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.