Scaling services with replicas in Docker - Mini Project: Build & Apply

Scaling services with replicas

📖 Scenario: You are managing a simple web service using Docker. To handle more visitors, you want to run multiple copies (replicas) of your service.This project will guide you to create a Docker Compose setup and scale the service replicas.

🎯 Goal: Build a Docker Compose file to define a web service and then scale it to run multiple replicas.

📋 What You'll Learn

Create a Docker Compose file with a web service using the nginx image

Add a configuration variable for the number of replicas

Use the docker-compose command to scale the service

Display the running containers to verify scaling

💡 Why This Matters

🌍 Real World

Scaling services with replicas is common when websites or apps get more visitors. Running multiple copies helps handle more users smoothly.

💼 Career

DevOps engineers often use Docker Compose and scaling commands to manage service availability and performance in real projects.

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 nginx:latest image.

Docker

# Create docker-compose.yml with a service named web using nginx:latest
# 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 variable for the number of replicas

Add a variable called REPLICAS in a .env file and set it to 3 to define how many replicas you want.

Docker

version: '3.8'
services:
  web:
    image: nginx:latest

# Create a .env file with REPLICAS=3
# Your code here

Hint

Create a file named .env in the same folder as docker-compose.yml and add the line REPLICAS=3.

3

Scale the web service using the replicas variable

Use the command docker-compose up -d --scale web=${REPLICAS} to start the web service with the number of replicas from the REPLICAS variable.

Docker

version: '3.8'
services:
  web:
    image: nginx:latest

# .env file content:
REPLICAS=3

# Run the docker-compose command to scale
# Your code here

Hint

Use the --scale option with docker-compose up -d and pass the replicas variable as ${REPLICAS}.

4

Display running containers to verify scaling

Run docker ps --filter name=web to list all running containers with the name web and verify that there are 3 replicas running.

Docker

version: '3.8'
services:
  web:
    image: nginx:latest

# .env file content:
REPLICAS=3

# Command to run:
docker-compose up -d --scale web=${REPLICAS}

# Run docker ps to check containers
# Your code here

Hint

Use docker ps --filter name=web to see all containers with web in their name. You should see 3 containers listed.

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