Defining services in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a service in Docker Compose?

A service in Docker Compose is a container configuration that defines how a container should run, including the image, ports, volumes, and environment variables.

beginner

Which file is used to define services in Docker Compose?

Services are defined in the docker-compose.yml file using YAML syntax.

beginner

How do you specify the image for a service in Docker Compose?

Use the image key under the service name to specify the Docker image to use, for example: image: nginx:latest.

beginner

What is the purpose of the ports key in a service definition?

The ports key maps container ports to host machine ports, allowing access to the service from outside the container.

beginner

How can you define environment variables for a Docker Compose service?

Use the environment key under the service to list environment variables as key-value pairs.

Where do you define services in Docker Compose?

A In the docker-compose.yml file

B In the Dockerfile

C In the .env file

D In the docker CLI command

Services are defined in the docker-compose.yml file using YAML syntax.

Which key specifies the Docker image for a service?

A image

B build

C container

D service

The 'image' key specifies which Docker image the service uses.

What does the 'ports' key do in a service definition?

A Sets environment variables

B Specifies the network

C Defines volumes

D Maps container ports to host ports

The 'ports' key maps container ports to host machine ports for access.

How do you add environment variables to a service?

A Using the 'env' key

B Using the 'environment' key

C Using the 'variables' key

D Using the 'config' key

The 'environment' key is used to define environment variables for a service.

Which file format is used for Docker Compose service definitions?

A JSON

B XML

C YAML

D INI

Docker Compose uses YAML format in the docker-compose.yml file.

Explain how to define a simple web service in Docker Compose.

Think about the minimum keys needed to run a web server container.

Describe the role of environment variables in Docker Compose services and how to set them.

Consider how environment variables customize container behavior.

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