Restart policies in Docker - Time & Space Complexity

Time Complexity: Restart policies

O(n)

Understanding Time Complexity

We want to understand how the time it takes for Docker to restart containers changes as the number of containers grows.

How does Docker handle restarting many containers, and how does that affect the time spent?

Scenario Under Consideration

Analyze the time complexity of the following Docker commands setting restart policies.

docker run --restart=always -d nginx

docker run --restart=on-failure:3 -d redis

docker run --restart=no -d alpine

docker ps --filter "restart=always"

This code runs three containers with different restart policies and lists containers with a specific restart policy.

Identify Repeating Operations

Look for repeated actions that affect time.

• Primary operation: Docker checks each container's restart policy when a container stops.
• How many times: Once per container stop event, repeated for each container that stops.

How Execution Grows With Input

As the number of containers increases, Docker must check more restart policies when containers stop.

Input Size (n)	Approx. Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The number of checks grows directly with the number of containers.

Final Time Complexity

Time Complexity: O(n)

This means the time to handle restart policies grows linearly with the number of containers.

Common Mistake

[X] Wrong: "Restart policies are handled instantly regardless of how many containers exist."

[OK] Correct: Docker must check each container's policy when it stops, so more containers mean more checks and more time.

Interview Connect

Understanding how Docker manages restart policies helps you reason about system behavior as it scales, a useful skill in real-world DevOps tasks.

Self-Check

"What if Docker handled restart policies in parallel for all containers? How would the time complexity change?"

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