Day 19 Docker for DevOps Engineers

Docker Volumes and Networks: Key Concepts for DevOps

What is a Docker Volume?

A Docker volume can persist and share container data beyond its lifecycle. It enables:

• Data persistence: Ensures data survives container restarts and removals.

• Data sharing: Allows multiple containers to read and write to the same data.

Ideal Use Cases:

• Storing database files, logs, or application data that needs to persist.

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What is a Docker Network?

Docker networks enable containers to communicate with each other and external systems securely and efficiently. By grouping containers in virtual networks, you can control how they interact.

Ideal Use Cases:

• Connecting a web application container to a database container.

• Isolating services for security and performance.

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Task 1: Multi-Container Application with Docker Compose

Objective: Deploy an application and database together using Docker Compose.

Step 1: Create docker-compose.yml

version: '3'  
services:  
  app:  
    image: my_app_image  
    ports:  
      - "5000:5000"  
    volumes:  
      - app_data:/usr/src/app  
    depends_on:  
      - db  

  db:  
    image: postgres:latest  
    environment:  
      POSTGRES_USER: user  
      POSTGRES_PASSWORD: password  
      POSTGRES_DB: mydatabase  
    volumes:  
      - db_data:/var/lib/postgresql/data  

volumes:  
  app_data:  
  db_data:

Key Features:

• App service: This is used my_app_image and linked to the database container.

• DB service: Runs PostgreSQL with persistent data storage in a volume.

Step 2: Manage the Multi-Container Setup

1. Start Containers:

    docker-compose up -d
   

   • Runs all services in detached mode.

2. Scale the App Service (e.g., 3 replicas):

    docker-compose up --scale app=3 -d
   

3. View Logs for troubleshooting:

    docker-compose logs app
   

4. Stop and Remove Containers:

    docker-compose down
   

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Task 2: Sharing Data Using Docker Volumes

Objective: Share data between two containers using a shared volume.

Step 1: Create Containers with a Shared Volume

docker run -d --name container1 --mount source=shared_volume,target=/data busybox  
docker run -d --name container2 --mount source=shared_volume,target=/data busybox

Step 2: Write Data in One Container

docker exec container1 sh -c "echo 'Hello from container1' > /data/file.txt"

Step 3: Read Data in Another Container

docker exec container2 cat /data/file.txt

Output:

Hello from container1

Step 4: Clean Up

1. List Volumes:

    docker volume ls
   

2. Remove Shared Volume:

    docker volume rm shared_volume
   

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Why Volumes and Networks Matter for DevOps

• Volumes: Enable data persistence and sharing, essential for stateful applications.

• Networks: Provide secure, efficient communication between services.

• Docker Compose: Simplifies the orchestration of multi-container applications, boosting productivity and reliability.

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Summary of Commands Used

Mastering these concepts helps DevOps engineers build scalable and resilient containerized applications. Happy Dockerizing! 🚀