docker-compose for services in Spring Boot - Mini Project: Build & Apply

docker-compose for services

📖 Scenario: You are building a simple Spring Boot application that needs to run alongside a database service. To make it easy to start both services together, you will use docker-compose to define and run these services.

🎯 Goal: Create a docker-compose.yml file that defines two services: a Spring Boot app and a PostgreSQL database. The Spring Boot app should connect to the database service using the correct hostname and port.

📋 What You'll Learn

Define a service named app using the Spring Boot Docker image springboot-app:latest

Define a service named db using the official PostgreSQL image postgres:15

Set environment variables for the db service: POSTGRES_USER as user, POSTGRES_PASSWORD as password, and POSTGRES_DB as mydb

Expose port 5432 on the db service

Configure the app service to depend on db

Set environment variables for the app service to connect to the database host db and port 5432

💡 Why This Matters

🌍 Real World

Docker Compose is widely used to run multi-service applications locally or in development environments. It simplifies starting and stopping related services together.

💼 Career

Understanding docker-compose is essential for developers and DevOps engineers to manage containerized applications efficiently, especially when working with microservices or databases.

1

Create the basic docker-compose structure

Create a file named docker-compose.yml and start with the version "3.8" and an empty services section.

Spring Boot

version: "3.8"
services:
  # Your services will go here

Hint

Start your docker-compose.yml with the version and services keys.

2

Add the database service configuration

Under services, add a service named db using the image postgres:15. Set environment variables POSTGRES_USER to user, POSTGRES_PASSWORD to password, and POSTGRES_DB to mydb. Expose port 5432 on the host.

Spring Boot

version: "3.8"
services:
  db:
    # Add image, environment, and ports here

Hint

Use the official PostgreSQL image and set the environment variables exactly as specified.

3

Add the Spring Boot app service with dependency

Add a service named app using the image springboot-app:latest. Make it depend on the db service using depends_on. Set environment variables SPRING_DATASOURCE_URL to jdbc:postgresql://db:5432/mydb, SPRING_DATASOURCE_USERNAME to user, and SPRING_DATASOURCE_PASSWORD to password.

Spring Boot

version: "3.8"
services:
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: mydb
    ports:
      - "5432:5432"
  app:
    # Add image, depends_on, and environment here

Hint

Use depends_on to ensure the app waits for the database. Set the Spring datasource environment variables to connect to the db service.

4

Complete the docker-compose file with network and restart policy

Add a restart policy with value always to the app service. Also, define a network named app-network and connect both app and db services to this network.

Spring Boot

version: "3.8"
services:
  db:
    image: postgres:15
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      POSTGRES_DB: mydb
    ports:
      - "5432:5432"
    networks:
      - app-network
  app:
    image: springboot-app:latest
    depends_on:
      - db
    environment:
      SPRING_DATASOURCE_URL: jdbc:postgresql://db:5432/mydb
      SPRING_DATASOURCE_USERNAME: user
      SPRING_DATASOURCE_PASSWORD: password
    networks:
      - app-network
    # Add restart policy here

networks:
  app-network:
    driver: bridge

Hint

Use the restart key under app and define the app-network at the bottom with driver bridge. Connect both services to this network.

Practice

1. What is the main purpose of using docker-compose with Spring Boot services?

easy

A. To run multiple related services together with one configuration file

B. To compile Java code faster

C. To replace the Spring Boot framework

D. To write unit tests automatically

Solution

1. Step 1: Understand what docker-compose does

   Docker Compose is a tool to run multiple containers (services) together using one YAML file.

2. Step 2: Relate to Spring Boot services

   It helps run your Spring Boot app alongside databases or other services easily.

3. Final Answer:

   To run multiple related services together with one configuration file -> Option A

4. Quick Check:

   docker-compose = run multiple services [OK]

Hint: Remember: docker-compose manages multiple containers easily [OK]

Common Mistakes:

• Thinking docker-compose compiles code
• Confusing docker-compose with testing tools
• Believing it replaces Spring Boot

2. Which of the following is the correct way to specify a service's port mapping in a docker-compose.yml file?

easy

A. ports: - "8080:8080"

B. ports: 8080:8080

C. port: 8080->8080

D. expose: 8080:8080

Solution

1. Step 1: Recall docker-compose port syntax

   Ports are listed as a YAML array with dash and quotes, like ports: - "hostPort:containerPort".

