Init container pattern in Docker - Mini Project: Build & Apply

Init Container Pattern in Docker

📖 Scenario: You are setting up a Docker environment where an application container depends on some initialization tasks to be completed first. This is common in real-world deployments where setup steps must finish before the main app runs.

🎯 Goal: Build a Docker Compose setup with an init container that runs a setup script before the main application container starts.

📋 What You'll Learn

Create a Docker Compose file with two services: init and app

The init service runs a simple shell command to create a file

The app service waits for the init service to complete before starting

Use the depends_on option to enforce startup order

💡 Why This Matters

🌍 Real World

Init containers are used in real deployments to prepare the environment, such as setting up config files or databases, before the main application starts.

💼 Career

Understanding init containers and service dependencies is important for DevOps roles managing containerized applications and orchestrations.

1

Create Docker Compose file with init and app services

Create a file named docker-compose.yml with two services: init and app. The init service should use the alpine image and run the command sh -c "echo Init done > /shared/init.txt". Add a top-level volumes section defining a named volume shared, and mount it to /shared in both services using volumes: - shared:/shared. The app service should use the alpine image and run sh -c "sleep 10" as a placeholder command.

Docker

# Your code here

Need a hint?

Use version: '3.8' at the top. Define two services named exactly init and app. Use the alpine image for both. Add top-level volumes: shared: and volumes: - shared:/shared under both services.

2

Add depends_on to app service

Modify the app service in docker-compose.yml to include depends_on using the map format so that the app service waits for the init service to complete successfully before starting. Use condition: service_completed_successfully.

Docker

version: '3.8'
services:
  init:
    image: alpine
    command: sh -c "echo Init done > /shared/init.txt"
    volumes:
      - shared:/shared
  app:
    image: alpine
    command: sh -c "sleep 10"
    volumes:
      - shared:/shared
    # Add depends_on here
    # Your code here
volumes:
  shared: {}

Need a hint?

Under the app service, add depends_on: as a map containing init: with condition: service_completed_successfully.

3

Change app command to check init file

Change the app service's command to sh -c "while [ ! -f /shared/init.txt ]; do sleep 1; done; echo Init file found; sleep 5" so it waits until the /shared/init.txt file created by the init service exists.

Docker

version: '3.8'
services:
  init:
    image: alpine
    command: sh -c "echo Init done > /shared/init.txt"
    volumes:
      - shared:/shared
  app:
    image: alpine
    # Change command to wait for /shared/init.txt
    command: sh -c "while [ ! -f /shared/init.txt ]; do sleep 1; done; echo Init file found; sleep 5"
    volumes:
      - shared:/shared
    depends_on:
      init:
        condition: service_completed_successfully
volumes:
  shared: {}

Need a hint?

Use a shell loop in the app command to wait for the file /shared/init.txt.

4

Run docker-compose and show app output

Run docker-compose up in your terminal to start both services. Then write the exact output line from the app service that says Init file found.

Docker

# Run 'docker-compose up' in your terminal
# Then write the output line from app service below
# Your code here

Need a hint?

Look for the line printed by the app service after it detects the init file.