Sharing volumes between containers in Docker - Commands & Configuration

Introduction

Sometimes, you want two or more containers to access the same files. Sharing volumes lets containers read and write the same data, like sharing a folder between friends.

When you want a web server container and a logging container to access the same log files.

When multiple app containers need to read and write shared configuration files.

When you want to persist data created by one container and use it in another container.

When you want to share a database data folder between backup and main database containers.

When you want to share uploaded files between a frontend and backend container.

Config File - docker-compose.yml

docker-compose.yml

version: '3.8'
services:
  app1:
    image: busybox
    command: sh -c "echo 'Hello from app1' > /shared/message.txt && sleep 3600"
    volumes:
      - shared-data:/shared
  app2:
    image: busybox
    command: sh -c "cat /shared/message.txt && sleep 3600"
    volumes:
      - shared-data:/shared
volumes:
  shared-data:

This file defines two containers, app1 and app2, sharing the same volume called shared-data.

app1 writes a message to a file inside the shared volume.

app2 reads the message from the same file.

The volumes section at the bottom creates the shared volume.

Commands

This command starts both containers in the background, creating the shared volume automatically.

Terminal

docker-compose up -d

Expected Output

Creating network "default" with the default driver

Creating volume "shared-data" with default driver

Creating app1 ... done

Creating app2 ... done

→

-d - Run containers in detached (background) mode

This command shows the logs of app2, which should display the message written by app1 to the shared volume.

Terminal

docker-compose logs app2

Expected Output

app2    | Hello from app1

This command lists all Docker volumes, showing that the shared volume exists.

Terminal

docker volume ls

Expected Output

DRIVER              VOLUME NAME

local               shared-data

This command stops and removes the containers and deletes the shared volume to clean up.

Terminal

docker-compose down -v

Expected Output

Stopping app2 ... done

Stopping app1 ... done

Removing app2 ... done

Removing app1 ... done

Removing network default

Removing volume shared-data

→

-v - Remove named volumes declared in the compose file

Key Concept

If you remember nothing else from this pattern, remember: Docker volumes let multiple containers share the same files safely and persistently.

Common Mistakes

Mounting the same host folder with different paths inside containers without using a named volume.

This can cause permission issues or inconsistent data because the host folder may not be properly shared or isolated.

Use a named Docker volume declared in the compose file to share data between containers.

Not using the same volume name in all containers that need to share data.

Containers will not share data if they mount different volumes or paths, so the data won't be visible across containers.

Use the exact same volume name in the volumes section for all containers that need to share files.

Not cleaning up volumes after testing, leaving unused volumes consuming disk space.

Unused volumes can accumulate and waste disk space over time.

Use 'docker-compose down -v' to remove volumes when they are no longer needed.

Summary

Define a named volume in docker-compose.yml to share files between containers.

Mount the same named volume path inside each container that needs access.

Use 'docker-compose up -d' to start containers and 'docker-compose logs' to verify shared data.

Clean up with 'docker-compose down -v' to remove containers and volumes.

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