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.