Container filesystem is ephemeral in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding container filesystem persistence

Which statement best describes why data stored inside a Docker container's filesystem is lost after the container stops?

A The container filesystem is ephemeral and resets to the original image state on restart.

B Docker automatically backs up container files to the host, so data is never lost.

C Containers use a permanent disk partition on the host that preserves all changes.

D Container filesystems are shared with the host and changes are instantly saved.

💡 Hint

Think about what happens when you stop and start a container without volumes.

Docker containers use a layered filesystem that resets to the original image state when restarted unless volumes are used. This means any changes made inside the container's filesystem are lost after it stops.

💻 Command Output

intermediate

Effect of file creation inside a container

What will be the output of the following commands executed on a Docker container?

docker run --rm ubuntu sh -c "echo hello > /tmp/greeting.txt && cat /tmp/greeting.txt"
docker run --rm ubuntu cat /tmp/greeting.txt

Assuming no volumes are used.

A Both commands output 'hello'.

B First command outputs 'hello', second command outputs nothing and returns an error.

C First command outputs nothing, second command outputs 'hello'.

D Both commands output nothing and return errors.

💡 Hint

Consider what happens to files created inside a container after it exits.

The first command creates a file and outputs 'hello'. The container is removed after running. The second command starts a new container with a fresh filesystem, so the file does not exist and cat returns an error.

🔀 Workflow

advanced

Preserving data across container restarts

You want to keep data created by a container even after it stops. Which Docker command option should you use to achieve this?

A Use <code>docker run --network host</code> to share the host network.

B Use <code>docker run --rm</code> to automatically remove the container after exit.

C Use <code>docker run -v /host/path:/container/path</code> to mount a host directory as a volume.

D Use <code>docker run --memory 512m</code> to limit container memory.

💡 Hint

Think about how to store data outside the container's ephemeral filesystem.

Mounting a host directory as a volume with -v allows data to persist on the host filesystem, surviving container restarts and removals.

❓ Troubleshoot

advanced

Data loss despite using volumes

You mounted a volume to persist data, but after restarting the container, the data is missing. What is the most likely cause?

A The volume was mounted to the wrong container path, so data was saved elsewhere.

B Docker volumes automatically delete data on container restart.

C The container image does not support volumes.

D Docker does not allow writing to volumes by default.

💡 Hint

Check the paths used in the volume mount.

If the volume is mounted to the wrong path inside the container, data written elsewhere will not be saved to the volume, causing apparent data loss.

✅ Best Practice

expert

Ensuring data durability in containerized applications

Which practice best ensures that important application data is not lost when containers are updated or recreated?

A Use ephemeral containers without any data storage to avoid complexity.

B Rely on container snapshots to save the container state before updates.

C Store all data inside the container image layers for persistence.

D Use Docker volumes or bind mounts to store data outside the container filesystem.

💡 Hint

Think about separating data from container lifecycle.

Using volumes or bind mounts stores data on the host or external storage, independent of container lifecycle, ensuring durability across updates and recreations.