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

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: {}

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: {}

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

Hint

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

Practice

1. What is the main purpose of an init container in Docker or Kubernetes?

easy

A. To run setup tasks before the main application starts

B. To replace the main application container

C. To run the main application continuously

D. To monitor the main container's logs

Solution

1. Step 1: Understand init container role

   Init containers run before the main container to prepare the environment.

2. Step 2: Compare options

   Only To run setup tasks before the main application starts describes running setup tasks before the main app starts, which is the init container's job.

3. Final Answer:

   To run setup tasks before the main application starts -> Option A

4. Quick Check:

   Init container purpose = run setup tasks before main app [OK]

Hint: Init containers run first to prepare environment [OK]

Common Mistakes:

• Thinking init containers replace main containers
• Assuming init containers run continuously
• Confusing init containers with monitoring tools

2. Which of the following is the correct way to define an init container in a Kubernetes Pod YAML?

easy

A. initContainers:
- name: init
image: busybox
command: ['sh', '-c', 'echo Init']

B. containers:
- name: init
image: busybox
command: ['sh', '-c', 'echo Init']

C. init:
- name: init
image: busybox
command: ['sh', '-c', 'echo Init']

D. preContainers:
- name: init
image: busybox
command: ['sh', '-c', 'echo Init']

Solution

1. Step 1: Recall Kubernetes Pod spec structure

   Init containers are defined under the initContainers field in the Pod spec.

2. Step 2: Match correct YAML key

   Only initContainers: - name: init image: busybox command: ['sh', '-c', 'echo Init'] uses initContainers correctly; others use invalid keys.

3. Final Answer:

   initContainers with proper container spec -> Option A

4. Quick Check:

   Init containers key = initContainers [OK]

Hint: Init containers use 'initContainers' key in Pod YAML [OK]

Common Mistakes:

• Using 'containers' instead of 'initContainers'
• Using invalid keys like 'init' or 'preContainers'
• Misplacing init container definition outside Pod spec

3. Given this Pod spec snippet, what will happen if the init container fails?

initContainers:
  - name: setup
    image: busybox
    command: ['sh', '-c', 'exit 1']
containers:
  - name: app
    image: nginx

medium

A. The main app container will start normally

B. The Pod will ignore the init container failure and continue

C. The Pod will restart the init container until it succeeds

D. The main app container will start but with errors

Solution

1. Step 1: Understand init container failure behavior

   If an init container fails, Kubernetes restarts it until it succeeds or the Pod is stopped.

2. Step 2: Analyze options

   Only The Pod will restart the init container until it succeeds correctly states the Pod restarts the init container on failure; main container won't start until success.

3. Final Answer:

   The Pod will restart the init container until it succeeds -> Option C

4. Quick Check:

   Init container failure = Pod restarts init container [OK]

Hint: Init container failure causes restart, blocks main container [OK]

Common Mistakes:

• Thinking main container starts despite init failure
• Assuming Pod ignores init container failure
• Believing main container runs with errors from init failure

4. You wrote this Pod YAML but the init container does not run before the main container:

containers:
  - name: app
    image: nginx
initContainers:
  - name: setup
    image: busybox
    command: ['sh', '-c', 'echo Setup']

What is the likely error?

medium

A. The initContainers section is placed after containers, order matters

B. The initContainers key is incorrectly indented or misplaced

C. The command syntax is invalid for init containers

D. The image busybox cannot be used as an init container

Solution

1. Step 1: Check YAML structure rules

   Init containers must be correctly indented and placed at the same level as containers under the Pod spec.

2. Step 2: Identify common YAML mistakes

   Misplaced or wrongly indented initContainers causes it to be ignored, so init container won't run.

3. Final Answer:

   The initContainers key is incorrectly indented or misplaced -> Option B

4. Quick Check:

   YAML indentation errors cause init container not to run [OK]

Hint: Check YAML indentation and placement for initContainers [OK]

Common Mistakes:

• Assuming order of keys matters in YAML
• Using wrong command syntax inside init container
• Believing image choice blocks init container

5. You want to use an init container to download a configuration file before your main app starts. Which approach correctly shares the file between the init container and the main container?

hard

A. Copy the file from init container to main container using Docker cp command

B. Run the init container after the main container starts to copy the file

C. Store the file inside the init container image and expect main container to access it

D. Use a shared emptyDir volume mounted in both init and main containers

Solution

1. Step 1: Understand file sharing between containers

   Containers in the same Pod can share files using a shared volume like emptyDir.

2. Step 2: Evaluate options for sharing files

   Only Use a shared emptyDir volume mounted in both init and main containers correctly uses a shared volume mounted in both containers to share files.

3. Final Answer:

   Use a shared emptyDir volume mounted in both init and main containers -> Option D

4. Quick Check:

   Shared volume = file sharing between init and main containers [OK]

Hint: Use shared volume like emptyDir for file sharing [OK]

Common Mistakes:

• Trying to copy files between containers manually
• Expecting files inside init container image to be accessible
• Running init container after main container starts