Backup and restore strategies in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Backup a Docker volume using a container

What is the output of the following command sequence to backup a Docker volume named mydata into a tar file?

Docker

docker run --rm -v mydata:/data -v $(pwd):/backup alpine tar czf /backup/mydata_backup.tar.gz -C /data .
docker run --rm -v $(pwd):/backup alpine ls /backup

A mydata_backup.tar.gz

B mydata_backup.tar

C backup.tar.gz

D data_backup.tar.gz

💡 Hint

Check the tar file name specified in the command.

The tar file is named mydata_backup.tar.gz as specified in the tar command inside the container.

🧠 Conceptual

intermediate

Choosing backup strategy for Docker containers

Which backup strategy is best to ensure data persistence for a database running inside a Docker container?

A Backing up the database data directory using a Docker volume backup

B Backing up the container filesystem using docker commit

C Backing up the container logs using docker logs command

D Backing up the container image using docker save

💡 Hint

Think about where the database stores its data persistently.

Backing up the database data directory stored in a Docker volume ensures the actual data is saved. Container filesystem or image backups do not guarantee data consistency.

❓ Troubleshoot

advanced

Restore failure from Docker volume backup

You restored a Docker volume backup using the command docker run --rm -v mydata:/data -v $(pwd):/backup alpine tar xzf /backup/mydata_backup.tar.gz -C /data but the application cannot read the data. What is the most likely cause?

A The backup file is corrupted and cannot be extracted

B The volume <code>mydata</code> was not empty before restore causing permission conflicts

C The tar extraction did not preserve file permissions and ownership

D The container image alpine does not support tar extraction

💡 Hint

Consider what happens to file permissions when extracting tar archives inside containers.

Tar extraction inside a container may not preserve original file ownership and permissions, causing the application to fail reading data.

🔀 Workflow

advanced

Automated backup workflow for Docker volumes

Which sequence of commands correctly automates backing up a Docker volume appdata daily to a timestamped file in /backups directory on the host?

A mkdir /backups && docker save appdata > /backups/appdata_$(date +%F).tar.gz

B docker volume create appdata_backup && docker run --rm -v appdata_backup:/backup alpine tar czf /backup/appdata_$(date +%F).tar.gz -C /appdata .

C docker run --rm -v appdata:/data alpine tar czf /data/appdata_$(date +%F).tar.gz -C /data .

D mkdir -p /backups && docker run --rm -v appdata:/data -v /backups:/backup alpine sh -c 'tar czf /backup/appdata_$(date +%F).tar.gz -C /data .'

💡 Hint

Check volume mounting and tar command correctness.

Option D correctly mounts the volume and host backup directory, creates the backup file with date, and ensures the backup directory exists.

✅ Best Practice

expert

Best practice for restoring Docker container state with minimal downtime

What is the best practice to restore a Docker container's data volume and minimize downtime in a production environment?

A Stop the container, restore the volume backup, then start the container immediately

B Create a new volume, restore backup there, then update the container to use the new volume and restart

C Restore the backup directly into the running container's volume using docker exec

D Delete the container and volume, recreate both, then restore the backup

💡 Hint

Consider how to avoid data loss and downtime during restore.

Creating a new volume and restoring backup there allows testing and switching volumes with minimal downtime and risk.

Practice

1. What is the main purpose of backing up Docker volumes?

easy

A. To update Docker to the latest version

B. To speed up container startup time

C. To reduce the size of Docker images

D. To save important data so it can be restored later

Solution

1. Step 1: Understand backup purpose

   Backing up means saving data safely so it is not lost.

2. Step 2: Relate to Docker volumes

   Docker volumes store important data outside containers, so backing them up protects that data.

3. Final Answer:

   To save important data so it can be restored later -> Option D

4. Quick Check:

   Backup = Save data safely [OK]

Hint: Backup means saving data to prevent loss [OK]

Common Mistakes:

• Confusing backup with speeding up containers
• Thinking backup reduces image size
• Assuming backup updates Docker itself

2. Which of the following commands correctly creates a backup of a Docker volume named mydata using tar?

easy

A. docker save mydata > /backup/mydata.tar

B. docker run --rm -v mydata:/data -v /backup:/backup busybox tar cvf /backup/mydata.tar /data

C. docker volume backup mydata /backup/mydata.tar

D. docker run --rm -v mydata:/data busybox tar xvf /backup/mydata.tar /data

Solution

1. Step 1: Identify backup command syntax

   Backing up a volume uses tar cvf to create an archive of the volume data.

2. Step 2: Check each option

   docker run --rm -v mydata:/data -v /backup:/backup busybox tar cvf /backup/mydata.tar /data runs a container mounting the volume and backup directory and uses tar cvf to save data. docker run --rm -v mydata:/data busybox tar xvf /backup/mydata.tar /data extracts instead of creating. docker volume backup mydata /backup/mydata.tar is invalid Docker syntax. docker save mydata > /backup/mydata.tar saves an image, not a volume.

