Compose file versioning in Docker - Mini Project: Build & Apply

Compose file versioning

📖 Scenario: You are setting up a Docker Compose file to run a simple web application. You want to specify the version of the Compose file format to ensure compatibility with your Docker environment.

🎯 Goal: Create a Docker Compose file with a specific version, add a service configuration, and display the version used.

📋 What You'll Learn

Create a Docker Compose file with version '3.8'

Add a service named 'web' using the 'nginx:latest' image

Print the Compose file version to confirm

💡 Why This Matters

🌍 Real World

Docker Compose files are used to define and run multi-container Docker applications. Specifying the version ensures compatibility with Docker features.

💼 Career

Understanding Compose file versioning helps in configuring containerized applications reliably, a key skill for DevOps roles.

1

Create the Compose file with version

Create a dictionary called compose_file with a key version set to the string '3.8'.

Docker

# Your code here

Hint

Use a Python dictionary with the key 'version' and value '3.8'.

2

Add a service configuration

Add a key services to the compose_file dictionary. Set it to a dictionary with a key web whose value is another dictionary with the key image set to 'nginx:latest'.

Docker

compose_file = {
    'version': '3.8'
}
# Your code here

Hint

Remember to nest dictionaries for services and the web service configuration.

3

Access the Compose file version

Create a variable called version_used and set it to the value of the version key in the compose_file dictionary.

Docker

compose_file = {
    'version': '3.8',
    'services': {
        'web': {
            'image': 'nginx:latest'
        }
    }
}
# Your code here

Hint

Use dictionary key access to get the version value.

4

Print the Compose file version

Write a print statement to display the text "Compose file version: " followed by the value of version_used.

Docker

compose_file = {
    'version': '3.8',
    'services': {
        'web': {
            'image': 'nginx:latest'
        }
    }
}
version_used = compose_file['version']
# Your code here

Hint

Use string concatenation or f-string to combine the text and variable.

Practice

1. What does the version field in a Docker Compose file specify?

easy

A. The version of the operating system hosting Docker

B. The version of the Docker Engine installed on the system

C. The version of the application running inside the container

D. The format and features Docker Compose should use to read the file

Solution

1. Step 1: Understand the role of the version field

   The version field tells Docker Compose which syntax and features to expect in the file.

2. Step 2: Differentiate from other version types

   This field is not about Docker Engine, app, or OS versions but about Compose file format.

3. Final Answer:

   The format and features Docker Compose should use to read the file -> Option D

4. Quick Check:

   Compose file version = file format and features [OK]

Hint: Version field controls Compose syntax, not Docker or app versions [OK]

Common Mistakes:

• Confusing Compose file version with Docker Engine version
• Thinking it sets app container version
• Assuming it relates to OS version

2. Which of the following is the correct way to specify the Compose file version in YAML?

easy

A. version: "3.9"

B. version : 3.9

C. version = "3.9"

D. "version" : 3.9

Solution

1. Step 1: Recall YAML syntax for key-value pairs

   YAML uses colon and space to assign values, and strings should be quoted if they contain dots.

2. Step 2: Identify correct quoting for version

   The version number with a decimal should be quoted as a string: "3.9".

3. Final Answer:

   version: "3.9" -> Option A

4. Quick Check:

   YAML version field quoted with colon = correct [OK]

Hint: Use colon and quotes for version like version: "3.9" [OK]

Common Mistakes:

• Using equals sign instead of colon
• Not quoting version with decimals
• Putting quotes around key instead of value

3. Given this Compose file snippet:

version: "2.4"
services:
  web:
    image: nginx
    ports:
      - "80:80"

What will happen if you run this with Docker Compose version 1.27.0?

medium

A. It will run successfully using version 2 features

B. It will fail because version 2.4 is not supported in 1.27.0

C. It will ignore the version and run with default settings

D. It will upgrade the Compose file version automatically

Solution

1. Step 1: Check Docker Compose 1.27.0 compatibility

   Docker Compose 1.27.0 supports Compose file version 2.4 syntax and features.

2. Step 2: Understand behavior on supported versions

   Since version 2.4 is supported, Compose will run the services as defined without errors.

3. Final Answer:

   It will run successfully using version 2 features -> Option A

4. Quick Check:

   Supported Compose version runs fine [OK]

Hint: Check Compose version supports file version before running [OK]

Common Mistakes:

• Assuming newer Compose versions are always required
• Thinking Compose auto-upgrades file versions
• Believing unsupported versions cause silent ignores

4. You have this Compose file:

version: "3.8
services:
  app:
    image: myapp
    ports:
      - "8080:80"

When running docker-compose up, you get an error. What is the likely cause?

medium

A. The ports mapping syntax is incorrect

B. The image name is invalid without a tag

C. The version number is not quoted, causing a YAML parsing error

D. Docker Compose does not support version 3.8

Solution

1. Step 1: Identify YAML syntax issue

   The version field with a decimal must be quoted as a string to avoid YAML parsing errors.

2. Step 2: Check other fields for errors

   Image name without tag is valid (defaults to latest), ports syntax is correct, and version 3.8 is supported.

3. Final Answer:

   The version number is not quoted, causing a YAML parsing error -> Option C

4. Quick Check:

   Unquoted decimal version causes YAML error [OK]

Hint: Always quote version numbers with decimals in Compose files [OK]

Common Mistakes:

• Not quoting version with decimals
• Assuming image needs explicit tag
• Misreading ports syntax as error

5. You want to use a new Compose feature available only in version 3.9, but your current Compose file uses version "3.7". What is the best way to update your Compose file to use the new feature safely?

hard

A. Add the new feature without changing the version field, it will work automatically

B. Change the version field to "3.9" and test the file with your Docker Compose version

C. Downgrade your Docker Compose to an older version that supports 3.7 features only

D. Remove the version field entirely to let Docker Compose decide

Solution

1. Step 1: Update the version field to match the new feature

   Set the version field to "3.9" to indicate use of new features introduced in that version.

2. Step 2: Test compatibility with your Docker Compose version

   Ensure your Docker Compose installation supports version 3.9 to avoid errors.

3. Final Answer:

   Change the version field to "3.9" and test the file with your Docker Compose version -> Option B

4. Quick Check:

   Update version field and verify Compose support [OK]

Hint: Match version field to features and test Compose version support [OK]

Common Mistakes:

• Using new features without updating version field
• Removing version field causing unpredictable behavior
• Ignoring Compose version compatibility