Building microservices with Netflix OSS, Apache Kafka and Spring Boot – Part 3: Email service and Gateway

After Building microservices with Netflix OSS, Apache Kafka and Spring Boot – Part 1: Service registry and Config server and Building microservices with Netflix OSS, Apache Kafka and Spring Boot – Part 2: Message Broker and User service here is what comes next: Email Service Next step with building our system is the email service. It will […]

by Iskren Ivanov

November 21, 2017

5 min read

Building microservices with Netflix OSS Apache kafka and Spring Boot1 - Building microservices with Netflix OSS, Apache Kafka and Spring Boot - Part 3: Email service and Gateway

After Building microservices with Netflix OSS, Apache Kafka and Spring Boot – Part 1: Service registry and Config server and Building microservices with Netflix OSS, Apache Kafka and Spring Boot – Part 2: Message Broker and User service here is what comes next:

Email Service

Next step with building our system is the email service. It will be a microservice configured to listen for the USER_CREATED_TOPIC that comes from the user service. Here we will build an UserDto and will configure the Kafka consumer to transform the incoming payload to it. Similarly to the user microservice here we will have EmailService where the business logic will be executed. This EmailService will be using the UserDto from the payload, will transform it to Mail entity, will save it in the database and will send the mail.

In addition to Eureka Discovery; JPA; H2; Kafka; Config Client; to the new Spring Boot project(ms-mail) in SPRING INITIALIZR it is necessary to add Mail dependency as well. So the pom.xml looks like:

/pom.xml

<dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-config</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-eureka</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.kafka</groupId>
        <artifactId>spring-kafka</artifactId>
    </dependency>
    <dependency>
        <groupId>com.h2database</groupId>
        <artifactId>h2</artifactId>
        <scope>runtime</scope>
    </dependency>
    <dependency>
        <groupId>javax.mail</groupId>
        <artifactId>mail</artifactId>
    </dependency>

Sending mails is configured to use gmail. It is necessary to replace username and password with real values. This is the mail which will be used as a sender of the confirmation.

/ms-mail.yml

mail:
    host: smtp.gmail.com
    port: 587
    username: username
    password: password
    properties.mail.smtp:
      auth: true
      starttls.enable: true

To be able to read the messages from Kafka, we need to configure ConsumerFactory and wrap KafkaListenerContainerFactory. As well as the ProducerFactory it needs some configuration properties to be set. @EnableKafka is needed to enable detection of @KafkaListener annotations on any Spring-managed beans.

/ReceiverConfig.java

@Configuration
@EnableKafka
public class ReceiverConfig {

    @Value("${spring.kafka.bootstrap-servers}")
    private String bootstrapServers;

    @Bean
    public Map<String, Object> consumerConfigs() {
        Map<String, Object> props = new HashMap<>();
        props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
        props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
        props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, JsonDeserializer.class);
        props.put(ConsumerConfig.GROUP_ID_CONFIG, "UserCreatedConsumer");

        return props;
    }

    @Bean
    public ConsumerFactory<String, UserDto> consumerFactory() {
        return new DefaultKafkaConsumerFactory<>(consumerConfigs(), new StringDeserializer(),
                new JsonDeserializer<>(UserDto.class));
    }

    @Bean
    public ConcurrentKafkaListenerContainerFactory<String, UserDto> kafkaListenerContainerFactory() {
        ConcurrentKafkaListenerContainerFactory<String, UserDto> factory =
                new ConcurrentKafkaListenerContainerFactory<>();
        factory.setConsumerFactory(consumerFactory());

        return factory;
    }

    @Bean
    public Receiver receiver() {
        return new Receiver();
    }
}

Then in the method annotated with @KafkaListener we add the logic we want to be invoked when a message is received.

/Receiver.java

private CountDownLatch latch = new CountDownLatch(1);

@KafkaListener(topics = "${spring.kafka.topic.userCreated}")
public void receive(UserDto payload) {
   emailService.sendSimpleMessage(payload);
   latch.countDown();
}

As mentioned above EmailService is the place where we transform the incoming payload, send the email and save it for future reference.

/EmailServiceImpl.java

public class EmailServiceImpl implements EmailService {

    @Override
    public void sendSimpleMessage(UserDto input) {
        try {

            Mail newMail = new Mail();
            newMail.setTo(input.getUsername());
            newMail.setSubject("TestSubject");
            newMail.setText("TestText");

            SimpleMailMessage message = new SimpleMailMessage();
            message.setTo(newMail.getTo());
            message.setSubject(newMail.getSubject());
            message.setText(newMail.getText());

            mailRepository.save(newMail);
            emailSender.send(message);
        } catch (MailException exception) {
            exception.printStackTrace();
        }
    }
}

To test if everything is fine, build and run the project:

  • 1. Verify Service registry (Eureka) is running (https://localhost:8761)
  • 2. Config server (Spring Cloud Config) is running and ms-user and ms-email configuration is available (https://localhost:8888/ms-user/default)
  • 3. Build the project: mvn clean install
  • 4. run java-jar ms-mail-0.0.1-SNAPSHOT.jar
  • 5. Create new user with the following:
POST https://localhost:8081/member 
{
"username": "email@example.com",
"password": "password"
}
  • 6. Verify user was created: check if an email was received on email@example.com

Gateway (Zuul)

As you can see from the previous example, to register an user you should know the details about its service (server ip/port) to query it. Imagine a really complex system with dozens of microservices working together. To solve this we will create a new service(Zuul) that will be the front door for all other microservices. Clients will call this microservice and it will delegate the requests to the appropriate one.

Building Zuul service with Spring Boot is as easy as for the previous microservices. With SPRING INITIALIZR create new (ms-gateway) project and Eureka Discovery; Config Client and Zuul dependencies

/prom.xml

<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-starter-config</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-zuul</artifactId>
</dependency>

We configure the microservice Cloud config client

/bootstrap.yml

server:
  port: 8765
spring:
  application:
    name: ms-gateway
  cloud:
    config:
      discovery:
        enabled: true
        service-id: ms-config-server

Add also @EnableEurekaClient to enable EurekaClient configuration as well for the other microservices. The Zuul functionality is enabled with @EnableZuulProxy annotation in the Application.java file.

Some routing configuration is necessary to be added, for the Zuul to know which request to which service should be redirected. We do this in git config file.

/ms-gateway.yml

zuul:
  routes:
    ms-user: /api/user/**

With this configuration any request to 8765:/api/user will be redirected to the ms-user microservice. And Zuul will take its address and port from Eureka.

To test if everything is fine, build and run the project:

  • 1. Create new user
POST https://localhost:8765/api/user/member
{
"username": "email@example.com",
"password": "password"
}
  • 2. Verify an user was created
GET https://localhost:8765/member

We saw how to easily build microservice system which sends emails on registration with Netflix OSS, Apache Kafka and Spring Boot. The full implementation of these 3 parts can be found in the GitHub. Feel free to give your comments and give suggestions based on your experience.

You can find more information on the topic in the next part of this blog:

Building microservices with Netflix OSS, Apache Kafka and Spring Boot – Part 4: Security

Java Expert at Dreamix