Why is it undesirable to store SQL data in a Docker container?

Because the data in the container is temporary and will be lost when the container is deleted. To store data permanently, external volumes should be used, which can be mounted to the container.

Advantages of using external volumes:

• Data independence from the container's lifecycle: Data persists even when the container is stopped, restarted, or deleted.
• Backup convenience: Backups can be made directly from the volume without affecting the container.
• Migration: Volumes can be easily moved to another host or environment.
• Performance: In certain scenarios, external volumes can demonstrate better disk performance compared to storing data inside the container.

Types of volumes in Docker:

• Bind mounts: Volumes that are part of the host's filesystem.
• Docker managed volumes: Volumes managed by Docker, usually stored in a special directory on the host (/var/lib/docker/volumes/ by default).

Example of defining a volume in Docker Compose for PostgreSQL:

version: '3.8'
services:
  db:
    image: postgres:14
    volumes:
      - db_data:/var/lib/postgresql/data // Mounting a named volume to the PostgreSQL data directory
    environment:
      POSTGRES_DB: mydatabase
      POSTGRES_USER: myuser
      POSTGRES_PASSWORD: mypassword

volumes:
  db_data: // Defining a named volume