Volumes in Compose in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a volume in Docker Compose?

A volume is a storage area outside the container's writable layer. It keeps data safe even if the container is deleted or recreated.

beginner

How do you define a named volume in a docker-compose.yml file?

You define it under the 'volumes:' key at the bottom of the file, like this:

volumes:
  mydata:

intermediate

What is the difference between a bind mount and a volume in Docker Compose?

A bind mount links a host folder directly to the container, while a volume is managed by Docker and stored in Docker's storage area.

beginner

How do you attach a volume to a service in docker-compose.yml?

Under the service, use the 'volumes:' key and specify the volume name and container path, like:

volumes:
  - mydata:/app/data

beginner

Why use volumes in Docker Compose?

Volumes keep data persistent, share data between containers, and separate data from container lifecycle.

Where are Docker volumes stored by default?

A Inside the container's writable layer

B In the container's memory

C On a remote cloud server

D In Docker's managed storage area on the host

Docker volumes are stored in a special area on the host managed by Docker, separate from the container's writable layer.

How do you declare a volume for a service in docker-compose.yml?

A In the Dockerfile

B Under the service's 'volumes:' key with volume_name:container_path

C Under the 'services:' key only

D By running a docker volume create command inside the container

Volumes are declared under the service's 'volumes:' key in docker-compose.yml using the format volume_name:container_path.

What happens to data in a volume if the container is deleted?

A Data is moved to a backup folder automatically

B Data is deleted with the container

C Data remains safe in the volume

D Data is lost permanently

Volumes keep data safe even if the container is deleted or recreated.

Which of these is NOT a benefit of using volumes?

A Increasing container CPU speed

B Sharing data between containers

C Separating data from container lifecycle

D Data persistence

Volumes help with data persistence and sharing but do not affect container CPU speed.

What is a bind mount in Docker Compose?

A A direct link between a host folder and container folder

B A volume managed by Docker

C A network connection between containers

D A special type of container

A bind mount links a host folder directly to a container folder.

Explain how to define and use a volume in a docker-compose.yml file.

Think about where volumes are declared and how services use them.

Describe the benefits of using volumes in Docker Compose.

Why do we want data to live outside containers?

Practice

1. What is the main purpose of using volumes in a Docker Compose file?

easy

A. To limit container CPU usage

B. To speed up container startup time

C. To automatically update container images

D. To keep data safe outside containers so it persists

Solution

1. Step 1: Understand volume role in Docker

   Volumes store data outside the container's writable layer, so data is not lost when containers stop or are removed.

2. Step 2: Compare options

   Options B, C, and D relate to performance or resource limits, not data persistence.

3. Final Answer:

   To keep data safe outside containers so it persists -> Option D

4. Quick Check:

   Volumes = Data persistence [OK]

Hint: Volumes = persistent data storage outside containers [OK]

Common Mistakes:

• Thinking volumes speed up container startup
• Confusing volumes with resource limits
• Assuming volumes update images automatically

2. Which of the following is the correct syntax to define a named volume in a Docker Compose file?

easy

A. services:
volumes:
- mydata:/app/data

B. volumes:
- mydata:/app/data

C. volumes:
mydata:
driver: local

D. volumes:
mydata: /app/data

Solution

1. Step 1: Identify volume declaration section

   Named volumes are declared under the top-level 'volumes:' key with a name and optional driver.

2. Step 2: Check syntax correctness

   volumes: mydata: driver: local correctly defines a named volume 'mydata' with driver 'local'. Options A, B, and D mix service and volume syntax incorrectly or have invalid mapping syntax.

3. Final Answer:

   volumes: mydata: driver: local -> Option C

4. Quick Check:

   Named volume syntax = volumes: mydata: driver: local [OK]

Hint: Define named volumes under top-level 'volumes:' with name and driver [OK]

Common Mistakes:

• Defining volumes under services instead of top-level
• Using list syntax for volume declaration
• Incorrect mapping format for volumes

3. Given this snippet in a Docker Compose file:

services:
  web:
    image: nginx
    volumes:
      - mydata:/usr/share/nginx/html
volumes:
  mydata:

What happens to the data inside /usr/share/nginx/html when the container restarts?

medium

A. Data persists because 'mydata' is a named volume

B. Data is shared with the host folder automatically

C. Data is reset to the image default every restart

D. Data is lost because volumes do not persist

Solution

1. Step 1: Understand named volume behavior

   Named volumes like 'mydata' store data outside the container and keep it between restarts.

