Let's start with the app.yml file under src/main/docker, which can be found inside your gateway application.

As we saw at the beginning of this chapter, the file starts with the Docker version that it supports:

version: '2'

This is followed by the services section, where various services, applications, or components that we will kick start with this Docker Compose file are defined. 

Under the services section, we have a name for the service – in our case, store-app – followed by the image that is used as the container. This image is generated with the help of JIB from our Gradle build.

This is followed by a series of environment variables that our application will depend on, including the following:

• SPRING_PROFILES_ACTIVE: This tells the application to run in production mode and expose Swagger endpoints.
• EUREKA_CLIENT_SERVICE_URL_DEFAULTZONE: This tells the application where to check for the JHipster Registry (which is the Eureka client that we are using. If we have chosen Consul here, then the application will point to the Consul URL).
• SPRING_CLOUD_CONFIG_URI: This tells the application where to look for the config service for the application.
• SPRING_DATASOURCE_URL: This tells the application where to look for the data source.
• JHIPSTER_SLEEP: This is the custom property that we have used to make sure that the JHipster Registry starts before the application starts up.

  store-app:
    image: store
    environment:
      - _JAVA_OPTIONS=-Xmx512m -Xms256m
      - SPRING_PROFILES_ACTIVE=prod,swagger
      - MANAGEMENT_METRICS_EXPORT_PROMETHEUS_ENABLED=true
      - EUREKA_CLIENT_SERVICE_URL_DEFAULTZONE=...
      - SPRING_CLOUD_CONFIG_URI=...
      - SPRING_DATASOURCE_URL=..
      - JHIPSTER_SLEEP=30 # gives time for other services to boot before the application
    ports:
      - 8080:8080

We have seen how the service is defined with the Docker Compose file. Now, let's look at two other services that are needed for our application to run. These are the database and JHipster Registry.

So, we have another service called store-mysql, which creates and starts the MySQL server. Since we already have MySQL as a separate Docker Compose file, it is linked here with an extends keyword, followed by the Docker Compose file and the service that we have to start from the specified file:

  store-mysql:
    extends:
      file: mysql.yml
      service: store-mysql

The compose file, mysql.yml, contains the following code:

version: '2'
services:
  store-mysql:
    image: mysql:8.0.18
    # volumes:
    # - ~/volumes/jhipster/store/mysql/:/var/lib/mysql/
    environment:
      - MYSQL_USER=root
      - MYSQL_ALLOW_EMPTY_PASSWORD=yes
      - MYSQL_DATABASE=store
    ports:
      - 3306:3306
    command: mysqld --lower_case_table_names=1 --skip-ssl --character_set_server=utf8mb4 --explicit_defaults_for_timestamp

This started with the version of Docker Compose, followed by the services keyword, and then specified the service name, store-mysql, that is used in the app.yml file.

This is followed by a set of environment variables, such as the username and the password (it is set to empty here, but for a real production application, it is recommended to set a more complex password), and then the database schema name.

The command that needs to run in order to start the MySQL server is defined next. Now, we need to go back to the app.yml file and look at the JHipster Registry service. This is again extending the jhipster-registry.yml file. One more thing to note here is that even though we extend the services from another Docker Compose file, we can override the environment variables that we specified in the original Docker Compose file. This comes in handy in certain cases where we have to kickstart our application with different or customized values. In this case, the Spring Cloud Config server file location is overridden:

  jhipster-registry:
    extends:
      file: jhipster-registry.yml
      service: jhipster-registry
    environment:
      - SPRING_CLOUD_CONFIG_SERVER_COMPOSITE_0_TYPE=native
      - SPRING_CLOUD_CONFIG_SERVER_COMPOSITE_0_SEARCH_LOCATIONS=file:
  ./central-config/docker-config/

The Jhipster-registry.yml file is as follows:

version: '2'
services:
  jhipster-registry:
    image: jhipster/jhipster-registry:v5.0.2
    volumes:
      - ./central-server-config:/central-config
    environment:
      - _JAVA_OPTIONS=-Xmx512m -Xms256m
      - SPRING_PROFILES_ACTIVE=dev,swagger
      - SPRING_SECURITY_USER_PASSWORD=admin
      - JHIPSTER_REGISTRY_PASSWORD=admin
      - SPRING_CLOUD_CONFIG_SERVER_COMPOSITE_0_TYPE=native
      - SPRING_CLOUD_CONFIG_SERVER_COMPOSITE_0_SEARCH_LOCATIONS=file:
  ./central-config/localhost-config/
      # - SPRING_CLOUD_CONFIG_SERVER_COMPOSITE_0_TYPE=git
      # - SPRING_CLOUD_CONFIG_SERVER_COMPOSITE_0_URI=https://github.com/
  jhipster/jhipster-registry/
      # - SPRING_CLOUD_CONFIG_SERVER_COMPOSITE_0_SEARCH_PATHS=central-config
    ports:
      - 8761:8761

# Common configuration shared between all applications
configserver:
    name: Docker JHipster Registry
    status: Connected to the JHipster Registry running in Docker
jhipster:
    security:
        authentication:
            jwt:
                secret: my-secret-token-to-change-in-production
eureka:
    client:
        service-url:
            defaultZone: http://admin:${jhipster.registry.
              password}@localhost:8761/eureka/