Service scaling in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to scale the service named webapp to 3 replicas.

Docker

docker service scale webapp=[1]

Drag options to blanks, or click blank then click option'

A4

B2

C3

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number less than 1, which stops the service.

Forgetting the equal sign.

The command docker service scale webapp=3 sets the number of replicas of the webapp service to 3.

2fill in blank

medium

Complete the command to list all running services and their replicas.

Docker

docker service [1]

Drag options to blanks, or click blank then click option'

Als

Bps

Clist

Dshow

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ps' which shows tasks, not services.

Using 'show' which is not a valid docker service command.

The command docker service ls lists all services with their current replicas and status.

3fill in blank

hard

Fix the error in the command to scale the service api to 5 replicas.

Docker

docker service scale [1]=5

Drag options to blanks, or click blank then click option'

Aservice-api

Bapi

Capi-service

Dapis

Attempts:

3 left

💡 Hint

Common Mistakes

Adding extra words to the service name.

Using plural or hyphenated names incorrectly.

The service name must match exactly. Here, the service is named api, so the correct command is docker service scale api=5.

4fill in blank

hard

Fill both blanks to create a command that updates the frontend service to have 4 replicas.

Docker

docker service update --replicas [1] [2]

Drag options to blanks, or click blank then click option'

A4

Bfrontend

Cwebapp

D--force

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong service name.

Putting the service name before the replicas number.

The command docker service update --replicas 4 frontend updates the frontend service to 4 replicas. The --force flag is optional here.

5fill in blank

hard

Fill all three blanks to write a command that forces update on the backend service and scales it to 6 replicas.

Docker

docker service update [1] [2] [3]

Drag options to blanks, or click blank then click option'

A--force

B--replicas

C6

Dbackend

Efrontend

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing service names.

Forgetting the --force flag.

The command docker service update --force --replicas 6 backend forces an update on the backend service and scales it to 6 replicas.

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