Day 19 Task: Docker for DevOps Engineers – Mastering Docker Volume & Docker Network

---

Docker Volumes

Docker Volumes are essential for persisting data that lives outside a container’s lifecycle. Here’s why they’re a powerful feature:

• Persistent Storage: By default, when a container is removed, all data inside it is lost. Volumes allow us to store data outside containers, so it persists even if the container is deleted or stopped.

• Data Sharing Across Containers: We can mount the same volume to multiple containers to allow them to share data seamlessly.

• Examples of Use: Storing database data in volumes is a common practice so that data is saved even when the database container is updated or restarted.

Docker Networks

Docker Networks enable containers to communicate with each other as well as the host machine. They offer several networking capabilities:

• Virtual Networks: Containers connected to the same network can communicate directly, making it easier to connect applications with their services (like a database).

• Isolation: By isolating containers on separate networks, you can add a layer of security and control.

• Use Cases: Docker networks are commonly used for multi-container applications, where services like web servers, databases, and application backends need to communicate seamlessly.

---

Task 1: Create a Multi-Container Docker-Compose File

Example docker-compose.yml

Below is an example of a docker-compose.yml file to set up a Node.js application and a MySQL database using Docker Compose.

version: '3.8'

services:
  app:
    image: node:14
    container_name: node_app
    ports:
      - "3000:3000"
    environment:
      DB_HOST: db
      DB_USER: root
      DB_PASSWORD: example
      DB_NAME: mydb
    volumes:
      - app-data:/usr/src/app
    depends_on:
      - db
    networks:
      - app-network

  db:
    image: mysql:5.7
    container_name: mysql_db
    environment:
      MYSQL_ROOT_PASSWORD: example
      MYSQL_DATABASE: mydb
    volumes:
      - db-data:/var/lib/mysql
    networks:
      - app-network

volumes:
  app-data:
  db-data:

networks:
  app-network:

Command Breakdown

• Bring Up Containers in Detached Mode:

    docker-compose up -d
  

• Scale the App Service:

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

• Check Container Status:

    docker-compose ps
  

• View Logs:

    docker-compose logs app
  

• Bring Down Containers and Clean Up:

    docker-compose down
  

Task 2: Using Docker Volumes and Named Volumes

1. Create Containers with Shared Volume Run two containers that share the same volume to read and write data.

    docker run -d --name container1 --mount source=shared-data,target=/data alpine
    docker run -d --name container2 --mount source=shared-data,target=/data alpine
   

2. Verify Data Sharing Across Containers Use the docker exec command to write and verify data across containers:

    docker exec container1 sh -c "echo 'Hello from container1' > /data/shared-file.txt"
    docker exec container2 cat /data/shared-file.txt
   

   If data is shared correctly, container2 should output:

    Hello from container1
   

3. List and Remove Volumes

   • List All Volumes:

       docker volume ls
     

   • Remove a Volume:

       docker volume rm shared-data
     

---

Project Opportunity

Use this task to build a portfolio project by combining Docker Volumes and Networks in a real-world scenario. Document your work and post on LinkedIn with #90DaysOfDevOps to showcase your skills to the community!