Restart policies in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the effect of the restart policy 'on-failure:3'?

You run a Docker container with the restart policy set as on-failure:3. What happens if the container exits with a failure code 1 four times in a row?

Docker

docker run --restart=on-failure:3 myapp

A Docker never restarts the container because the failure count is exceeded before starting.

B Docker restarts the container indefinitely regardless of failure count.

C Docker restarts the container only the first 3 times it fails, then stops trying after the 4th failure.

D Docker restarts the container exactly 4 times, including the initial run.

💡 Hint

Think about how the number after 'on-failure' limits restart attempts.

The on-failure:3 policy tells Docker to restart the container only if it exits with a failure code, and to do this at most 3 times. After the 3rd restart, Docker stops trying.

🧠 Conceptual

intermediate

Which restart policy restarts a container always, even if it exits normally?

You want a Docker container to restart no matter how it stops, including normal exits. Which restart policy should you use?

A on-failure

B always

C unless-stopped

D no

💡 Hint

Consider which policy ignores exit status.

The always policy restarts the container regardless of exit status, including normal exits.

❓ Troubleshoot

advanced

Why does a container with restart policy 'unless-stopped' not restart after a manual stop?

You set a container with --restart=unless-stopped. You manually stop it with docker stop. What happens if Docker daemon restarts?

A The container does not restart because it was manually stopped.

B The container restarts only if Docker daemon was restarted manually.

C The container restarts only if it exited with failure.

D The container restarts because 'unless-stopped' ignores manual stops.

💡 Hint

Think about how 'unless-stopped' treats manual stops.

The unless-stopped policy restarts containers unless they were manually stopped. After a manual stop, Docker will not restart the container on daemon restart.

🔀 Workflow

advanced

Order the steps to set a restart policy on an existing container

You have a running container named 'webapp'. You want to set its restart policy to 'always'. What is the correct order of commands?

A 2,3,1,4

B 1,2,3,4

C 1,4,2,3

D 2,1,3,4

💡 Hint

You must stop the container before updating restart policy.

You stop the container first, then update the restart policy, then start it again. Finally, inspect to verify the change.

✅ Best Practice

expert

Which restart policy is best for a critical service that must always run unless explicitly stopped?

You deploy a critical service in Docker that must restart automatically on failure or daemon restart, but should not restart if you manually stop it. Which restart policy fits best?

A unless-stopped

B always

C no

D on-failure

💡 Hint

Consider manual stops and automatic restarts on daemon restart.

The unless-stopped policy restarts containers on failure or daemon restart, but respects manual stops by not restarting then.