Why Rolling updates in Docker? - Purpose & Use Cases

The Big Idea

What if you could update your app without anyone noticing a thing?

The Scenario

Imagine you have a website running on a server. To update it, you stop the server, replace the files, and start it again. During this time, visitors see an error or a blank page.

The Problem

This manual update causes downtime. Visitors get frustrated because the site is unavailable. Also, if something goes wrong, you must fix it quickly or the site stays broken. Doing this repeatedly is slow and risky.

The Solution

Rolling updates let you update parts of your system one by one without stopping everything. This way, some parts keep working while others update, so users don't notice any downtime.

Before vs After

✗ Before

docker stop myapp
replace files
docker start myapp

✓ After

docker service update --image newimage myapp

What It Enables

Rolling updates make it possible to improve your app without interrupting users, keeping your service smooth and reliable.

Real Life Example

A popular online store updates its product page code. Using rolling updates, customers keep shopping without any interruptions or errors during the update.

Key Takeaways

Manual updates cause downtime and risk errors.

Rolling updates update parts gradually to avoid downtime.

This keeps services available and users happy during changes.

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