Docker Compose for dev environment - Mini Project: Build & Apply

Docker Compose for dev environment

📖 Scenario: You are a developer who wants to run a simple web application and a database together on your computer. Instead of starting each service manually, you want to use Docker Compose to manage both services easily.

🎯 Goal: Build a docker-compose.yml file that defines a web service using the nginx image and a database service using the mysql image. Then start the services together with Docker Compose.

📋 What You'll Learn

Create a docker-compose.yml file with two services: web and db

The web service uses the nginx:latest image and maps port 8080 on the host to port 80 in the container

The db service uses the mysql:5.7 image with environment variables MYSQL_ROOT_PASSWORD set to example

Use Docker Compose commands to start the services

Display the running services using Docker Compose

💡 Why This Matters

🌍 Real World

Developers often need to run multiple services like web servers and databases together on their local machines. Docker Compose helps manage these services easily with one file and simple commands.

💼 Career

Knowing Docker Compose is essential for developers and DevOps engineers to create reproducible development environments and simplify running multi-container applications.

1

Create the basic Docker Compose file

Create a file named docker-compose.yml with a version set to '3.8' and define a service called web that uses the image nginx:latest.

Docker

# Create docker-compose.yml with version '3.8' and a service 'web' using image 'nginx:latest'
# Your code here

Hint

Start with the version key, then add services and under it the web service with the image set to nginx:latest.

2

Add port mapping to the web service

In the docker-compose.yml file, add a ports section under the web service that maps port 8080 on the host to port 80 in the container.

Docker

version: '3.8'
services:
  web:
    image: nginx:latest
    # Add ports mapping 8080:80 below
    # Your code here

Hint

Use the ports key as a list and add the string "8080:80" to map the ports.

3

Add the database service with environment variables

Add a new service called db to the docker-compose.yml file. Use the image mysql:5.7 and set the environment variable MYSQL_ROOT_PASSWORD to example under the environment section.

Docker

version: '3.8'
services:
  web:
    image: nginx:latest
    ports:
      - "8080:80"
  # Add db service with mysql:5.7 image and MYSQL_ROOT_PASSWORD environment variable
  # Your code here

Hint

Under services, add db with the image and an environment dictionary setting MYSQL_ROOT_PASSWORD.

4

Start services and display running containers

Run the command docker-compose up -d to start the services in detached mode. Then run docker-compose ps to display the running services.

Docker

# Run 'docker-compose up -d' to start services
# Run 'docker-compose ps' to show running containers
# Your code here

Hint

Use docker-compose up -d to start the containers in the background. Then use docker-compose ps to list the running containers.

Practice

1. What is the main purpose of using docker-compose.yml in a development environment?

easy

A. To build Docker images only

B. To define and run multiple containers with one command

C. To replace Docker CLI commands completely

D. To monitor container resource usage

Solution

1. Step 1: Understand Docker Compose role

   Docker Compose uses a YAML file to define multiple containers and their settings.

2. Step 2: Identify main benefit in dev

   It allows running all containers together with a single command, simplifying setup.

3. Final Answer:

   To define and run multiple containers with one command -> Option B

4. Quick Check:

   Compose = multiple containers, one command [OK]

Hint: Compose runs many containers with one file and command [OK]

Common Mistakes:

• Thinking Compose only builds images
• Believing Compose replaces all Docker CLI commands
• Confusing Compose with monitoring tools

2. Which of the following is the correct syntax to expose port 8080 on the host mapped to port 80 in a docker-compose.yml service?

easy

A. ports:
- "80:8080"

B. expose:
- "80:8080"

C. expose:
- "8080:80"

D. ports:
- "8080:80"

Solution

1. Step 1: Understand ports syntax in Compose

   The format is "host_port:container_port" to map host port to container port.

2. Step 2: Match ports for host 8080 to container 80

   To expose container port 80 on host port 8080, use "8080:80" under ports.

3. Final Answer:

   ports: - "8080:80" -> Option D

4. Quick Check:

   HostPort:ContainerPort = 8080:80 [OK]

Hint: Ports use host:container format [OK]

Common Mistakes:

• Reversing host and container ports
• Using expose instead of ports for host mapping
• Incorrect YAML indentation

3. Given this docker-compose.yml snippet, what will be the effect of the volume configuration?

services:
  app:
    image: myapp
    volumes:
      - ./src:/app/src

medium

A. The host's ./src folder is shared with /app/src inside the container

B. The container's /app/src folder is copied to host's ./src folder once

C. The container will have an empty /app/src folder

D. The volume will cause a syntax error

Solution

1. Step 1: Understand volume syntax in Compose

   Format "host_path:container_path" mounts host folder inside container.

