Service scaling in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is service scaling in Docker?

Service scaling means increasing or decreasing the number of containers running a service to handle more or less work.

beginner

Which Docker command is used to scale a service?

The command docker service scale SERVICE=NUMBER changes the number of containers for a service.

beginner

What happens if you scale a service down in Docker?

Docker stops some containers of the service to reduce the total number running, saving resources.

beginner

How can you check the current number of containers running for a service?

Use docker service ls to see services and docker service ps SERVICE to see containers running for that service.

beginner

Why is service scaling useful in real life?

It helps handle more users or traffic by adding containers, or save money by reducing containers when less work is needed.

Which command scales a Docker service to 5 containers?

A docker container scale myservice 5

B docker service scale myservice=5

C docker scale service myservice 5

D docker service update --scale 5 myservice

The correct syntax to scale a service is docker service scale SERVICE=NUMBER.

What does scaling a service up mean?

A Adding more containers

B Stopping containers

C Updating container images

D Removing the service

Scaling up means increasing the number of containers running the service.

How do you see the current number of containers for a service?

A docker service inspect SERVICE

B docker container ls

C docker service ps SERVICE

D docker ps -a

The command docker service ps SERVICE lists containers running for that service.

If you scale a service down, what happens?

A Containers stop to reduce count

B Nothing changes

C Service is deleted

D More containers start

Scaling down stops some containers to reduce the total number running.

Why might you want to scale a service automatically?

A To backup data

B To save disk space

C To update software versions

D To handle changing user demand

Automatic scaling helps match the number of containers to user demand or traffic.

Explain how you would scale a Docker service named 'webapp' to 3 containers and check the result.

Think about the command to change container count and how to verify it.

Describe why service scaling is important in managing applications with changing traffic.

Consider what happens when many or few users visit your app.

Practice

1. What does the command docker service scale myservice=3 do?

easy

A. It stops the service named 'myservice'.

B. It runs 3 copies of the service named 'myservice'.

C. It updates the service 'myservice' to version 3.

D. It removes the service named 'myservice'.

Solution

1. Step 1: Understand the command structure

   The command docker service scale SERVICE_NAME=NUMBER sets the number of replicas for a service.

2. Step 2: Apply to the given command

   Here, myservice=3 means run 3 replicas of the service named 'myservice'.

3. Final Answer:

   It runs 3 copies of the service named 'myservice'. -> Option B

4. Quick Check:

   Scaling sets replicas = 3 [OK]

Hint: Scale sets number of service copies with SERVICE=NUMBER [OK]

Common Mistakes:

• Confusing scaling with stopping or removing services
• Thinking scale changes service version
• Using wrong syntax without '=' sign

2. Which of these is the correct syntax to scale a Docker service named 'webapp' to 5 replicas?

easy

A. docker service scale webapp 5

B. docker service scale -replicas=5 webapp

C. docker service scale webapp=5

D. docker scale service webapp=5

Solution

1. Step 1: Recall correct scale command syntax

   The correct syntax is docker service scale SERVICE_NAME=NUMBER with an equals sign.

2. Step 2: Match options to syntax

   docker service scale webapp=5 uses webapp=5 which matches the correct syntax.

3. Final Answer:

   docker service scale webapp=5 -> Option C

4. Quick Check:

   Correct syntax uses '=' sign [OK]

Hint: Use '=' between service name and replica count [OK]

Common Mistakes:

• Omitting '=' sign between service and number
• Using wrong command order
• Confusing 'docker scale' with 'docker service scale'

3. Given the command docker service scale backend=2 was run on a service with 1 replica, what will be the output of docker service ls for the 'backend' service's REPLICAS column?

medium

A. 2/2

B. 1/2

C. 2/1

D. 1/1

Solution

1. Step 1: Understand scaling effect on replicas

   Scaling to 2 means the service should have 2 running replicas out of 2 desired.

2. Step 2: Interpret REPLICAS column format

   The format is running/desired. After scaling, both running and desired should be 2.

3. Final Answer:

   2/2 -> Option A

4. Quick Check:

   Replicas show running/desired = 2/2 [OK]

Hint: Replicas show running/desired count after scaling [OK]

Common Mistakes:

• Mixing running and desired counts order
• Assuming replicas don't update immediately
• Confusing scale number with current replicas

4. You run docker service scale frontend=abc but get an error. What is the likely cause?

medium

A. The replica count must be a number, not 'abc'.

B. The service name 'frontend' is invalid.

C. You need to use a space instead of '='.

D. Scaling requires the service to be stopped first.

Solution

1. Step 1: Check the replica count format

   The replica count must be a positive integer number, not a string like 'abc'.

2. Step 2: Validate error cause

   Using 'abc' causes a syntax or validation error because replicas must be numeric.

3. Final Answer:

   The replica count must be a number, not 'abc'. -> Option A

4. Quick Check:

   Replica count must be numeric [OK]

Hint: Replica count must be a number, not letters [OK]

Common Mistakes:

• Using letters instead of numbers for replicas
• Thinking service must be stopped to scale
• Using space instead of '=' sign

5. You have a service 'api' currently running 4 replicas. You want to handle a sudden traffic spike by doubling the replicas temporarily. Which command correctly doubles the replicas?

hard

A. docker service scale api=4

B. docker service scale api=16

C. docker service scale api=2

D. docker service scale api=8

Solution

1. Step 1: Identify current replica count

   The service 'api' currently has 4 replicas running.

2. Step 2: Calculate double replicas

   Doubling 4 replicas means setting replicas to 8.

3. Step 3: Choose correct scale command

   The command docker service scale api=8 sets replicas to 8, doubling the current count.

4. Final Answer:

   docker service scale api=8 -> Option D

5. Quick Check:

   Double 4 replicas = 8 [OK]

Hint: Double replicas by setting scale to twice current count [OK]

Common Mistakes:

• Setting replicas to current count instead of double
• Setting replicas to half or quadruple by mistake
• Using wrong syntax for scaling