Docker & VM: Introduction

Learn the key concepts of Docker, including the differences between containers and virtual machines, and how Docker simplifies application deployment with lightweight, fast, and isolated environments.

1. Docker vs. Virtual Machine (VM)

Feature	Docker (Containers)	Virtual Machine (VM)
Kernel Sharing	Yes, all containers use a single host kernel.	No, each VM has its own separate kernel and operating system.
Size	Lightweight, as they only contain the application and its dependencies.	Heavyweight, as each VM includes a full operating system.
Speed	Fast startup, in seconds or milliseconds.	Slow startup, in minutes.
Isolation	Processes are isolated, but at the operating system kernel level.	Full hardware isolation, controlled by a hypervisor.

2. The Container as an Application Environment

A container is a running instance of a Docker image, acting as a complete, isolated environment for your application.

Dockerfile: This is the blueprint for building an image, containing instructions (e.g., where to get the base OS, what to install).
Docker Image: It is a static, ready-to-use package that contains the entire application and its environment (files, dependencies, libraries). An image is immutable once built.
Container: This is a dynamic process that runs the environment defined in the image. File modifications in a container are ephemeral by default. For persistent data, you must use volumes or bind mounts.
Communication: It happens via port mapping or Unix domain sockets.

3. VMM (Hypervisor) Types

A hypervisor is software that enables and manages multiple virtual machines on a single physical server.

Hypervisor Type	Description	Use Case
Type 1 (Bare-Metal)	Runs directly on the physical hardware without an underlying operating system. It is very performant and secure.	Data centers, production servers.
Type 2 (Hosted)	Runs as an application on an existing host operating system (e.g., Windows). It is easier to use, but less performant.	Development and testing environments.