2. Step 2: Analyze the mount point

   The volume is mounted to '/usr/share/nginx/html', so data written there is saved in 'mydata' volume.

3. Final Answer:

   Data persists because 'mydata' is a named volume -> Option A

4. Quick Check:

   Named volume mount = data persistence [OK]

Hint: Named volumes keep data safe across container restarts [OK]

Common Mistakes:

• Assuming data resets to image default
• Confusing named volumes with bind mounts
• Thinking volumes do not persist data

4. You wrote this in your Docker Compose file but get an error:

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

What is the problem?

medium

A. You cannot use relative paths under 'volumes:' top-level key

B. The image name 'myapp' is invalid

C. Bind mounts must be declared under 'volumes:' top-level key

D. The service name 'app' is reserved

Solution

1. Step 1: Understand volume declaration rules

   Top-level 'volumes:' is for named volumes only, not for bind mounts with paths.

2. Step 2: Identify misuse of relative path

   Using './data:' under 'volumes:' top-level is invalid syntax; relative paths belong only in service volume mounts.

3. Final Answer:

   You cannot use relative paths under 'volumes:' top-level key -> Option A

4. Quick Check:

   Relative paths only in service volumes [OK]

Hint: Declare bind mounts only under service volumes, not top-level [OK]

Common Mistakes:

• Putting bind mount paths under top-level volumes
• Assuming image name causes this error
• Thinking service names are restricted

5. You want to share a local folder ./config with a container's /etc/config path and also keep a database's data persistent using a named volume dbdata. Which is the correct Docker Compose snippet?

hard

A. services:
app:
image: myapp
volumes:
- dbdata:/etc/config
db:
image: postgres
volumes:
- ./config:/var/lib/postgresql/data
volumes:
dbdata:

B. services:
app:
image: myapp
volumes:
- ./config:/etc/config
db:
image: postgres
volumes:
- dbdata:/var/lib/postgresql/data
volumes:
dbdata:

C. services:
app:
image: myapp
volumes:
- /etc/config:./config
db:
image: postgres
volumes:
- /var/lib/postgresql/data:dbdata
volumes:
dbdata:

D. services:
app:
image: myapp
volumes:
- ./config:/etc/config
db:
image: postgres
volumes:
- ./dbdata:/var/lib/postgresql/data
volumes:
dbdata:

Solution

1. Step 1: Identify bind mount for local folder sharing

   Bind mounts use local paths like './config:/etc/config' under service volumes.

2. Step 2: Identify named volume for database persistence

   Named volumes like 'dbdata' are declared top-level and mounted as 'dbdata:/var/lib/postgresql/data'.

3. Step 3: Check options for correct syntax

   services: app: image: myapp volumes: - ./config:/etc/config db: image: postgres volumes: - dbdata:/var/lib/postgresql/data volumes: dbdata: correctly uses bind mount for app and named volume for db. Options B and C swap paths incorrectly. services: app: image: myapp volumes: - ./config:/etc/config db: image: postgres volumes: - ./dbdata:/var/lib/postgresql/data volumes: dbdata: uses a bind mount for dbdata instead of named volume.

4. Final Answer:

   services: app: image: myapp volumes: - ./config:/etc/config db: image: postgres volumes: - dbdata:/var/lib/postgresql/data volumes: dbdata: -> Option B

5. Quick Check:

   Bind mount + named volume = services: app: image: myapp volumes: - ./config:/etc/config db: image: postgres volumes: - dbdata:/var/lib/postgresql/data volumes: dbdata: [OK]

Hint: Use bind mounts for local folders, named volumes for persistent data [OK]

Common Mistakes:

• Swapping source and target paths
• Using bind mounts instead of named volumes for DB data
• Declaring bind mounts under top-level volumes