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.

Practice

1. What does the --restart=always policy do when used with docker run?

easy

A. It restarts the container only if it exits with an error.

B. It never restarts the container automatically.

C. It restarts the container automatically whenever it stops, no matter the reason.

D. It restarts the container only if manually triggered.

Solution

1. Step 1: Understand the restart policy options

   The --restart flag controls when Docker restarts a container automatically.

2. Step 2: Analyze the always policy

   The always policy means Docker restarts the container whenever it stops, regardless of exit status or manual stop.

3. Final Answer:

   It restarts the container automatically whenever it stops, no matter the reason. -> Option C

4. Quick Check:

   --restart=always = restart always [OK]

Hint: Always means restart no matter what, even manual stops [OK]

Common Mistakes:

• Confusing 'always' with 'on-failure'
• Thinking 'always' ignores manual stops
• Mixing 'no' restart with 'always'

2. Which of the following is the correct syntax to run a container with a restart policy that restarts only on failure?

easy

A. docker run --restart=on-failure:5 myimage

B. docker run --restart=always myimage

C. docker run --restart unless-stopped myimage

D. docker run --restart=no myimage

Solution

1. Step 1: Recall the syntax for restart policies

   The correct syntax uses --restart=policy[:max-retries] with an equals sign and optional retry count.

2. Step 2: Identify the option that matches restart on failure with max retries

   on-failure:5 means restart only if the container fails, up to 5 times.

3. Final Answer:

   docker run --restart=on-failure:5 myimage -> Option A

4. Quick Check:

   Correct syntax uses equals sign and policy [OK]

Hint: Use --restart=policy with equals sign, e.g. --restart=on-failure [OK]

Common Mistakes:

• Missing equals sign (=) after --restart
• Using wrong policy names or formats
• Confusing 'always' with 'on-failure'

3. What will happen if you run this command?

docker run --restart=unless-stopped myapp

medium

A. The container restarts unless you manually stop it.

B. The container restarts always, even if stopped manually.

C. The container never restarts automatically.

D. The container restarts only if it fails, not if stopped manually.

Solution

1. Step 1: Understand the 'unless-stopped' policy

   This policy restarts the container automatically except when you stop it manually.

2. Step 2: Apply the policy to the command

   Since the command uses --restart=unless-stopped, the container will restart on failures or Docker daemon restarts, but not if manually stopped.

3. Final Answer:

   The container restarts unless you manually stop it. -> Option A

4. Quick Check:

   unless-stopped means restart except manual stop [OK]

Hint: 'unless-stopped' means restart except manual stop [OK]

Common Mistakes:

• Thinking it restarts always even after manual stop
• Confusing 'unless-stopped' with 'on-failure'
• Assuming no restart happens

4. You tried to run a container with this command:

docker run --restart always:3 myimage

Why does it fail?

medium

A. The container name is missing.

B. The image name is invalid.

C. The '--restart' flag is misspelled.

D. The 'always' policy does not accept a retry count.

Solution

1. Step 1: Check the restart policy syntax

   The always policy does not support a retry count suffix like ':3'. Only on-failure accepts that.

2. Step 2: Identify the cause of failure

   Using always:3 is invalid syntax, so Docker rejects the command.

3. Final Answer:

   The 'always' policy does not accept a retry count. -> Option D

4. Quick Check:

   always policy has no retry count [OK]

Hint: 'always' policy never uses retry count suffix [OK]

Common Mistakes:

• Adding retry count to 'always' policy
• Assuming all policies accept retry counts
• Ignoring error messages from Docker

5. You want a container to restart automatically only if it crashes, but not if you stop it manually. Which restart policy and command should you use?

hard

A. docker run --restart=always mycontainer

B. docker run --restart=on-failure mycontainer

C. docker run --restart=unless-stopped mycontainer

D. docker run --restart=no mycontainer

Solution

1. Step 1: Understand the requirement

   The container should restart only if it crashes (fails), but not if stopped manually.

2. Step 2: Match the restart policy

   on-failure restarts the container only when it exits with an error, ignoring manual stops.

3. Step 3: Verify the command

   The command docker run --restart=on-failure mycontainer correctly applies this policy.

4. Final Answer:

   docker run --restart=on-failure mycontainer -> Option B

5. Quick Check:

   on-failure = restart only on crash [OK]

Hint: Use 'on-failure' to restart only on crashes, not manual stops [OK]

Common Mistakes:

• Using 'always' which restarts even after manual stop
• Using 'unless-stopped' which restarts after manual stop
• Choosing 'no' which disables restart