Docker for DevOps Engineers

Docker Compose is a powerful tool for managing multi-container Docker applications. It simplifies defining and running multi-container Docker applications by allowing you to use a single YAML file to configure all the services, networks, and volumes your application needs. In this blog post, we'll dive deeper into Docker Volumes and Docker Networks, and provide practical examples to help you understand how to leverage these features for effective containerized applications.

Introduction to Docker Volumes

Docker Volumes provide a way to persist data generated by and used by Docker containers. Without volumes, data stored in a container is lost when the container is removed. Volumes solve this problem by storing data in a specific location on the host machine, separate from the container’s filesystem. This means the data persists even if the container is stopped or removed.

Benefits of Using Docker Volumes

1. Persistence: Data stored in volumes persists even if the container is deleted.

2. Sharing Data: Multiple containers can access and share the same volume.

3. Data Management: Volumes can be managed independently from containers.

4. Backup & Restore: Volumes can be easily backed up and restored.

Creating and Using Docker Volumes

Here’s how you can use Docker Volumes:

1. Creating a Volume

    docker volume create my-volume
   

   This command creates a named volume called my-volume.

2. Using a Volume with docker run

    docker run -d -v my-volume:/data --name my-container alpine
   

   In this command:

   • -d runs the container in detached mode.

   • -v my-volume:/data mounts the volume my-volume to the /data directory in the container.

3. Listing Volumes

    dedocker volume ls
   

   This lists all volumes on your system.

4. Inspecting a Volume

    dedocker volume inspect my-volume
   

   This command provides detailed information about the volume.

5. Removing a Volume

    docker volume rm my-volume
   

   This removes the specified volume.

Introduction to Docker Networks

Docker Networks allow you to create virtual networks for your containers, enabling them to communicate with each other and with the host machine. Networks are essential for setting up isolated environments for different applications or services while allowing them to interact.

Benefits of Using Docker Networks

1. Isolation: Networks isolate container traffic, improving security.

2. Service Discovery: Containers on the same network can communicate using container names.

3. Custom Network Configurations: You can define network settings and controls for container communications.

Creating and Using Docker Networks

1. Creating a Network

    docker network create my-network
   

   This command creates a new network called my-network.

2. Connecting Containers to a Network

    docker run -d --network my-network --name my-container1 alpine
    docker run -d --network my-network --name my-container2 alpine
   

   Both containers are connected and can communicate with each other using their container names.

3. Listing Networks

    docker network ls
   

   This command lists all networks.

4. Inspecting a Network

    docker network inspect my-network
   

   This provides detailed information about the network.

5. Removing a Network

    docker network rm my-network
   

   This removes the specified network.

Task 1: Multi-Container Docker Compose File

In this task, we will create a Docker Compose file that brings up a multi-container application with an application container and a database container.

Creating docker-compose.yml

Here's a basic example of a docker-compose.yml file that defines a web application and a MySQL database:

version: '3.8'

services:
  web:
    image: nginx:latest
    ports:
      - "8080:80"
    networks:
      - webnet

  db:
    image: mysql:5.7
    environment:
      MYSQL_ROOT_PASSWORD: example
    networks:
      - webnet
    volumes:
      - dbdata:/var/lib/mysql

networks:
  webnet:

volumes:
  dbdata:

In this file:

• The web service uses the nginx image and maps port 8080 on the host to port 80 in the container.

• The db service uses the mysql image and set a root password.

• Both services are connected to the webnet network.

• The db service uses a named volume dbdata for persistent storage.

Commands to Manage the Multi-Container Application

• Start the Application:

    docker-compose up -d
  

  This command starts the containers defined in docker-compose.yml in detached mode.

• Scale the Application:

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

  This command scales the web service to 3 replicas.

• View Container Status:

    docker-compose ps
  

  This command shows the status of all containers.

• View Logs:

    docker-compose logs web
  

  This command shows logs for the web service.

• Stop and Remove Containers:

    docker-compose down
  

  This command stops and removes all containers, networks, and volumes defined in the docker-compose.yml file.

Task 2: Using Docker Volumes with Multiple Containers

In this task, we will create two containers that share a volume and verify data consistency.

Creating Containers with Shared Volume

1. Run Containers with Shared Volume

    docker run -d --name container1 -v shared-volume:/data alpine
    docker run -d --name container2 -v shared-volume:/data alpine
   

   Both containers use the shared-volume volume mounted to the /data directory.

2. Write Data to the Volume

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

   This command writes a message to a file inside the volume from container1.

3. Read Data from the Volume

    docker exec container2 cat /data/message.txt
   

   This command reads the message from the file inside the volume from container2, verifying that both containers share the same data.

4. List Volumes

    docker volume ls
   

   This command lists all volumes, including shared-volume.

5. Remove the Volume

    docker volume rm shared-volume
   

   This command removes the volume when you are done.