Defining services in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a service named 'web' using the nginx image.

Docker

services:
  web:
    image: [1]

Drag options to blanks, or click blank then click option'

Anginx

Bmysql

Credis

Dubuntu

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a database image like mysql instead of a web server image.

Using an OS image like ubuntu which doesn't run a web server by default.

The image field specifies which Docker image the service uses. Here, 'nginx' is the correct image for a web server.

2fill in blank

medium

Complete the code to expose port 80 on the 'web' service.

Docker

services:
  web:
    image: nginx
    ports:
      - "[1]:80"

Drag options to blanks, or click blank then click option'

A8080

B443

C80

D3306

Attempts:

3 left

💡 Hint

Common Mistakes

Using port 8080 on the host without knowing if it is intended.

Using port 443 which is for HTTPS, not HTTP.

The ports mapping format is host_port:container_port. To expose port 80 on the host to port 80 in the container, use '80:80'.

3fill in blank

hard

Fix the error in the service definition by completing the restart policy.

Docker

services:
  web:
    image: nginx
    restart: [1]

Drag options to blanks, or click blank then click option'

Aalways

Bnever

Csometimes

Don-failure

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sometimes' which is not a valid restart policy.

Using 'never' which disables restart.

The restart policy 'always' ensures the container restarts automatically if it stops.

4fill in blank

hard

Fill both blanks to define environment variables for the 'db' service.

Docker

services:
  db:
    image: mysql
    environment:
      - [1]=root
      - [2]=password123

Drag options to blanks, or click blank then click option'

AMYSQL_USER

BMYSQL_ROOT_PASSWORD

CMYSQL_PASSWORD

DMYSQL_DATABASE

Attempts:

3 left

💡 Hint

Common Mistakes

Using MYSQL_ROOT_PASSWORD for a non-root user password.

Confusing MYSQL_DATABASE with user credentials.

Environment variables for MySQL include MYSQL_USER for the username and MYSQL_PASSWORD for the user's password.

5fill in blank

hard

Fill all three blanks to define a volume mount for the 'web' service.

Docker

services:
  web:
    image: nginx
    volumes:
      - [1]:[2]:[3]

Drag options to blanks, or click blank then click option'

A./html

B/usr/share/nginx/html

Cro

Drw

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'rw' instead of 'ro' when read-only is intended.

Swapping host and container paths.

The volume mount syntax is host_path:container_path:mode. Here, ./html is the folder on the host, /usr/share/nginx/html is the container folder, and ro means read-only.

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