docker-compose.yml structure - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of docker-compose up with multiple services

Given this docker-compose.yml file, what will be the output when running docker-compose up?

Docker

version: '3.8'
services:
  web:
    image: nginx:alpine
    ports:
      - "8080:80"
  db:
    image: postgres:13
    environment:
      POSTGRES_PASSWORD: example

A Starts two containers: one running nginx accessible on port 8080, and one running postgres with password set

B Starts one container combining nginx and postgres in a single service

C Fails with error: 'image not found' because alpine tag is invalid

D Starts only the nginx container; postgres is ignored

💡 Hint

Think about how docker-compose handles multiple services defined under 'services'.

docker-compose starts each service as a separate container. The 'web' service runs nginx on port 8080, and the 'db' service runs postgres with the specified environment variable.

❓ Configuration

intermediate

Correct syntax for environment variables in docker-compose.yml

Which option shows the correct way to set environment variables for a service in docker-compose.yml?

A

environment:
  - POSTGRES_PASSWORD: example
  - POSTGRES_USER: admin

B

environment:
  POSTGRES_PASSWORD: example
  POSTGRES_USER: admin

C

environment:
  POSTGRES_PASSWORD=example
  POSTGRES_USER=admin

D

environment:
  - POSTGRES_PASSWORD=example
  - POSTGRES_USER=admin

💡 Hint

Look for the YAML mapping style used for environment variables.

In docker-compose.yml, environment variables can be set as a mapping (key: value) or as a list of strings with key=value. Option B uses mapping correctly.

❓ Troubleshoot

advanced

Error caused by incorrect indentation in docker-compose.yml

What error will occur if the following docker-compose.yml snippet is used?

Docker

version: '3.8'
services:
  web:
  image: nginx:alpine
  ports:
    - "8080:80"

A docker-compose fails with 'image not found' error

B docker-compose runs successfully with no errors

C docker-compose ignores the 'ports' section silently

D yaml.parser.ParserError due to incorrect indentation under 'web'

💡 Hint

YAML is sensitive to spaces and indentation.

The 'image' and 'ports' keys must be indented under 'web'. Incorrect indentation causes a YAML parsing error.

🔀 Workflow

advanced

Order of steps to add a new service to an existing docker-compose.yml

What is the correct order of steps to add a new service called 'cache' running Redis to an existing docker-compose.yml and start it?

A 1,4,2,3

B 4,1,2,3

C 1,2,4,3

D 2,1,4,3

💡 Hint

Think about editing the file before starting services and verifying after.

First add the service, then add ports if needed, then start services, then verify running containers.

✅ Best Practice

expert

Choosing the best way to share environment variables securely in docker-compose.yml

Which option is the best practice to securely provide environment variables like passwords to services in docker-compose.yml?

A Hardcode passwords directly in the environment section of docker-compose.yml

B Pass passwords as command line arguments when running docker-compose up

C Use an external .env file and reference variables in docker-compose.yml

D Store passwords in the image and do not specify in docker-compose.yml

💡 Hint

Think about separating secrets from code files.

Using an external .env file keeps secrets out of the main compose file and allows better security and flexibility.

Practice

1. What is the main purpose of the services section in a docker-compose.yml file?

easy

A. To define the containers that will run and their settings

B. To specify the Docker version to use

C. To list the volumes to be created

D. To set environment variables globally

Solution

1. Step 1: Understand the role of services

   The services section lists all containers to run and their configurations.

2. Step 2: Compare with other sections

   version sets compose file version, volumes defines shared storage, environment variables are per service or global but not the main purpose of services.

3. Final Answer:

   To define the containers that will run and their settings -> Option A

4. Quick Check:

   services = container definitions [OK]

Hint: Services section lists containers and their configs [OK]

Common Mistakes:

• Confusing version with services
• Thinking volumes are defined inside services only
• Assuming environment variables are global by default

2. Which of the following is the correct way to specify a port mapping in docker-compose.yml under a service named web?

easy

A. port:
- "80:80"

B. ports:
- "80:80"

C. ports:
- 80:80

D. mapping:
- "80:80"

Solution

1. Step 1: Check correct key for port mapping

   The correct key is ports (plural), not port or mapping.

