Docker: Multiple services in docker compose

So far we have only seen one service being run inside docker compose. In this exercise we will define two services, that can communicate with each other, and be started using a single command.

We will define a spring boot application and a MySQL database.

Goal:

Have a MySQL and Spring Boot application running together using Docker Compose.

Step 2: Adding docker compose support

We will reuse the docker-compose.yml file from the previous exercise and add a new service for the Spring Boot application.

services:
  db:
    image: mysql:8.0
    environment:
      MYSQL_ROOT_PASSWORD: root
      MYSQL_DATABASE: mydb
      MYSQL_USER: user
      MYSQL_PASSWORD: secret
    ports:
      - "3307:3306"
    volumes:
      - db_data:/var/lib/mysql

  app:
    build:
      context: .
      dockerfile: Dockerfile
    environment:
      SPRING_DATASOURCE_URL: jdbc:mysql://db:3306/mydb
      SPRING_DATASOURCE_USERNAME: user
      SPRING_DATASOURCE_PASSWORD: secret
      SPRING_JPA_HIBERNATE_DDL_AUTO: update
    ports:
      - "8080:8080"
    depends_on:
      - db

volumes:
  db_data:

To start the services, use the following command to start the containers in detached mode:

docker compose up -d

To check if everything is running, list the containers:

docker ps -a

You will notice that the Spring Boot application has exited with an error. To inspect the logs, use the following command:

docker logs <app_container_id>

Going through the logs, you will notice the following exception:

 org.hibernate.exception.JDBCConnectionException: unable to obtain isolated JDBC connection [Communications link failure
The last packet successfully received from the server was 0 milliseconds ago. The driver has not received any packets from the server.]

Spring Data JPA is trying to create a connection to the database, but it is failing because the database is not ready yet.

We can try to restart only the Spring Boot application service using the following command:

docker compose up -d app

Check the logs again:

docker logs <app_container_id>

You should see something similar to this:

---------------
Initializing Database using CommandLineRunner
---------------
DB URL: jdbc:mysql://db:3306/mydb
Active Profiles: default
---------------

This means that the Spring Boot application has successfully established a connection to the MySQL database.

Step 3: Adding a health check

First stop the two containers by running:

docker compose down -v

To make sure that the database is ready before the Spring Boot application starts, we can add a health check to the MySQL service in the docker-compose.yml file.

services:
  db:
    image: mysql:8.0
    environment:
      MYSQL_ROOT_PASSWORD: root
      MYSQL_DATABASE: mydb
      MYSQL_USER: user
      MYSQL_PASSWORD: secret
    ports:
      - "3307:3306"
    volumes:
      - db_data:/var/lib/mysql
    healthcheck:
      test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]
      interval: 20s
      retries: 5

  app:
    build:
      context: .
      dockerfile: Dockerfile
    environment:
      SPRING_DATASOURCE_URL: jdbc:mysql://db:3306/mydb
      SPRING_DATASOURCE_USERNAME: user
      SPRING_DATASOURCE_PASSWORD: secret
      SPRING_JPA_HIBERNATE_DDL_AUTO: update
    ports:
      - "8080:8080"
    depends_on:
      db:
        condition: service_healthy

volumes:
  db_data:

The healthcheck section defined under the db service, defines how to check if the MySQL database is ready to accept connections. The command test: ["CMD", "mysqladmin", "ping", "-h", "localhost"] is the command that will be run to check the health of the database. timeout is the maximum time to wait for the command to complete, and retries is the number of times to retry the command before considering the service unhealthy.

The depends_on section in the app service ensures that the Spring Boot application will only start after the MySQL database is considered healthy (we defined that under the healthcheck section).

Step 4: Tear down

Run the following command to stop and remove the containers, and volumes defined in the docker-compose.yml file:

docker compose down -v