Named volumes for data sharing in Docker - Mini Project: Build & Apply

Named volumes for data sharing

📖 Scenario: You are setting up two Docker containers that need to share data. To do this safely and easily, you will use a named volume. This volume will store files that both containers can access.

🎯 Goal: Create a Docker named volume called shared_data. Then, configure two containers to use this volume so they can share files. Finally, verify the shared data by listing the files inside the volume from one container.

📋 What You'll Learn

Create a named volume called shared_data

Run a container named writer that uses the shared_data volume and writes a file inside it

Run a container named reader that uses the shared_data volume and lists the files inside it

Print the output of the reader container showing the shared file

💡 Why This Matters

🌍 Real World

Named volumes are used in real projects to share data between containers, like sharing database files or configuration.

💼 Career

Understanding named volumes is important for DevOps roles to manage persistent data and container communication.

1

Create a named volume called shared_data

Write the Docker command to create a named volume called shared_data.

Docker

# Your code here

Hint

Use docker volume create followed by the volume name.

2

Run a container named writer that uses the shared_data volume and writes a file

Write the Docker command to run a container named writer using the shared_data volume mounted at /data. Inside the container, create a file named hello.txt with the content Hello from writer. Use the alpine image and run the command sh -c "echo 'Hello from writer' > /data/hello.txt".

Docker

docker volume create shared_data
# Your code here

Hint

Use docker run --rm --name writer -v shared_data:/data alpine sh -c "echo 'Hello from writer' > /data/hello.txt".

3

Run a container named reader that uses the shared_data volume and lists files

Write the Docker command to run a container named reader using the shared_data volume mounted at /data. Use the alpine image and run the command ls /data to list files inside the volume.

Docker

docker volume create shared_data
docker run --rm --name writer -v shared_data:/data alpine sh -c "echo 'Hello from writer' > /data/hello.txt"
# Your code here

Hint

Use docker run --rm --name reader -v shared_data:/data alpine ls /data.

4

Print the output of the reader container showing the shared file

Run the command to list files inside the shared_data volume from the reader container and print the output. The output should show hello.txt.

Docker

docker volume create shared_data
docker run --rm --name writer -v shared_data:/data alpine sh -c "echo 'Hello from writer' > /data/hello.txt"
docker run --rm --name reader -v shared_data:/data alpine ls /data
# Your code here

Hint

Run the reader container with ls /data and check the output.

Practice

1. What is the main purpose of using named volumes in Docker?

easy

A. To isolate network traffic between containers

B. To speed up container startup time

C. To share and persist data between containers safely

D. To limit CPU usage of containers

Solution

1. Step 1: Understand what named volumes do

   Named volumes store data outside the container's writable layer, allowing data to persist beyond container life.

2. Step 2: Identify the main benefit

   This persistence and sharing of data between containers is the key purpose of named volumes.

3. Final Answer:

   To share and persist data between containers safely -> Option C

4. Quick Check:

   Named volumes = data sharing and persistence [OK]

Hint: Named volumes keep data safe and share it [OK]

Common Mistakes:

• Confusing volumes with network settings
• Thinking volumes improve CPU or speed
• Assuming volumes delete data with containers

2. Which of the following is the correct command to create a named volume called mydata?

easy

A. docker volume create mydata

B. docker create volume mydata

C. docker volume new mydata

D. docker volume make mydata

Solution

1. Step 1: Recall Docker volume creation syntax

   The correct command to create a volume is docker volume create [name].

2. Step 2: Match the command with options

   Only docker volume create mydata uses the correct syntax: docker volume create mydata.

3. Final Answer:

   docker volume create mydata -> Option A

4. Quick Check:

   Volume create command = docker volume create [OK]

Hint: Use 'docker volume create' to make named volumes [OK]

Common Mistakes:

• Swapping order of words in the command
• Using 'new' or 'make' instead of 'create'
• Omitting the 'volume' keyword

3. Given the command:

docker run -d --name app1 -v myvol:/app/data busybox sleep 1000

What happens to the data inside /app/data if the container app1 is removed?

medium

A. Data inside /app/data is saved in the named volume 'myvol'

B. Data inside /app/data is lost unless backed up manually

C. Data inside /app/data is copied to the host's /var/lib/docker folder automatically

D. Data inside /app/data is deleted with the container

Solution

1. Step 1: Understand volume mounting in the command

   The -v myvol:/app/data option mounts the named volume 'myvol' to the container path '/app/data'.

2. Step 2: Know volume persistence behavior

   Named volumes keep data even if the container is removed, so data inside '/app/data' is saved in 'myvol'.

3. Final Answer:

   Data inside /app/data is saved in the named volume 'myvol' -> Option A

4. Quick Check:

   Named volume keeps data after container removal [OK]

Hint: Named volumes keep data safe after container removal [OK]

Common Mistakes:

• Thinking data deletes with container
• Confusing named volumes with anonymous volumes
• Assuming data copies to host automatically

4. You run this command to share data between two containers:

docker run -d --name c1 -v sharedvol:/data busybox sleep 1000
docker run -d --name c2 -v sharedvol:/data busybox sleep 1000

But container c2 cannot see files created by c1 inside /data. What is the likely problem?

medium

A. The volume mount syntax is incorrect

B. Containers must be on the same network to share volumes

C. The volume 'sharedvol' was not created before running containers

D. Files created by c1 are not saved because the container is stopped

Solution

1. Step 1: Check volume creation requirement

   Named volumes must exist or be created automatically. If 'sharedvol' was never created, Docker creates an empty volume.

2. Step 2: Understand volume persistence and sharing

   If 'sharedvol' was not created or data was not saved properly, c2 won't see c1's files. Network does not affect volume sharing.

3. Final Answer:

   Files created by c1 are not saved because the container is stopped -> Option D

4. Quick Check:

   Stopped container does not save files to volume [OK]

Hint: Files must be saved and container running to share data [OK]

Common Mistakes:

• Thinking network affects volume sharing
• Assuming mount syntax is wrong
• Believing stopped containers save files automatically

5. You want to share a folder /var/log/app on your host with multiple containers using a named volume. Which sequence of commands correctly creates and uses the named volume to achieve this?

hard

A. docker volume create app_logs

docker run -d -v /var/log/app:/app/log busybox sleep 1000

B. docker volume create --opt type=none --opt device=/var/log/app --opt o=bind app_logs

docker run -d -v app_logs:/app/log busybox sleep 1000

C. docker volume create app_logs

docker run -d -v app_logs:/var/log/app busybox sleep 1000

D. docker run -d -v app_logs:/app/log busybox sleep 1000

Solution

1. Step 1: Create a named volume that binds to host folder

   Use docker volume create with options type=none, device=/var/log/app, and o=bind to bind the host folder to the volume.

2. Step 2: Run container mounting the named volume

   Mount the named volume app_logs to the container path /app/log with -v app_logs:/app/log.

3. Final Answer:

   docker volume create --opt type=none --opt device=/var/log/app --opt o=bind app_logs docker run -d -v app_logs:/app/log busybox sleep 1000 -> Option B

4. Quick Check:

   Bind host folder with volume create options [OK]

Hint: Use volume create with bind options for host folder sharing [OK]

Common Mistakes:

• Mounting host folder directly without volume create
• Mounting volume to wrong container path
• Skipping volume creation step