Rolling updates in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a rolling update command in Docker Swarm

What is the output of the following command when updating a service with rolling update strategy in Docker Swarm?

Docker

docker service update --image nginx:1.19 my_web_service

A

my_web_service
rolling update started
3/3 tasks updated

B Error: service not found

C

my_web_service
service updated successfully

D

my_web_service
overall progress: 1 out of 3 tasks
1/3 updated, 0 tasks running

💡 Hint

Think about how Docker Swarm reports progress during rolling updates.

The docker service update command shows the progress of the rolling update, including how many tasks have been updated and how many are running.

🧠 Conceptual

intermediate

Understanding rolling update behavior in Docker Swarm

Which statement correctly describes the rolling update behavior in Docker Swarm?

A Docker Swarm updates all service tasks simultaneously, causing downtime.

B Docker Swarm updates tasks one by one, ensuring some tasks remain running during update.

C Docker Swarm stops all tasks before starting new ones to avoid conflicts.

D Docker Swarm requires manual restart of each task during update.

💡 Hint

Think about how rolling updates minimize downtime.

Rolling updates in Docker Swarm update tasks one at a time or in batches, keeping the service available during the update.

❓ Configuration

advanced

Configuring rolling update parameters in Docker service

Which Docker service update command correctly sets the rolling update to update 2 tasks at a time with a 10-second delay between updates?

A docker service update --update-parallelism 2 --update-delay 10s my_service

B docker service update --parallelism 2 --delay 10 my_service

C docker service update --update-parallel 2 --update-delay 10 my_service

D docker service update --update-parallelism=2 --update-delay=10 my_service

💡 Hint

Check the exact option names and units for delay.

The correct flags are --update-parallelism and --update-delay with the delay specified in seconds with 's'.

❓ Troubleshoot

advanced

Troubleshooting a failed rolling update in Docker Swarm

After running a rolling update, some tasks fail to start and the service is stuck updating. What is the most likely cause?

A The service has no published ports.

B The update delay is set too high.

C The new image tag does not exist or is invalid.

D The Docker daemon is running an outdated version.

💡 Hint

Think about what would prevent new tasks from starting.

If the new image tag is invalid or missing, tasks cannot start with the updated image, causing the update to fail.

🔀 Workflow

expert

Order the steps for performing a safe rolling update in Docker Swarm

Arrange the following steps in the correct order to perform a safe rolling update of a Docker Swarm service.

A 1,2,3,4

B 2,1,3,4

C 1,3,2,4

D 3,1,2,4

💡 Hint

Think about preparation, execution, monitoring, and finalizing steps.

First, ensure the new image is ready. Then update the service. Monitor the update and finally rollback if needed or confirm success.

Practice

1. What is the main purpose of a rolling update in Docker?

easy

A. To restart the Docker daemon automatically

B. To delete all containers and recreate them at once

C. To backup container data before updating

D. To update containers gradually without downtime

Solution

1. Step 1: Understand rolling update concept

   A rolling update means updating parts of a system bit by bit instead of all at once.

2. Step 2: Apply to Docker containers

   In Docker, rolling updates replace containers one by one to avoid downtime.

3. Final Answer:

   To update containers gradually without downtime -> Option D

4. Quick Check:

   Rolling update = gradual update without downtime [OK]

Hint: Rolling update means gradual replacement, not all at once [OK]

Common Mistakes:

• Thinking rolling update deletes all containers simultaneously
• Confusing rolling update with backup process
• Assuming rolling update restarts Docker daemon

2. Which Docker command option is used to perform a rolling update on a service?

easy

A. docker service update --rolling-update

B. docker service update --rollback

C. docker service update --update-parallelism

D. docker service update --force

Solution

1. Step 1: Identify rolling update options

   Docker uses --update-parallelism to control how many containers update at once.

2. Step 2: Match option to rolling update

   This option allows gradual updates, which is the essence of rolling updates.

3. Final Answer:

   docker service update --update-parallelism -> Option C

4. Quick Check:

   Rolling update option = --update-parallelism [OK]

Hint: Use --update-parallelism to control rolling update speed [OK]

Common Mistakes:

• Using --force which restarts all containers immediately
• Assuming --rollback triggers rolling update
• Inventing a non-existent --rolling-update flag

3. Given the command:

docker service update --image myapp:v2 --update-parallelism 2 myservice

What will happen during this update?

medium

A. All containers of myservice update at once to v2

B. Two containers update at a time to v2 until all are updated

C. Only two containers update and the rest stay on old version

D. The service rolls back to the previous version

Solution

1. Step 1: Understand --update-parallelism effect

   The option --update-parallelism 2 means update 2 containers at once.

2. Step 2: Apply to service update

   Docker updates 2 containers to image v2, then moves to next 2, until all are updated.

3. Final Answer:

   Two containers update at a time to v2 until all are updated -> Option B

4. Quick Check:

   --update-parallelism 2 means update 2 containers at once [OK]

Hint: Parallelism number controls how many containers update simultaneously [OK]

Common Mistakes:

• Thinking all containers update simultaneously
• Believing only two containers update permanently
• Confusing update with rollback

4. You run:

docker service update --image myapp:v3 --update-parallelism 3 --update-delay 10s myservice

But the update fails and leaves some containers on old version. What is a likely cause?

medium

A. The image myapp:v3 does not exist or is invalid

B. The update-delay is too short causing timeout

C. The update-parallelism value must be 1

D. Docker service update does not support update-delay

Solution

1. Step 1: Check update-delay and parallelism validity

   Both --update-delay and --update-parallelism are valid options and can be used together.

2. Step 2: Consider image availability

   If the new image myapp:v3 is missing or invalid, update fails and some containers remain old.

3. Final Answer:

   The image myapp:v3 does not exist or is invalid -> Option A

4. Quick Check:

   Update fails if new image is missing or bad [OK]

Hint: Check image existence if update leaves old containers [OK]

Common Mistakes:

• Assuming update-delay causes failure
• Thinking parallelism must be 1
• Believing update-delay is unsupported

5. You want to update a Docker service with 10 replicas using rolling updates. You want to update 3 containers at a time and wait 5 seconds between batches. Which command correctly achieves this?

hard

A. docker service update --image myapp:v2 --update-parallelism 3 --update-delay 5s myservice

B. docker service update --image myapp:v2 --update-parallelism 5 --update-delay 3s myservice

C. docker service update --image myapp:v2 --update-parallelism 10 --update-delay 0s myservice

D. docker service update --image myapp:v2 --update-delay 5s myservice

Solution

1. Step 1: Match update-parallelism to batch size

   To update 3 containers at a time, use --update-parallelism 3.

2. Step 2: Set delay between batches

   To wait 5 seconds between batches, use --update-delay 5s.

3. Step 3: Verify full command

   Combining these with the image and service name gives the correct command.

4. Final Answer:

   docker service update --image myapp:v2 --update-parallelism 3 --update-delay 5s myservice -> Option A

5. Quick Check:

   Parallelism 3 + delay 5s = correct rolling update command [OK]

Hint: Use --update-parallelism for batch size and --update-delay for wait time [OK]

Common Mistakes:

• Using wrong parallelism number
• Omitting update-parallelism option
• Setting delay but not parallelism