Virtual Machines

The most basic thing you can rent in the cloud is a computer to run your software. In most cases, that computer is not a whole physical machine to yourself — it's a virtual machine: a slice of a larger real machine, handed to you as if it were a complete computer of its own.

Understanding what "virtual" means here unlocks a lot: it explains why cloud servers can appear and disappear in seconds, why you pay for a fraction of a machine, and why thousands of customers can share a single building full of hardware without interfering with each other.

Think of a large apartment building. One big structure is divided into separate, self-contained flats — each tenant's space is private and independent, even though they share the walls, roof, and foundations. A virtual machine works the same way: private and self-contained, built from a shared physical structure.

What Is a Server?

Before getting to "virtual," it helps to know what a server is. A server is simply a computer that runs software and waits for requests — a website, an app, a database. When you open a web page, a server somewhere receives your request and sends back the page. The word "server" just describes the role: this computer serves other computers.

In the old world, a company that needed to run software had to buy a physical server, put it in a room, plug it in, and keep it running. The cloud lets you rent that server instead, and a virtual machine is the unit of rental.

Virtual, Not Physical

A cloud provider owns enormous physical machines — computers with far more processing power and memory than any one customer needs. Special software, called a hypervisor, splits each physical machine into many isolated slices. Each slice looks and behaves like its own separate computer. That slice is a virtual machine.

You rent one of those slices. From your point of view, it's a whole computer: it has its own processing power, its own memory, its own storage, and its own connection to the network. It runs in complete isolation from the other slices on the same physical hardware. The fact that it shares a building doesn't affect you any more than your apartment neighbor's cooking affects your kitchen.

What You Control

When you rent a virtual machine, you get the keys to the whole thing — down to the operating system, the foundational software (like Windows or Linux) that every other program runs on top of. You choose the operating system, install whatever software you need, configure it, and run it how you like.

That level of control is the VM's defining feature. It means the most flexibility — and also the most responsibility. Someone has to keep the operating system updated, manage security, and handle the configuration. The provider keeps the physical hardware running; everything above the hardware is yours to manage. (This is the IaaS layer from Chapter 1, applied to compute.)

Sizing: Picking How Much Power You Rent

Not all virtual machines are the same size. Providers offer a menu of configurations — more processing power, more memory, more storage — each at a different price. A small website might run on the most modest option; a large database might need a very large one. You pick the size that fits, and pay accordingly. If your needs grow, you can switch to a bigger size; if they shrink, you can move down.

This is the cloud's pay-for-what-you-use idea applied to raw computing: you're not stuck with the size of a machine you already bought.