Service scaling in Docker - Mini Project: Build & Apply

Service scaling with Docker Compose

📖 Scenario: You are managing a simple web application using Docker Compose. To handle more users, you want to run multiple copies of the web service.

🎯 Goal: Learn how to scale a Docker Compose service by increasing the number of running containers.

📋 What You'll Learn

Create a Docker Compose file with a web service

Add a scale variable to control the number of containers

Use the Docker Compose command to scale the service

Display the number of running containers after scaling

💡 Why This Matters

🌍 Real World

Scaling services with Docker Compose helps handle more users by running multiple copies of the same service.

💼 Career

Understanding service scaling is essential for DevOps roles to manage application availability and load.

1

Create a Docker Compose file with a web service

Create a file named docker-compose.yml with a service called web that uses the image nginx:latest.

Docker

# Your code here

Hint

Use YAML syntax to define the service under services with the name web and specify the image nginx:latest.

2

Add a scale variable to control the number of containers

Create a variable called scale_count and set it to 3 to represent how many containers you want to run.

Docker

version: '3'
services:
  web:
    image: nginx:latest

# Your code here

Hint

Define a variable named scale_count and assign it the value 3.

3

Use the Docker Compose command to scale the service

Write a shell command string called scale_command that uses docker-compose up -d --scale web=3 to start 3 containers of the web service using the scale_count variable.

Docker

version: '3'
services:
  web:
    image: nginx:latest

scale_count = 3

# Your code here

Hint

Use an f-string to insert scale_count into the command string.

4

Display the number of running containers after scaling

Write a print statement that outputs exactly: Running 3 containers of web service using the scale_count variable.

Docker

version: '3'
services:
  web:
    image: nginx:latest

scale_count = 3

scale_command = f"docker-compose up -d --scale web={scale_count}"

# Your code here

Hint

Use a print statement with an f-string to show the number of containers running.

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