @Override
    public List listAllUsers(){
        return userRepo.findAll();
    }
}

• Rest Service Endpoints

@RestController
@RequestMapping("/users")
@RefreshScope
public class UserMgmtEndpoint {

    @Autowired
    private UserMgmtService userMgmtService;

    @Value("${message}")
    private String message;

    @RequestMapping(method = RequestMethod.GET, produces = "application/json")
    public String welcome() {
        return message;
    }

    @RequestMapping(value="/{userId}", method = RequestMethod.GET, produces = "application/json")
    public User findUserById(@PathVariable("userId") String userId) {
        User user = userMgmtService.findByUserId(userId);
        return user;
    }

    @RequestMapping(value = "/create", method = RequestMethod.POST, consumes = "application/json")
    public String createUser(@RequestBody User user) {
        userMgmtService.saveUser(user);
        return "UserId: "+user.getId();
    }

    @RequestMapping(value = "/listAll", method = RequestMethod.GET, produces = "application/json")
    public List<User> listUsers() {
        return userMgmtService.listAllUsers();
    }
}

Source Code

Complete source code for this microservice is available in below Git location.
https://github.com/thetechnojournals/microservices/tree/master/user-mgmt-service


4. Cloud Config Server (Configuration management)

Cloud Config server is used here to provide the distributed configuration management. Client applications connect to config server and access related configurations. To develop a microservice, configurations must be stored in separate place and available on demand. Using config server microservices can subscribe to config server and pull the latest configurations immediately after any config change in config server.
For complete implementation and more details please refer below post on cloud config server.
Cloud config server implementation with example 

Running and Testing service

To start the service we need to run it in below order.

1. Start RabbitMQ and Cloud config server
2. Start Discovery server
3. Start MongoDB and Microservice
4. Start API Gateway

Now you can enter below URL to test the service directly.
http://localhost:8080/users
To test the service through API gateway and discovery server, you need to open below URL in browser.
http://localhost:9999/users/

Both of the above URLs will give you same results.