Sharing volumes between containers in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Docker volume?

A Docker volume is a storage area managed by Docker that containers can use to save and share data persistently outside the container's writable layer.

beginner

How do you share data between two Docker containers using volumes?

You create a Docker volume and mount it into both containers at the desired path. This way, both containers can read and write to the same data location.

beginner

What is the command to create a Docker volume named 'shared-data'?

docker volume create shared-data

intermediate

How do you run two containers sharing the same volume 'shared-data' at path '/app/data'?

docker run -d --name container1 -v shared-data:/app/data image1 docker run -d --name container2 -v shared-data:/app/data image2

beginner

Why is sharing volumes between containers useful?

It allows containers to share files and data easily, such as logs, configuration files, or databases, without copying data between containers or rebuilding images.

Which Docker command creates a volume named 'data-vol'?

A docker volume create data-vol

B docker create volume data-vol

C docker volume new data-vol

D docker volume add data-vol

The correct command to create a volume is 'docker volume create '.

How do you mount a volume named 'shared-vol' to '/data' inside a container?

A docker run -v /data:shared-vol image

B docker run --mount shared-vol:/data image

C docker run --volume /data:shared-vol image

D docker run -v shared-vol:/data image

The syntax to mount a volume is '-v volume-name:container-path'.

What happens if two containers share the same Docker volume?

A They cannot access the volume simultaneously.

B They share the same data stored in the volume.

C Each container gets a copy of the volume data.

D The volume is deleted after the first container stops.

Sharing a volume means both containers access the same data location.

Which of these is NOT a benefit of sharing volumes between containers?

A Persistent data storage

B Data sharing between containers

C Automatic container networking

D Avoiding data duplication

Sharing volumes does not handle networking between containers.

To share a volume between containers, you must:

A Create a volume and mount it in both containers

B Copy files manually between containers

C Use the same image for both containers

D Run containers on the same host only

Sharing volumes requires creating a volume and mounting it into each container.

Explain how to share data between two Docker containers using volumes.

Think about creating a common storage space both containers can use.

Describe the benefits of using Docker volumes to share data between containers.

Consider why sharing data directly is better than copying files.

Practice

1. What is the main purpose of sharing volumes between Docker containers?

easy

A. To automatically update container images

B. To increase the CPU performance of containers

C. To allow containers to access and modify the same files

D. To isolate containers completely from each other

Solution

1. Step 1: Understand volume sharing concept

   Sharing volumes means multiple containers can read and write the same files stored in a volume.

2. Step 2: Identify the purpose of sharing volumes

   This allows containers to share data easily, like sharing notes or files in a folder between friends.

3. Final Answer:

   To allow containers to access and modify the same files -> Option C

4. Quick Check:

   Sharing volumes = shared files access [OK]

Hint: Sharing volumes means sharing files between containers [OK]

Common Mistakes:

• Thinking volumes improve CPU or performance
• Confusing volume sharing with container isolation
• Assuming volumes update container images automatically

2. Which Docker command syntax correctly shares a volume named shared_data between two containers app1 and app2?

easy

A. docker run --volumes shared_data:/data --name app1 busybox

B. docker run -v shared_data:/data --name app1 busybox

C. docker run -v /shared_data:/data --name app1 busybox

D. docker run -v shared_data --name app1 --link app2 busybox

Solution

1. Step 1: Check volume syntax for sharing

   The correct syntax to share a named volume is -v volume_name:container_path. docker run -v shared_data:/data --name app1 busybox uses -v shared_data:/data which is correct.

2. Step 2: Verify other options

   docker run --volumes shared_data:/data --name app1 busybox uses invalid --volumes flag (should be --volume or -v). docker run -v /shared_data:/data --name app1 busybox uses a host path, not a named volume. docker run -v shared_data --name app1 --link app2 busybox uses incomplete volume syntax (missing :/data) and --link which is unrelated to volume sharing.

3. Final Answer:

   docker run -v shared_data:/data --name app1 busybox -> Option B

4. Quick Check:

   Use -v volume_name:path to share volumes [OK]

Hint: Use -v volume_name:container_path to share volumes [OK]

Common Mistakes:

• Using host paths instead of named volumes
• Confusing --link with volume sharing
• Using incorrect volume syntax

3. Given these commands, what will be the output of docker exec app2 cat /data/message.txt?

docker volume create shared_vol
docker run -d -v shared_vol:/data --name app1 busybox sh -c "echo Hello > /data/message.txt"
docker run -d -v shared_vol:/data --name app2 busybox sleep 1000

medium

A. Empty output

B. Error: file not found

C. Container app2 does not exist

D. Hello

Solution

1. Step 1: Understand volume sharing effect

   Both containers use the same volume shared_vol mounted at /data. Writing in app1 creates the file in the shared volume.

2. Step 2: Check file access from app2

   Since app2 mounts the same volume, it can read the file /data/message.txt created by app1. So the command outputs "Hello".

3. Final Answer:

   Hello -> Option D

4. Quick Check:

   Shared volume files visible in all containers [OK]

Hint: Shared volumes show same files in all containers [OK]

Common Mistakes:

• Assuming files are isolated per container
• Expecting file not found error
• Confusing container names or existence

4. You tried to share a volume between two containers but the second container cannot see the files created by the first. Which of these is the most likely cause?

medium

A. The volume was not named the same in both containers

B. The containers are running different images

C. The containers are on different networks

D. The first container did not expose ports

Solution

1. Step 1: Check volume naming consistency

   For containers to share data, they must use the exact same volume name. Different names create separate volumes.

2. Step 2: Evaluate other options

   Different images or networks do not affect volume sharing. Port exposure is unrelated to file sharing.

3. Final Answer:

   The volume was not named the same in both containers -> Option A

4. Quick Check:

   Same volume name needed to share data [OK]

Hint: Use identical volume names to share data [OK]

Common Mistakes:

• Thinking network or image affects volume sharing
• Confusing port exposure with volumes
• Using different volume names by mistake

5. You want two containers to share a volume but keep their own separate data initially, then merge changes later. Which Docker feature or approach helps achieve this?

hard

A. Use named volumes with different initial content and copy files manually

B. Use a bind mount to a host directory shared by both containers

C. Use Docker networks to share data between containers

D. Run containers with --rm to remove volumes after exit

Solution

1. Step 1: Understand volume sharing with initial separate data

   Named volumes start empty by default. To have separate initial data, create volumes separately and copy files manually.

2. Step 2: Consider merging changes later

   After initial separate data, you can copy or sync files between volumes or containers to merge changes.

3. Step 3: Evaluate other options

   Bind mounts share host directories but do not isolate initial data. Networks do not share files. --rm removes containers and volumes, not helpful here.

4. Final Answer:

   Use named volumes with different initial content and copy files manually -> Option A

5. Quick Check:

   Named volumes + manual copy = separate then merge [OK]

Hint: Start with separate volumes, copy files to merge later [OK]

Common Mistakes:

• Assuming networks share files
• Using --rm which deletes volumes
• Thinking bind mounts isolate initial data