Scaling services with replicas in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Scaling a Docker service to 3 replicas

You run the command docker service scale webapp=3 on a Docker Swarm cluster. What is the expected state of the service webapp after this command?

A The service <code>webapp</code> will have 3 replicas on the same node only.

B The service <code>webapp</code> will be removed and recreated with 3 replicas.

C The service <code>webapp</code> will have exactly 3 running replicas across the swarm nodes.

D The command will fail because scaling requires a Docker Compose file.

💡 Hint

Think about how Docker Swarm manages service replicas across nodes.

The docker service scale command adjusts the number of replicas for the service. Docker Swarm ensures the replicas are distributed across available nodes, not just one node. The service is updated in place, not removed and recreated.

🧠 Conceptual

intermediate

Understanding replica distribution in Docker Swarm

In Docker Swarm, when you scale a service to multiple replicas, how does Swarm decide where to place these replicas?

A Swarm places all replicas on the manager node by default.

B Swarm distributes replicas evenly across worker nodes based on resource availability.

C Swarm places replicas only on nodes with the least CPU usage, ignoring memory.

D Swarm randomly places replicas without considering node resources.

💡 Hint

Think about how a cluster balances workload.

Docker Swarm schedules replicas by considering node resources like CPU and memory to balance the workload evenly across worker nodes.

❓ Configuration

advanced

Docker Compose file for scaling a service

Which Docker Compose snippet correctly defines a service named api with 4 replicas in a Docker Swarm stack?

A

services:
  api:
    image: myapi:latest
    deploy:
      replicas: 4

B

services:
  api:
    image: myapi:latest
    scale: 4

C

services:
  api:
    image: myapi:latest
    deploy:
      scale: 4

D

services:
  api:
    image: myapi:latest
    replicas: 4

💡 Hint

Look for the correct key under deploy for replicas.

In Docker Compose for Swarm, the deploy section controls deployment settings like replicas. The correct key is replicas.

❓ Troubleshoot

advanced

Troubleshooting failed scaling of a service

You tried to scale a service to 5 replicas using docker service scale, but only 3 replicas are running. What is the most likely cause?

A The <code>docker service scale</code> command only supports scaling up to 3 replicas.

B The service image is missing from the Docker Hub registry.

C The swarm manager node is offline.

D There are not enough nodes or resources available in the swarm to run 5 replicas.

💡 Hint

Consider cluster resource limits and node availability.

If the swarm does not have enough nodes or resources (CPU, memory), it cannot run all requested replicas. Docker will run as many as possible.

🔀 Workflow

expert

Correct sequence to scale a Docker Swarm service

Arrange the steps in the correct order to scale a Docker Swarm service named frontend from 2 to 5 replicas.

A 1,2,3,4

B 2,1,3,4

C 1,3,2,4

D 1,2,4,3

💡 Hint

Think about checking before and after scaling, then confirming service health.

First, check current replicas, then scale the service, verify the updated replicas, and finally confirm the service runs well on nodes.