Volumes are Docker’s mechanism for persisting and sharing data between the host machine and containers, or between containers. By default, container filesystems are ephemeral (data is lost when the container is removed). Volumes solve this by providing persistent storage that exists outside the container lifecycle.

Three Types of Volume

1. Bind Mount: ./local:/container

• Mounts host directory into container.
• Host content overwrites container content.
• Use case: Development, for hot reloading.
• It should start by ”./“.

volumes:
  - ./frontend:/app # Local code visible in container (not a copy)

2. Named Volume: volume_name:/container

• Docker-managed persistent storage.
• Survives container deletion.
• Use case: Databases, persistent data.

volumes:
  - postgres_data:/var/lib/postgresql/data

3. Anonymous Volume: /container/path

• Protects specific path from parent bind mount
• It creates an anonymous volume using the data in the container image in the specific path.
• It gets deleted once the image is deleted.
• Use case: Isolating node_modules from host

volumes:
  - ./frontend:/app
  - /app/node_modules # Keeps container dependencies

Why Isolate node_modules?

Native binaries compiled for macOS will not run in Linux containers. We can use anonymous volume to preserve Linux-compiled dependencies inside container while mounting source code from host.

frontend:
  volumes:
    - ./frontend:/app # Source code (node_modules is inside this folder too)
    - /app/node_modules # "Overwriting" container dependencies (Linux)
  environment:
    - CHOKIDAR_USEPOLLING=true # Enable file watching