Service scaling in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Scaling a Docker service with the CLI

You run the command docker service scale web=3 on a Docker Swarm cluster. What is the expected output or system state change?

A The command fails with a syntax error due to missing flags

B The service named 'web' is scaled to 3 replicas, but the CLI outputs an error 'service not found'

C The service named 'web' is scaled to 1 replica, ignoring the number 3

D The service named 'web' is scaled to 3 replicas, and the CLI outputs 'web scaled to 3'

💡 Hint

Remember that docker service scale changes the number of replicas for an existing service.

The command docker service scale web=3 sets the number of replicas of the service 'web' to 3. The CLI confirms this by outputting 'web scaled to 3'.

🧠 Conceptual

intermediate

Understanding service scaling impact on containers

If you scale a Docker service from 2 to 5 replicas, what happens to the containers running the service?

A All containers are stopped and replaced by 5 new containers

B Three new containers are created to reach 5 replicas, and the existing 2 continue running

C The number of containers stays at 2, but each container handles more load

D The service is paused until manual restart

💡 Hint

Think about how replicas relate to containers in Docker services.

Scaling up a service increases the number of container instances (replicas). Existing containers keep running, and new ones are added to reach the desired count.

❓ Troubleshoot

advanced

Troubleshooting failed service scaling in Docker Swarm

You try to scale a service with docker service scale api=10, but only 5 replicas run. What could be a likely cause?

A The cluster has insufficient resources to run 10 replicas

B The command syntax is incorrect and ignored

C Docker Swarm limits replicas to 5 by default

D The service name 'api' is invalid

💡 Hint

Consider resource availability on the cluster nodes.

If the cluster nodes do not have enough CPU or memory, Docker Swarm cannot start all requested replicas, resulting in fewer running containers.

🔀 Workflow

advanced

Correct sequence to scale a Docker service safely

Arrange the steps to safely scale a Docker service from 1 to 4 replicas:

A 1,3,2,4

B 2,1,3,4

C 1,2,3,4

D 1,2,4,3

💡 Hint

Think about checking status before and after scaling, then monitoring.

First check the service status, then scale it, verify the new replicas, and finally monitor logs to catch any issues.

✅ Best Practice

expert

Best practice for scaling stateful services in Docker Swarm

Which approach is best when scaling a stateful service in Docker Swarm to avoid data inconsistency?

A Use persistent volumes and ensure the service supports multiple replicas safely

B Scale the service rapidly to the desired number without volume considerations

C Avoid using Docker Swarm and run containers manually for stateful apps

D Use ephemeral storage and rely on container restart to recover data

💡 Hint

Think about data persistence and consistency in stateful applications.

Stateful services require persistent storage and careful design to handle multiple replicas without data conflicts. Using persistent volumes and ensuring the app supports scaling is best practice.

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