Docker for DevOps Engineer

Docker Volume

In Docker, a volume is a managed data storage area that exists independently of a container's life cycle. It allows for persistent data storage, detached from a specific container, and can be shared among multiple containers. Volumes are managed by Docker and can be easily backed up, moved, and restored. They are designed to facilitate data sharing and persistence across containers within the Docker environment. Volumes do not expand the size of containers that use them. Instead, they act as separate storage entities outside the container's main structure. This means that even if a container is removed or replaced, the data within the volume remains intact and unaffected.

Using the -v flag in Docker commands is concise but combines all options into a single field, making it less explicit. On the other hand, the --mount flag provides a more explicit and verbose approach by separating the options. The key distinction lies in how the syntax is structured. Here's a comparison illustrating the syntax for each flag:

-v Syntax:

docker run -v /host/path:/container/path -v /data:/data:ro -v /more/data

--mount Syntax:

docker run --mount type=bind,source=/host/path,target=/container/path --mount type=bind,source=/data,target=/data,readonly --mount type=volume,source=/more/data

The --mount flag separates the options explicitly, making it clearer and more organized, while the -v syntax combines them into a single field, sacrificing some readability for brevity.

There are different types of volumes in Docker

Named Volume: These are named entities managed by Docker. They are more convenient and easier to manage as Docker takes care of the location and lifecycle.
```
 docker volume create my_volume
 docker run -v my_volume:/path/in/container my image
```
Anonymous Volumes: These are created when a container needs a private volume that doesn't need to be shared or managed outside that container.
```
 docker run -v /path/in/container my_image
```
Bind Mounts: These allow you to specify a directory on the host machine that will be mounted into the container.
```
 docker run -v /host/path:/container/path my_image
```

Common Docker Volume Commands

Create a volume

docker volume create my_volume
List volumes

docker volume ls
Inspect a volume

docker volume inspect my_volume
Remove a volume

docker volume rm my_volume

Docker Network

In Docker, a network is a virtual communication bridge that allows containers to interact with each other and with other resources. It provides a means for containers to communicate within the Docker environment. Docker supports different types of networks, such as bridge, host, overlay, and macvlan, each serving specific connectivity purposes. Networks enable containers to share data and communicate in isolated environments, offering flexibility in managing how containers interact and exchange information.

Common Docker Network Commands

Create a Network

docker network create <network_name>
List Networks

docker network ls
Inspect a Network

docker network inspect <network_name>
Connect a Container to a network

docker network connect <network_name> <container_name>
Disconnect a Container from a network

docker network disconnect <network_name> <container_name>
Remove a Network

docker network rm <network_name>

Task 1

Create a multi-container docker-compose file which will bring UP and bring DOWN containers in a single shot ( Example - Create application and database container )

version : "3.3"
services:
  web:
    image: varsha0108/local_django:latest
    deploy:
        replicas: 2
    ports:
      - "8001-8005:8001"
    volumes:
      - my_django_volume:/app
  db:
    image: mysql
    ports:
      - "3306:3306"
    environment:
      - "MYSQL_ROOT_PASSWORD=test@123"
volumes:
  my_django_volume:
    external: true

The configuration implies that the web service container requires the my_django_volume volume to persist data within the /app directory. This volume is assumed to exist beforehand or be created externally. If the my_django_volume volume does not exist, you can create it using the following command.

docker volume create my_django_volume

Use the docker-compose up command with the -d flag to start a multi-container application in detached mode.

Use the docker-compose scale command to increase or decrease the number of replicas for a specific service. You can also add replicas in the deployment file for auto-scaling.

Use the docker-compose ps command to view the status of all containers, and docker-compose logs to view the logs of a specific service.

Use the docker volume ls command to list all volumes and the docker volume rm command to remove the volume when you're done.

Task 2

Learn how to use Docker Volumes and Named Volumes to share files and directories between multiple containers.

Create a new volume - docker volume create ubuntu_volume

Create two or more containers that read and write data to the same volume using the docker run --mount command.

docker run -d --name nginx_container_1 -v ubuntu_volume:/app nginx:latest docker run -d --name nginx_container_2 -v ubuntu_volume:/app nginx:latest

Verify that the data is the same in all containers by using the docker exec command to run commands inside each container.

Use the docker volume ls command to list all volumes and the docker volume rm command to remove the volume when you're done.

Further insights on Docker volumes can be found on SpaceLift's blog. Explore more on this topic here.

Thank you!