📖 8 min read · By ['[“DungNQ49”]']

Review Questions - Container Orchestration & Compose#

Test your understanding of Docker Compose concepts with these review questions.

Multiple Choice#

1. How do services communicate with each other in Docker Compose by default?#

A) Via IP addresses only
B) Using service names as hostnames
C) Through localhost
D) They cannot communicate by default

Answer

B) Using service names as hostnames

Docker Compose creates a default network where services can reach each other using their service names as DNS hostnames. For example, a service named postgres can be reached at hostname postgres.

2. What does the `depends_on` directive do?#

A) Ensures services are on the same network
B) Controls the order in which services start
C) Shares volumes between services
D) Links environment variables between services

Answer

B) Controls the order in which services start

depends_on specifies that a service should wait for another service to start before starting itself. With conditions like service_healthy, it can also wait for health checks to pass.

3. What is the difference between `ports` and `expose` in docker-compose.yml?#

A) There is no difference
B) ports publishes to host, expose only exposes to other services in the network
C) expose publishes to host, ports only exposes to other services
D) ports is for TCP, expose is for UDP

Answer

B) ports publishes to host, expose only exposes to other services in the network

ports maps container ports to host ports, making them accessible from outside
expose only documents the port and makes it accessible to linked services, not the host

4. What happens to named volumes when you run `docker compose down`?#

A) They are always deleted
B) They are preserved unless you use -v flag
C) They are moved to a backup location
D) They are converted to bind mounts

Answer

B) They are preserved unless you use -v flag

Named volumes persist data even when containers are removed. To delete volumes along with containers, use docker compose down -v.

5. What file is automatically loaded with docker-compose.yml?#

A) docker-compose.local.yml
B) docker-compose.override.yml
C) docker-compose.dev.yml
D) docker-compose.default.yml

Answer

B) docker-compose.override.yml

Docker Compose automatically loads docker-compose.override.yml if it exists alongside docker-compose.yml. This is useful for development-specific configurations.

6. Which dependency condition waits for a service’s health check to pass?#

A) service_started
B) service_running
C) service_healthy
D) service_ready

Answer

C) service_healthy

depends_on:
  database:
    condition: service_healthy

This waits for the database service’s health check to pass before starting the dependent service.

7. How do you scale a service to 5 instances?#

A) docker compose scale service=5
B) docker compose up -d --scale service=5
C) docker compose replicas service 5
D) docker compose start service -n 5

Answer

B) docker compose up -d --scale service=5

The --scale flag allows you to run multiple instances of a service. Note that when scaling, you should use expose instead of fixed ports to avoid port conflicts.

8. What is the purpose of profiles in Docker Compose?#

A) To define security settings
B) To selectively start certain services
C) To configure logging
D) To set resource limits

Answer

B) To selectively start certain services

Profiles allow you to group services and only start them when that profile is activated:

services:
  debug-tools:
    profiles:
      - debug

Start with: docker compose --profile debug up

Scenario Questions#

9. Database Connection Refused#

Your application container keeps failing with “connection refused” when trying to connect to PostgreSQL, even though depends_on: [postgres] is configured.

What’s the likely issue and how do you fix it?

Answer

Issue: depends_on: [postgres] only waits for the container to start, not for PostgreSQL to be ready to accept connections.

Solution: Use health check condition:

services:
  app:
    depends_on:
      postgres:
        condition: service_healthy

  postgres:
    image: postgres:15
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 3s
      retries: 5

This ensures the app only starts after PostgreSQL is actually ready to accept connections.

10. Port Already in Use#

You try to start your compose stack but get an error saying port 5432 is already in use.

What are your options?

Answer

Options:

Change the host port:

ports:
  - "5433:5432" # Use 5433 on host, 5432 in container

Stop the conflicting service:

# Find what's using the port
sudo lsof -i :5432
# Stop local PostgreSQL
sudo systemctl stop postgresql

Don’t expose to host (if only needed internally):

# Remove ports section, use expose instead
expose:
  - "5432"

Use a different project name to isolate networks:
```
docker compose -p myproject up
```

11. Data Loss on Rebuild#

Every time you rebuild your containers, all database data is lost.

How do you fix this?

Answer

Issue: Data is stored inside the container instead of a persistent volume.

Solution: Use named volumes for database data:

services:
  postgres:
    image: postgres:15
    volumes:
      - postgres_data:/var/lib/postgresql/data

volumes:
  postgres_data: # Named volume - persists across rebuilds

Important notes:

Named volumes persist unless explicitly removed with docker compose down -v
Use docker volume ls to see existing volumes
Use docker volume inspect volume_name to see where data is stored

12. Environment Variables Not Working#

You set environment variables in .env file but they’re not being picked up by your application.

What could be wrong?

Answer

Possible issues and solutions:

Not referencing .env file:

services:
  app:
    env_file:
      - .env # Explicitly reference

Variable substitution vs passing:

# This substitutes at compose time (from shell or .env)
environment:
  - DATABASE_URL=${DATABASE_URL}

# This passes the file contents to the container
env_file:
  - .env

Wrong file name: Ensure it’s exactly .env not .env.txt
Rebuild required:
```
docker compose up -d --build
```

Check variable is set:

docker compose exec app env | grep DATABASE_URL

Command Identification#

13. Match the command to its purpose:#

Command	Purpose
`docker compose logs -f app`	?
`docker compose exec app bash`	?
`docker compose run --rm app pytest`	?
`docker compose config`	?