2. Step 2: Validate syntax for port mapping

   Port mappings must be strings with quotes like "80:80" to avoid YAML parsing issues.

3. Final Answer:

   ports: - "80:80" -> Option B

4. Quick Check:

   Use ports with quoted strings [OK]

Hint: Use 'ports' key with quoted strings for mappings [OK]

Common Mistakes:

• Using singular 'port' instead of 'ports'
• Not quoting port mappings causing YAML errors
• Using wrong key like 'mapping'

3. Given this docker-compose.yml snippet:

version: '3'
services:
  app:
    image: myapp:latest
    environment:
      - DEBUG=true
    ports:
      - "5000:5000"

What will happen when you run docker-compose up?

medium

A. The container runs but port 5000 is not exposed

B. An error occurs because environment variables must be in key:value format

C. The image 'myapp:latest' is ignored and default image is used

D. A container named 'app' runs with DEBUG=true and port 5000 exposed

Solution

1. Step 1: Analyze environment variable format

   Environment variables can be listed as strings like "DEBUG=true" in docker-compose.yml.

2. Step 2: Check port mapping and image usage

   Port 5000 is correctly mapped and image 'myapp:latest' will be used to run the container.

3. Final Answer:

   A container named 'app' runs with DEBUG=true and port 5000 exposed -> Option D

4. Quick Check:

   Env vars as strings and ports mapped = container runs correctly [OK]

Hint: Env vars as strings and ports quoted work fine [OK]

Common Mistakes:

• Thinking environment variables must be key:value pairs
• Assuming ports need special syntax to expose
• Believing image name is ignored if not official

4. Identify the error in this docker-compose.yml snippet:

version: '3'
services:
  db:
    image: postgres
    ports:
      - 5432:5432
    environment:
      POSTGRES_PASSWORD: example

medium

A. Port mapping should be quoted strings like "5432:5432"

B. Environment variables must be listed as a sequence, not mapping

C. The version '3' is invalid and should be '3.8'

D. The image name 'postgres' must include a tag

Solution

1. Step 1: Check port mapping syntax

   Port mappings should be quoted strings like "5432:5432" to avoid YAML parsing errors.

2. Step 2: Validate environment variable format

   Environment variables can be mappings like POSTGRES_PASSWORD: example, which is correct here.

3. Final Answer:

   Port mapping should be quoted strings like "5432:5432" -> Option A

4. Quick Check:

   Port mappings need quotes to parse correctly [OK]

Hint: Always quote port mappings to avoid YAML errors [OK]

Common Mistakes:

• Not quoting port mappings
• Confusing environment variable formats
• Thinking version '3' is invalid

5. You want to run two services, web and db, where web depends on db. Which docker-compose.yml snippet correctly expresses this dependency and shares a volume named data between them?

hard

A. version: '3'
services:
web:
image: webapp
depends_on:
- db
db:
image: postgres
volumes:
data:
external: true

B. version: '3'
services:
web:
image: webapp
volumes:
- data:/var/www/data
db:
image: postgres
depends_on:
- web
volumes:
- data:/var/lib/postgresql/data
volumes:
data:

C. version: '3'
services:
web:
image: webapp
depends_on:
- db
volumes:
- data:/var/www/data
db:
image: postgres
volumes:
- data:/var/lib/postgresql/data
volumes:
data:

D. version: '3'
services:
web:
image: webapp
depends_on:
- db
volumes:
- /data:/var/www/data
db:
image: postgres
volumes:
- /data:/var/lib/postgresql/data

Solution

1. Step 1: Check dependency declaration

   web depends on db using depends_on with - db under web.

2. Step 2: Verify volume sharing and declaration

   Both services use the named volume data mapped to their paths, and volumes: section declares data properly.

3. Final Answer:

   version: '3' services: web: image: webapp depends_on: - db volumes: - data:/var/www/data db: image: postgres volumes: - data:/var/lib/postgresql/data volumes: data: -> Option C

4. Quick Check:

   depends_on + named volume declared under volumes = correct [OK]

Hint: Use depends_on and declare shared volumes under volumes [OK]

Common Mistakes:

• Reversing depends_on order
• Using host paths instead of named volumes
• Not declaring volumes section for named volumes