Docker Volumes

Definition: Docker volumes are used to persist data generated by and used by Docker containers. Volumes are stored on the host filesystem and are independent of the container’s lifecycle.
Usage: Volumes allow data to be shared between containers and the host system, making data persistent across container restarts and updates.
Lifecycle: Volumes are created using the docker volume create command or automatically when specified in a Docker Compose file. They can be managed (listed, removed) using Docker CLI commands.

Volumes have 2 different uses:

Code Synchronisation (Development):
- During development, it’s common to mount your project directory to the container to reflect code changes immediately without rebuilding the container.
- This allows you to edit your code on your host machine and see changes instantly within the container, which is useful for rapid development and debugging.
Persistence:
- If your backend service writes data to the filesystem that needs to persist across container restarts, using volumes ensures this data is not lost when the container is stopped or recreated.

How to configure volumes

We can configure containers using docker-compose

Example

services:
	service-1:
		volumes:
			- ./path/on/host:./path/on/container

When the container makes a change in the files in the directory, there will be a corresponding change on the linked path on the host machine

Feature	Docker Images	Docker Volumes	Docker Containers
Purpose	Blueprint for containers	Persistent storage for container data	Running instances of images
Mutable	No (immutable once created)	Yes (data can be changed)	Yes (state changes during runtime)
Creation	Built from Dockerfiles	Created via Docker CLI or Docker Compose	Created from Docker images
Storage	Stored in image registries (e.g., Docker Hub)	Stored on host filesystem	Exist in memory and disk as long as running
Isolation	Provides consistent environment	Shares data across containers and host	Isolated environment for application
Usage	Basis for creating containers	Persistent data storage	Run applications in isolated environments

Docker Images are the templates used to create containers.
Docker Volumes provide persistent storage that can be shared between containers and the host.
Docker Containers are the running instances of Docker images that execute applications in isolated environments.