🚀 Docker Volumes & Networks for DevOps Engineers.

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🗄️ Docker Volumes (Persistent Storage)

By default, container data is deleted when a container is removed. Volumes store data outside the container so it persists even if the container is deleted.

📌 Key Benefits:

✔ Data stays even if the container is removed.
✔ Multiple containers can share the same volume.
✔ Better performance than bind mounts.

🔗 Useful Commands:

✅ Create a volume:

docker volume create my_volume

✅ List all volumes:

docker volume ls

✅ Inspect volume details:

docker volume inspect my_volume

✅ Remove a volume:

docker volume rm my_volume

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🌐 Docker Network (Container Communication)

Containers by default are isolated. Docker Networks allow containers to communicate with each other securely.

📌 Network Types:

✔ Bridge (Default) – Connects containers on the same host.
✔ Host – Uses the host’s network directly.
✔ Overlay – Used in multi-host Docker Swarm setups.

🔗 Useful Commands:

✅ Create a network:

docker network create my_network

✅ List all networks:

docker network ls

✅ Inspect network details:

docker network inspect my_network

✅ Remove a network:

docker network rm my_network

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🏗️ Task 1: Multi-Container Setup Using Docker Compose

Let's create a multi-container setup with an application and a database using docker-compose which uses volume and network concept.

🔗 Example docker-compose.yml file:

version: '3.8'

services:
  app:
    image: my-app
    networks:
      - my_network
    depends_on:
      - db

  db:
    image: postgres
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
    volumes:
      - db_data:/var/lib/postgresql/data
    networks:
      - my_network

volumes:
  db_data:

networks:
  my_network:

📌 Running the Setup:

✅ Start containers in detached mode:

docker-compose up -d

✅ Stop and clean up everything:

docker-compose down

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📂 Task 2: Share Data Between Containers Using Volumes

Let's create two containers that share the same volume and verify data persistence.

✅ Run two containers with a shared volume:

docker run -d --name c1 --mount source=shared_vol,target=/data busybox  
docker run -d --name c2 --mount source=shared_vol,target=/data busybox

🔹 Explanation:

• docker run → Starts a new container.

• -d → Runs the container in detached mode (in the background).

• --name c1 → Assigns the name c1 to the first container.

• --mount source=shared_vol,target=/data →

  • source=shared_vol → Uses a named volume called shared_vol.

  • target=/data → Mounts it inside the container at /data.

• busybox → A lightweight Linux-based container image.

Both c1 and c2 use the same volume, meaning they can share data.

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✅ Write data in container1:

docker exec c1 sh -c "echo 'Hello from container1' > /data/message.txt"

🔹 Explanation:

• docker exec c1 → Runs a command inside the running container c1.

• sh -c → Executes the following shell command.

• echo 'Hello from container1' > /data/message.txt →

  • Writes "Hello from container1" into the file /data/message.txt.

  • Since /data is a shared volume, this file is accessible from c2.

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✅ Read data from container2:

docker exec c2 cat /data/message.txt

🔹 Explanation:

• docker exec c2 → Runs a command inside the c2 container.

• cat /data/message.txt → Reads and displays the content of message.txt.

• Since c1 wrote the file to the shared volume, c2 can now read it! ✅

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✅ List all volumes:

docker volume ls

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✅ Remove the volume when done:

docker volume rm shared_vol

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