Docker Containers vs Virtual Machines

For decades, servers used Virtual Machines (VMs) to isolate applications. Today, we use Containers (Docker). Why the shift?

Virtual Machines (VMs)

A VM is a computer inside a computer.

• Hardware: The physical server.
• Hypervisor: Software (like VMware) that splits the hardware.
• Guest OS: Each VM runs its own full Operating System (Windows, Linux, etc.).

The Cost: If you run 3 VMs, you are running 3 Kernels, 3 sets of boot files, and 3 background processes. This wastes RAM and CPU.

Containers (Docker)

A Container is an isolated process, not a full computer.

• Hardware: The physical server.
• Host OS: One single Operating System (Linux).
• Container Engine: Splits the OS resources.
• App: The app runs directly on the Host Kernel.

The Benefit: Containers share the Linux Kernel. They start in milliseconds (not minutes) because they don't need to boot an OS.

Comparison