Why Docker Compose simplifies multi-container apps - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start all services defined in a Docker Compose file.

Docker

docker-compose [1]

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

Abuild

Brun

Cup

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'build' instead of 'up' will only build images but not start containers.

Using 'stop' will stop running containers, not start them.

The docker-compose up command starts all the services defined in the docker-compose.yml file, making it easy to run multi-container apps with one command.

2fill in blank

medium

Complete the code to define a service named 'web' in a Docker Compose YAML file.

Docker

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

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

A80

B8080

C443

D3000

Attempts:

3 left

💡 Hint

Common Mistakes

Using 8080 or 3000 as container port will not work with default nginx.

Using 443 is for HTTPS, not default HTTP.

Port mapping 80:80 means the host's port 80 maps to the container's port 80, which is the default HTTP port for nginx.

3fill in blank

hard

Fix the error in the Docker Compose command to rebuild images before starting containers.

Docker

docker-compose [1] --build

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

Arun

Bup

Cstart

Dbuild

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' ignores the --build flag.

Using 'build' only builds images but does not start containers.

The docker-compose up --build command rebuilds images before starting containers, which is useful after changing Dockerfiles.

4fill in blank

hard

Fill both blanks to define a volume named 'db-data' and mount it to '/var/lib/mysql' in the 'db' service.

Docker

services:
  db:
    image: mysql
    volumes:
      - [1]:[2]

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

Adb-data

B/var/lib/mysql

C/data

Dmysql-data

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/data' instead of '/var/lib/mysql' will not persist MySQL data correctly.

Using 'mysql-data' instead of 'db-data' changes the volume name.

Volumes are named storage areas. Here, 'db-data' is the volume name, and '/var/lib/mysql' is the path inside the container where MySQL stores data.

5fill in blank

hard

Fill all three blanks to define a network named 'app-net', attach 'web' and 'db' services to it in Docker Compose.

Docker

networks:
  [1]:

services:
  web:
    image: nginx
    networks:
      - [2]

  db:
    image: mysql
    networks:
      - [3]

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

Aapp-net

Bdefault

Dbackend-net

Attempts:

3 left

💡 Hint

Common Mistakes

Using different network names breaks service communication.

Using 'default' network name when a custom network is defined.

Defining a network 'app-net' and attaching both 'web' and 'db' services to it allows them to communicate easily.

Practice

1. What is the main benefit of using Docker Compose for multi-container applications?

easy

A. It allows managing multiple containers with a single configuration file and command.

B. It replaces Docker Engine for running containers.

C. It automatically updates container images without user input.

D. It only works with single-container applications.

Solution

1. Step 1: Understand Docker Compose purpose

   Docker Compose uses one file to define and run multiple containers together easily.

2. Step 2: Compare options

   Options B, C, and D describe incorrect or unrelated features. Only It allows managing multiple containers with a single configuration file and command. correctly states the main benefit.

3. Final Answer:

   It allows managing multiple containers with a single configuration file and command. -> Option A

4. Quick Check:

   Single file + command = easier multi-container management [OK]

Hint: Think: one file controls many containers [OK]

Common Mistakes:

• Confusing Docker Compose with Docker Engine
• Believing Compose updates images automatically
• Thinking Compose works only for one container

2. Which of the following is the correct command to start all services defined in a docker-compose.yml file?

easy

A. docker-compose run

B. docker-compose up

C. docker-compose build

D. docker-compose start-all

Solution

1. Step 1: Identify the command to launch services

   The docker-compose up command starts all containers defined in the compose file.

2. Step 2: Review other commands

   run starts a one-off container, build builds images, and start-all is invalid.

3. Final Answer:

   docker-compose up -> Option B

4. Quick Check:

   Start all services = docker-compose up [OK]

Hint: Use 'up' to launch all services together [OK]

Common Mistakes:

• Using 'run' instead of 'up' to start all containers
• Confusing 'build' with starting containers
• Using a non-existent command like 'start-all'

3. Given this docker-compose.yml snippet:

version: '3'
services:
  web:
    image: nginx
    ports:
      - "8080:80"
  db:
    image: mysql
    environment:
      MYSQL_ROOT_PASSWORD: example

What happens when you run docker-compose up?

medium

A. Only the web service starts, exposing port 80 on 8080.

B. The db service starts but fails due to missing password.

C. Both web and db services start; web is accessible on port 8080.

D. No containers start because the file version is incorrect.

Solution

1. Step 1: Analyze the compose file

   Two services: 'web' with nginx exposing port 8080:80, and 'db' with mysql and a root password set.

2. Step 2: Understand the command effect

   docker-compose up starts all services defined, so both 'web' and 'db' start successfully.

3. Final Answer:

   Both web and db services start; web is accessible on port 8080. -> Option C

4. Quick Check:

   All services start with 'up' if config is valid [OK]

Hint: All services in file start with 'docker-compose up' [OK]

Common Mistakes:

• Thinking only one service starts by default
• Assuming missing environment variables cause failure here
• Believing file version '3' is invalid

4. You wrote a docker-compose.yml but running docker-compose up gives an error: ERROR: yaml.scanner.ScannerError: mapping values are not allowed here. What is the most likely cause?

medium

A. Incorrect indentation or syntax in the YAML file.

B. Docker Compose is not installed on the system.

C. The Docker daemon is not running.

D. The image names are missing in the file.

Solution

1. Step 1: Understand the error message

   The error indicates a YAML syntax problem, usually caused by wrong indentation or misplaced characters.

2. Step 2: Evaluate other options

   Options B and C cause different errors, and missing image names cause different warnings, not YAML syntax errors.

3. Final Answer:

   Incorrect indentation or syntax in the YAML file. -> Option A

4. Quick Check:

   YAML errors = check indentation and syntax [OK]

Hint: YAML errors usually mean bad indentation [OK]

Common Mistakes:

• Assuming Docker Compose is missing when error is YAML
• Thinking Docker daemon status causes YAML errors
• Ignoring indentation rules in YAML files

5. You want to share your multi-container app setup with a teammate. Which Docker Compose feature best helps you do this efficiently?

hard

A. Using Docker Swarm instead of Compose for local development.

B. Running each container manually with separate docker run commands.

C. Building images locally and sending them as tar files.

D. Using a single docker-compose.yml file to define all services and their settings.

Solution

1. Step 1: Identify sharing method for multi-container apps

   A single docker-compose.yml file contains all service definitions and configurations, making sharing easy.

2. Step 2: Compare other options

   Manual docker run commands are error-prone and complex; sending images as tar files is cumbersome; Docker Swarm is for orchestration, not simple sharing.

3. Final Answer:

   Using a single docker-compose.yml file to define all services and their settings. -> Option D

4. Quick Check:

   One file shares whole app setup easily [OK]

Hint: Share one compose file, not many commands [OK]

Common Mistakes:

• Thinking manual commands are easier to share
• Believing image tar files are best for sharing setup
• Confusing Docker Swarm with Compose for local sharing