Restart policies in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a restart policy in Docker?

A restart policy tells Docker when to automatically restart a container if it stops or crashes.

beginner

Name the four main Docker restart policies.

The four main restart policies are: no, on-failure, always, and unless-stopped.

beginner

What does the 'always' restart policy do?

The 'always' policy restarts the container whenever it stops, no matter the exit status, even after Docker daemon restarts.

beginner

How does the 'on-failure' restart policy work?

The 'on-failure' policy restarts the container only if it exits with a non-zero exit code, indicating an error.

intermediate

What is the difference between 'unless-stopped' and 'always' restart policies?

'Unless-stopped' restarts containers unless you manually stop them. 'Always' restarts containers even after manual stops.

Which Docker restart policy restarts a container only if it crashes with an error?

A always

B on-failure

C no

D unless-stopped

The 'on-failure' policy restarts containers only when they exit with a non-zero error code.

What happens if you set the restart policy to 'no'?

A Docker never restarts the container automatically

B Docker always restarts the container

C Docker restarts only on failure

D Docker restarts unless manually stopped

The 'no' policy means Docker will not restart the container automatically.

Which restart policy will restart a container even after Docker daemon restarts?

A no

B on-failure

C always

D none

The 'always' policy restarts containers even after Docker daemon restarts.

If you want a container to restart unless you manually stop it, which policy do you choose?

A unless-stopped

B always

C on-failure

D no

'Unless-stopped' restarts containers unless you manually stop them.

How do you set a restart policy when running a Docker container?

A docker create --restart-policy=<policy> <image>

B docker start --restart=<policy> <container>

C docker restart --policy=<policy> <container>

D docker run --restart=<policy> <image>

You use 'docker run --restart= ' to set the restart policy.

Explain the purpose of Docker restart policies and list the main types.

Think about what happens when a container stops or crashes.

Describe a scenario where you would use the 'on-failure' restart policy instead of 'always'.

Consider when you want to avoid restarting containers that stopped on purpose.

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