3. Final Answer:

   docker run --rm -v mydata:/data -v /backup:/backup busybox tar cvf /backup/mydata.tar /data -> Option B

4. Quick Check:

   Backup = tar cvf inside container [OK]

Hint: Use tar cvf to create backup archive inside container [OK]

Common Mistakes:

• Using tar xvf which extracts instead of backs up
• Trying non-existent docker volume backup command
• Using docker save which backs up images, not volumes

3. What will be the result of running this command?

docker run --rm -v mydata:/data -v /backup:/backup busybox tar xvf /backup/mydata.tar -C /data

medium

A. It restores the volume data from the backup archive

B. It creates a backup archive of the volume data

C. It lists the contents of the backup archive without extracting

D. It deletes the volume data

Solution

1. Step 1: Understand the tar command

   The tar xvf command extracts files from an archive.

2. Step 2: Analyze the Docker command

   The command mounts the volume mydata at /data and the backup directory at /backup and extracts the archive /backup/mydata.tar into it using -C /data.

3. Final Answer:

   It restores the volume data from the backup archive -> Option A

4. Quick Check:

   tar xvf = extract = restore [OK]

Hint: tar xvf extracts files, so it restores data [OK]

Common Mistakes:

• Confusing xvf with cvf (backup vs restore)
• Thinking it lists archive contents
• Assuming it deletes data

4. You tried to restore a Docker volume backup with this command:

docker run --rm -v mydata:/data busybox tar cvf /backup/mydata.tar /data

But the backup file is empty. What is the likely mistake?

medium

A. Using tar cvf instead of tar xvf to restore

B. Mounting the volume at the wrong path

C. Creating a backup instead of restoring

D. Not specifying the -C option to change directory

Solution

1. Step 1: Identify the command purpose

   tar cvf creates an archive (backup), not restore.

2. Step 2: Understand the mistake

   The command tries to create a backup but is used when intending to restore, so the backup file is empty or incorrect.

3. Final Answer:

   Creating a backup instead of restoring -> Option C

4. Quick Check:

   tar cvf = backup, not restore [OK]

Hint: tar cvf creates backup; use tar xvf to restore [OK]

Common Mistakes:

• Confusing backup and restore tar options
• Mounting volume incorrectly
• Forgetting to use -C option

5. You want to backup multiple Docker volumes data1 and data2 into separate tar files and then restore them later. Which sequence of commands correctly achieves this?

hard

A. docker run --rm -v data1:/data -v /backup:/backup busybox tar cvf /backup/data1.tar /datadocker run --rm -v data2:/data -v /backup:/backup busybox tar cvf /backup/data2.tar /datadocker run --rm -v data1:/data -v /backup:/backup busybox tar xvf /backup/data1.tar -C /datadocker run --rm -v data2:/data -v /backup:/backup busybox tar xvf /backup/data2.tar -C /data

B. docker volume backup data1 /backup/data1.tardocker volume backup data2 /backup/data2.tardocker volume restore data1 /backup/data1.tardocker volume restore data2 /backup/data2.tar

C. docker save data1 > /backup/data1.tardocker save data2 > /backup/data2.tardocker load < /backup/data1.tardocker load < /backup/data2.tar

D. docker export data1 > /backup/data1.tardocker export data2 > /backup/data2.tardocker import /backup/data1.tardocker import /backup/data2.tar

Solution

1. Step 1: Backup each volume separately

   Use tar cvf inside a container mounting each volume and the backup directory to create separate tar files.

2. Step 2: Restore each volume separately

   Use tar xvf with -C option inside a container mounting each volume and the backup directory to extract data back.

3. Step 3: Verify other options

   Other sequences use invalid Docker commands like docker volume backup, commands meant for images like docker save/load, or for containers like docker export/import.

4. Final Answer:

   docker run --rm -v data1:/data -v /backup:/backup busybox tar cvf /backup/data1.tar /data<br>docker run --rm -v data2:/data -v /backup:/backup busybox tar cvf /backup/data2.tar /data<br>docker run --rm -v data1:/data -v /backup:/backup busybox tar xvf /backup/data1.tar -C /data<br>docker run --rm -v data2:/data -v /backup:/backup busybox tar xvf /backup/data2.tar -C /data -> Option A

5. Quick Check:

   Backup and restore volumes with tar inside containers [OK]

Hint: Backup and restore each volume with tar inside containers [OK]

Common Mistakes:

• Using docker save/load which works for images, not volumes
• Using non-existent docker volume backup commands
• Confusing export/import with volume backup