2. Step 2: Analyze given volume mapping

   Host folder ./src is mounted to container path /app/src, sharing files live.

3. Final Answer:

   The host's ./src folder is shared with /app/src inside the container -> Option A

4. Quick Check:

   Volume mount = host folder shared inside container [OK]

Hint: Volumes share host folder inside container path [OK]

Common Mistakes:

• Thinking volume copies files only once
• Confusing source and target paths
• Assuming volume creates empty folder

4. You wrote this docker-compose.yml but get an error when running docker-compose up. What is wrong?

services:
  web:
    image: nginx
    ports:
      - "80:80"
    volumes:
      - ./html:/usr/share/nginx/html
    environment:
      - DEBUG=true
    command: nginx -g 'daemon off;'
    restart: always
  db:
    image: postgres
    environment:
      POSTGRES_PASSWORD: example
    volumes:
      - pgdata:/var/lib/postgresql/data
volumes:
  pgdata:

medium

A. The environment variable for db should use list syntax or key-value mapping consistently

B. The volume pgdata is not defined properly under volumes

C. The command for web service is invalid

D. The environment variable for db uses wrong syntax, should be a list

Solution

1. Step 1: Inspect YAML syntax in command field

   The command: nginx -g 'daemon off;' lacks quotes around the full string, causing YAML to parse it as separate unquoted scalars, leading to a syntax error.

2. Step 2: Confirm other sections are valid

   Environment uses valid list/map styles (mixed OK), volumes properly defined.

3. Final Answer:

   The command for web service is invalid -> Option C

4. Quick Check:

   Multi-word command needs full quotes or YAML list [OK]

Hint: YAML command with spaces: quote whole or use list [OK]

Common Mistakes:

• Blaming environment syntax mixing (both styles valid)
• Misreading pgdata volume definition
• Overlooking unquoted multi-word command

5. You want to set up a development environment with a web app and a database using Docker Compose. The web app needs live code updates from your host machine, and the database should persist data between restarts. Which docker-compose.yml configuration correctly achieves this?

hard

A. services:
web:
image: mywebapp
volumes:
- ./code:/app
ports:
- "3000:3000"
db:
image: mysql
environment:
MYSQL_ROOT_PASSWORD: rootpass
volumes:
- dbdata:/var/lib/mysql
volumes:
dbdata:

B. services:
web:
image: mywebapp
ports:
- "3000:3000"
db:
image: mysql
environment:
MYSQL_ROOT_PASSWORD: rootpass
volumes:
dbdata:
driver: local

C. services:
web:
image: mywebapp
volumes:
- /app:/code
ports:
- "3000:3000"
db:
image: mysql
environment:
MYSQL_ROOT_PASSWORD: rootpass
volumes:
- dbdata:/var/lib/mysql
volumes:
dbdata:

D. services:
web:
image: mywebapp
volumes:
- ./code:/app
ports:
- "3000:3000"
db:
image: mysql
environment:
MYSQL_ROOT_PASSWORD: rootpass

Solution

1. Step 1: Ensure live code updates for web app

   Mount host folder ./code to container /app using volumes for live updates.

2. Step 2: Ensure database data persistence

   Use a named volume (dbdata) mounted to /var/lib/mysql to keep data between restarts.

3. Step 3: Check ports and environment

   Expose port 3000 for web and set MYSQL_ROOT_PASSWORD for db environment.

4. Final Answer:

   Configuration with volumes for web and named volume for db -> Option A

5. Quick Check:

   Volumes for live code and persistent db = services: web: image: mywebapp volumes: - ./code:/app ports: - "3000:3000" db: image: mysql environment: MYSQL_ROOT_PASSWORD: rootpass volumes: - dbdata:/var/lib/mysql volumes: dbdata: [OK]

Hint: Use volumes for code and named volume for DB persistence [OK]

Common Mistakes:

• Not mounting host code folder for live updates
• Missing volume for database persistence
• Incorrect volume paths or missing volumes section