2. Step 2: Check options

   ports: - "8080:8080" matches the correct YAML syntax for port mapping.

3. Final Answer:

   ports: - "8080:8080" -> Option A

4. Quick Check:

   Port mapping uses dash and quotes in YAML [OK]

Hint: Use dash and quotes for ports in docker-compose YAML [OK]

Common Mistakes:

• Omitting dash for list items
• Using wrong key 'port' instead of 'ports'
• Confusing 'expose' with 'ports'

3. Given this docker-compose.yml snippet:

services:
  app:
    image: springboot-app
    ports:
      - "8080:8080"
    depends_on:
      - db
  db:
    image: postgres
    environment:
      POSTGRES_PASSWORD: example

What happens when you run docker-compose up?

medium

A. docker-compose throws a syntax error

B. Only app service starts, db is ignored

C. db starts but app fails due to missing environment

D. Both app and db services start, app waits for db to start

Solution

1. Step 1: Understand depends_on behavior

   The app service depends on db, so docker-compose starts db first, then app.

2. Step 2: Check environment and ports

   db has required environment variable, app exposes port 8080 correctly.

3. Final Answer:

   Both app and db services start, app waits for db to start -> Option D

4. Quick Check:

   depends_on starts dependencies first [OK]

Hint: depends_on starts services in order, no syntax errors here [OK]

Common Mistakes:

• Assuming depends_on waits for full readiness
• Thinking db is ignored without explicit start
• Confusing environment variables with ports

4. You wrote this docker-compose.yml but your Spring Boot app fails to connect to the database:

services:
  app:
    image: springboot-app
    ports:
      - "8080:8080"
    environment:
      SPRING_DATASOURCE_URL: jdbc:postgresql://localhost:5432/mydb
  db:
    image: postgres
    environment:
      POSTGRES_PASSWORD: example

What is the likely problem?

medium

A. POSTGRES_PASSWORD is incorrect environment variable

B. Missing port mapping for db service

C. The app tries to connect to localhost instead of the db service hostname

D. The app image name is wrong

Solution

1. Step 1: Check datasource URL in app environment

   The app uses jdbc URL with 'localhost', which inside container means itself, not db service.

2. Step 2: Understand docker-compose networking

   Services communicate by service name, so URL should use 'db' hostname, not 'localhost'.

3. Final Answer:

   The app tries to connect to localhost instead of the db service hostname -> Option C

4. Quick Check:

   Use service name, not localhost, for inter-service DB URL [OK]

Hint: Use service names, not localhost, inside docker-compose networks [OK]

Common Mistakes:

• Assuming localhost works inside containers
• Forgetting to map ports for external access
• Misnaming environment variables

5. You want to run a Spring Boot app with a MySQL database using docker-compose. You want the database to keep data even if the container stops. Which snippet correctly adds persistent storage for the database?

hard

A. volumes:
- /var/lib/mysql:/db-data

volumes:
db-data:

B. volumes:
- db-data:/var/lib/mysql

volumes:
db-data:

C. volumes:
- db-data:/mysql/data

volumes:
mysql-data:

D. volumes:
- /mysql/data:/db-data

volumes:
db-data:

Solution

1. Step 1: Understand volume syntax for persistence

   Docker volumes map a named volume to the container path where data is stored, e.g., db-data:/var/lib/mysql.

2. Step 2: Check volume declaration

   The named volume must be declared under top-level volumes: with the same name.

3. Final Answer:

   volumes: - db-data:/var/lib/mysql volumes: db-data: -> Option B

4. Quick Check:

   Named volume mapped to container path with declaration [OK]

Hint: Map named volume to container data path and declare it below [OK]

Common Mistakes:

• Reversing host and container paths
• Not declaring the named volume
• Using wrong container data directory