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.

Practice

1. What does scaling a Docker service with replicas do?

easy

A. Runs multiple copies of the service to handle more work

B. Deletes the service completely

C. Changes the service's network settings

D. Updates the Docker engine version

Solution

1. Step 1: Understand the purpose of replicas

   Replicas are copies of a service running simultaneously to share the workload.

2. Step 2: Identify the effect of scaling replicas

   Scaling increases or decreases the number of these copies to improve performance and availability.

3. Final Answer:

   Runs multiple copies of the service to handle more work -> Option A

4. Quick Check:

   Scaling = multiple copies [OK]

Hint: Scaling means running more copies of your service [OK]

Common Mistakes:

• Thinking scaling deletes the service
• Confusing scaling with network changes
• Assuming scaling updates Docker itself

2. Which is the correct command to scale a Docker service named webapp to 5 replicas?

easy

A. docker service scale webapp -replicas 5

B. docker scale service webapp 5

C. docker service scale webapp --replicas=5

D. docker service scale webapp=5

Solution

1. Step 1: Recall the correct syntax for scaling

   The correct syntax is docker service scale SERVICE_NAME=NUMBER.

2. Step 2: Match the syntax with the options

   docker service scale webapp=5 matches the exact syntax: docker service scale webapp=5.

3. Final Answer:

   docker service scale webapp=5 -> Option D

4. Quick Check:

   Correct syntax = docker service scale NAME=NUM [OK]

Hint: Use SERVICE_NAME=NUMBER format with docker service scale [OK]

Common Mistakes:

• Using wrong command order
• Adding extra flags not needed
• Using spaces instead of '='

3. Given the command docker service scale api=3, what will be the number of running containers for the api service?

medium

A. 3 containers

B. 1 container

C. 0 containers

D. Depends on the Docker version

Solution

1. Step 1: Understand the scale command effect

   The command sets the number of replicas to 3, so Docker runs 3 containers for the service.

2. Step 2: Confirm the number of containers

   Scaling to 3 means exactly 3 containers will be running for api.

3. Final Answer:

   3 containers -> Option A

4. Quick Check:

   Scale api=3 means 3 containers [OK]

Hint: Scale number equals running containers count [OK]

Common Mistakes:

• Assuming only 1 container runs by default
• Thinking scaling depends on Docker version
• Confusing replicas with CPU cores

4. You run docker service scale backend=two but get an error. What is the problem?

medium

A. The service name cannot be 'backend'

B. The number of replicas must be a numeric value

C. The command should be run as root user

D. Docker service scale does not support scaling

Solution

1. Step 1: Check the scale command syntax

   The replicas count must be a number, not a word.

2. Step 2: Identify the error cause

   Using 'two' instead of '2' causes a syntax error.

3. Final Answer:

   The number of replicas must be a numeric value -> Option B

4. Quick Check:

   Replicas must be numeric [OK]

Hint: Use digits, not words, for replica counts [OK]

Common Mistakes:

• Using words instead of numbers
• Assuming service name is invalid
• Thinking root user is required

5. You have a service app running with 2 replicas. You want to update it to 4 replicas but keep the service running without downtime. Which command should you use?

hard

A. docker service scale app 4

B. docker container run app --replicas 4

C. docker service update --replicas 4 app

D. docker service restart app --replicas 4

Solution

1. Step 1: Understand how to update replicas without downtime

   Using docker service update --replicas 4 app changes replicas smoothly.

2. Step 2: Compare options for scaling

   docker service scale can scale but docker service update is preferred for zero downtime updates.

3. Step 3: Identify incorrect commands

   docker container run and docker service restart do not scale replicas properly.

4. Final Answer:

   docker service update --replicas 4 app -> Option C

5. Quick Check:

   Use service update for zero downtime scaling [OK]

Hint: Use 'docker service update --replicas' to avoid downtime [OK]

Common Mistakes:

• Using docker service scale causing brief downtime
• Trying to run containers manually
• Restarting service instead of updating replicas