Answer

Command	Purpose
`docker compose logs -f app`	Follow (stream) logs for the app service
`docker compose exec app bash`	Open interactive shell in running app container
`docker compose run --rm app pytest`	Run one-off command (pytest) and remove container after
`docker compose config`	Validate and view the merged compose configuration

14. What commands would you use to:#

Rebuild images and start all services
Stop all services but keep volumes
View resource usage of all containers
Run database migrations before starting app

Answer

# 1. Rebuild and start
docker compose up -d --build

# 2. Stop without removing volumes
docker compose down  # (default behavior preserves volumes)

# 3. View resource usage
docker compose top
# or
docker stats

# 4. Run migrations before app
docker compose run --rm app alembic upgrade head
docker compose up -d

Short Answer#

15. Explain the difference between bind mounts and named volumes#

Answer

Bind Mounts:

Map a host directory to container path
Format: ./host/path:/container/path
Managed by user on host filesystem
Good for: development (hot reload), config files
Example: ./src:/app/src

Named Volumes:

Managed by Docker
Format: volume_name:/container/path
Stored in Docker’s data directory
Persist until explicitly deleted
Good for: database data, persistent application data
Example: postgres_data:/var/lib/postgresql/data

volumes:
  - ./app:/app # Bind mount
  - db_data:/var/lib/postgresql/data # Named volume

16. How do you configure different environments (dev/staging/prod) with Docker Compose?#

Answer

Method 1: Override files

# docker-compose.yml (base)
# docker-compose.override.yml (dev - auto-loaded)
# docker-compose.prod.yml (production)

# Development (automatic override)
docker compose up

# Production (explicit files)
docker compose -f docker-compose.yml -f docker-compose.prod.yml up

Method 2: Environment files

# .env.dev, .env.prod
docker compose --env-file .env.prod up

Method 3: Profiles

services:
  debug-tools:
    profiles: [dev]
  monitoring:
    profiles: [prod]

docker compose --profile dev up

17. What happens when you scale a service that has a fixed port mapping?#

Answer

Problem: Port conflicts occur because you cannot bind the same host port to multiple containers.

# This will fail when scaled
ports:
  - "8000:8000"

Solutions:

Use port ranges:
```
ports:
  - "8000-8010:8000"
```
Use expose only (internal access):
```
expose:
  - "8000"
```

Use a load balancer:

nginx:
  ports:
    - "80:80"
app:
  expose:
    - "8000"
  deploy:
    replicas: 3

18. What is the purpose of health checks in Docker Compose?#

Answer

Health checks serve multiple purposes:

Dependency coordination: With condition: service_healthy, dependent services wait until the dependency is actually ready, not just started
Container orchestration: Docker can restart unhealthy containers (with appropriate restart policy)
Load balancer integration: Load balancers can route traffic only to healthy containers
Monitoring: Easily identify service health with docker compose ps

Example:

services:
  postgres:
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 10s
      timeout: 5s
      retries: 5
      start_period: 30s

States: starting → healthy/unhealthy

Self-Assessment Checklist#

Rate your confidence (1-5) on each skill:

Skill	Confidence
Writing docker-compose.yml files	⬜
Configuring multi-service applications	⬜
Setting up service dependencies	⬜
Managing volumes and data persistence	⬜
Environment configuration	⬜
Scaling services	⬜
Debugging compose issues	⬜
Production configuration	⬜

Scoring:

35-40: Expert level - ready to orchestrate complex applications
25-34: Proficient - comfortable with multi-service deployments
15-24: Intermediate - practice more complex scenarios
Below 15: Beginner - review documentation and redo exercises

Review Questions - Container Orchestration & Compose#

Multiple Choice#

1. How do services communicate with each other in Docker Compose by default?#

2. What does the depends_on directive do?#

3. What is the difference between ports and expose in docker-compose.yml?#

4. What happens to named volumes when you run docker compose down?#

5. What file is automatically loaded with docker-compose.yml?#

6. Which dependency condition waits for a service’s health check to pass?#

7. How do you scale a service to 5 instances?#

8. What is the purpose of profiles in Docker Compose?#

Scenario Questions#

9. Database Connection Refused#

10. Port Already in Use#

11. Data Loss on Rebuild#

12. Environment Variables Not Working#

Command Identification#

13. Match the command to its purpose:#

14. What commands would you use to:#

Short Answer#

15. Explain the difference between bind mounts and named volumes#

16. How do you configure different environments (dev/staging/prod) with Docker Compose?#

17. What happens when you scale a service that has a fixed port mapping?#

18. What is the purpose of health checks in Docker Compose?#

Self-Assessment Checklist#

2. What does the `depends_on` directive do?#

3. What is the difference between `ports` and `expose` in docker-compose.yml?#

4. What happens to named volumes when you run `docker compose down`?#