Defining services in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of docker-compose service start command

What is the output when you run docker-compose up -d with this docker-compose.yml file?

Docker

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

A Creating network "default" with the default driver\nCreating default_web_1 ... done

B Error: Service 'web' has no container_name specified

C Creating network "default" with the default driver\nCreating volume "default_web_data" with default driver\nCreating default_web_1 ... done

D Starting web ... done

💡 Hint

Focus on what docker-compose creates by default when starting a service without volumes.

The command creates a default network and starts the container for the 'web' service. No volumes are defined, so no volume creation message appears.

❓ Configuration

intermediate

Correct service definition for environment variables

Which service definition correctly sets environment variables for a service in docker-compose.yml?

A

services:
  app:
    image: myapp
    environment:
      - DEBUG=true
      - PORT=3000

B

services:
  app:
    image: myapp
    env:
      DEBUG: true
      PORT: 3000

C

services:
  app:
    image: myapp
    environment:
      DEBUG: true
      PORT: 3000

D

services:
  app:
    image: myapp
    env_vars:
      - DEBUG=true
      - PORT=3000

💡 Hint

Check the correct key name and format for environment variables in docker-compose.

The correct key is 'environment' with key-value pairs or list format. Option C uses key-value pairs correctly.

❓ Troubleshoot

advanced

Troubleshooting service port binding error

You defined this service in docker-compose.yml but get an error when starting it: Bind for 0.0.0.0:80 failed: port is already allocated. What is the cause?

Docker

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

A Another process on the host is already using port 80, so Docker cannot bind it.

B The image 'nginx' does not expose port 80, causing the error.

C The ports syntax is incorrect; it should be '80->80' instead of '80:80'.

D Docker daemon is not running, so ports cannot be bound.

💡 Hint

Check if any other application is using the host port before Docker tries to bind it.

Port binding errors occur when the host port is already in use by another process, preventing Docker from binding it.

🔀 Workflow

advanced

Order of steps to update a running service with new image

What is the correct order of commands to update a running service to use a new image version in Docker Compose?

A 3,2,1,4

B 1,3,2,4

C 1,2,4,3

D 3,1,2,4

💡 Hint

You want to stop the old containers before pulling and starting new ones.

First stop containers (down), then pull new images, then start containers (up -d), then check status (ps).

✅ Best Practice

expert

Best practice for defining multiple services sharing a network

Which docker-compose service definition best ensures two services share the same user-defined network named 'appnet'?

A

networks:
  appnet:
services:
  service1:
    image: alpine
    networks:
      - appnet
  service2:
    image: alpine
    networks:
      - appnet

B

services:
  service1:
    image: alpine
    networks:
      - appnet
  service2:
    image: alpine
    networks:
      - appnet
networks:
  appnet:
    driver: bridge

C

services:
  service1:
    image: alpine
  service2:
    image: alpine
networks:
  appnet:
    driver: bridge

D

services:
  service1:
    image: alpine
  service2:
    image: alpine
    networks:
      - appnet

💡 Hint

Define the network under 'networks' and attach it explicitly to each service.

Option B defines the network with a driver and attaches it to both services explicitly, ensuring they share the same network.

Practice

1. What is the main purpose of defining services in a Docker Compose file?

easy

A. To install software inside containers manually

B. To write shell scripts for container startup

C. To group containers with their settings for easy management

D. To create virtual machines inside Docker

Solution

1. Step 1: Understand the role of services in Docker Compose

   Services define how containers run, including images, ports, and environment variables.

2. Step 2: Recognize the benefit of grouping containers

   Grouping containers with their settings allows easy start, stop, and management of multiple containers together.

3. Final Answer:

   To group containers with their settings for easy management -> Option C

4. Quick Check:

   Services group containers = D [OK]

Hint: Services group containers and settings in one file [OK]

Common Mistakes:

• Confusing services with scripts
• Thinking services create virtual machines
• Believing services install software manually

2. Which of the following is the correct way to define a service named web using the image nginx in a Docker Compose YAML file?

easy

A. services:
web:
image: nginx

B. services:
web:
image nginx

C. service:
web:
image: nginx

D. services:
web:
img: nginx

Solution

1. Step 1: Check the correct YAML key for services

   The top-level key must be services (plural), not service.

2. Step 2: Verify the syntax for image definition

   The correct syntax is image: nginx with a colon and proper indentation.

3. Final Answer:

   services: web: image: nginx -> Option A

4. Quick Check:

   Correct YAML keys and syntax = B [OK]

Hint: Use 'services:' plural and 'image:' with colon [OK]

Common Mistakes:

• Using 'service' instead of 'services'
• Missing colon after 'image'
• Using 'img' instead of 'image'

3. Given this Docker Compose snippet, what port will be accessible on the host machine?

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

medium

A. Port 80 on the host

B. Port 8080 on the host

C. Port 80 inside the container only

D. Port 8080 inside the container only

Solution

1. Step 1: Understand port mapping syntax

   The format "host_port:container_port" means host port 8080 maps to container port 80.

2. Step 2: Identify which port is exposed on the host

   Port 8080 on the host machine will forward traffic to port 80 inside the container.

3. Final Answer:

   Port 8080 on the host -> Option B

4. Quick Check:

   Host port is first in mapping = A [OK]

Hint: Host port is before colon in ports mapping [OK]

Common Mistakes:

• Confusing host and container ports
• Thinking container port is exposed on host
• Ignoring quotes around ports

4. Identify the error in this Docker Compose service definition:

services:
  db:
    image: postgres
    ports:
      - "5432:"
    environment:
      POSTGRES_PASSWORD: example

medium

A. Missing container port in ports mapping

B. Environment variable name is incorrect

C. Image name should be capitalized

D. Service name cannot be 'db'

Solution

1. Step 1: Check ports syntax

   Ports must be mapped as "host_port:container_port" or just container port. Here, "5432:" is invalid because missing container port after colon.

2. Step 2: Confirm environment and image correctness

   Environment variable and image name are correct; service name 'db' is valid.

3. Final Answer:

   Missing container port in ports mapping -> Option A

4. Quick Check:

   Ports need host:container or container port only = A [OK]

Hint: Ports need host:container or container port only [OK]

Common Mistakes:

• Using only one port number with quotes
• Assuming environment variable names are wrong
• Thinking service names have restrictions

5. You want to define two services, frontend and backend, where frontend depends on backend to start first. Which is the correct way to express this dependency in Docker Compose?

hard

A. services:
frontend:
image: frontend-image
backend:
image: backend-image
depends_on:
- frontend

B. services:
backend:
image: backend-image
depends_on:
- frontend
frontend:
image: frontend-image

C. services:
frontend:
image: frontend-image
links:
- backend
backend:
image: backend-image

D. services:
frontend:
image: frontend-image
depends_on:
- backend
backend:
image: backend-image

Solution

1. Step 1: Understand 'depends_on' usage

   The depends_on key under a service lists services that must start before it.

2. Step 2: Identify correct dependency direction

   Since frontend depends on backend, frontend should have depends_on: - backend.

3. Step 3: Check other options for correctness

   The incorrect options either reverse the dependency direction or use the deprecated links for dependency control.

4. Final Answer:

   services: frontend: image: frontend-image depends_on: - backend backend: image: backend-image -> Option D

5. Quick Check:

   depends_on under dependent service = C [OK]

Hint: Put depends_on under service that waits for others [OK]

Common Mistakes:

• Reversing dependency direction
• Using links instead of depends_on
• Placing depends_on under wrong service