Servers you can't see

One computer, many machines

Virtualization lets one physical computer act like several separate computers. Each one is a virtual machine (VM): it has its own operating system, its own apps and its own slice of the real computer's CPU, memory and storage. To the software inside, a VM looks just like a real computer. Sam can run four test servers on one powerful machine instead of buying four.

Mr. Barkley: One power bill instead of four. Go on.

Sam: And if a test server breaks, I delete it and make a new one in minutes.

The hypervisor

The hypervisor is the software that creates and runs virtual machines. It shares the physical computer's hardware among the VMs and keeps them apart, so one VM crashing doesn't take down the others. Some hypervisors run directly on the hardware, which is common for servers. Others run as an app on a normal desktop, which is handy for testing on a laptop.

You: So the hypervisor is like the warehouse manager handing out shelf space.

Rosa: Hey, that's my job!

Host and guest operating systems

The physical computer is the host. An operating system running inside a VM is a guest operating system (OS). Sam can run a Linux guest on his Windows laptop to test the label app, or a Windows guest on a Linux server. Each guest thinks it has a whole computer to itself, while the hypervisor quietly shares the real one.

Sam: My laptop runs Windows. The Linux test box inside it is the guest.

Ranger: a very good guest himself, sits politely

Exam tip Inside the VM = guest OS. The software that runs VMs = hypervisor. The physical machine = host.

Infrastructure as a Service (IaaS)

With Infrastructure as a Service (IaaS), a cloud provider rents you the basic building blocks: virtual servers, storage and networks. The provider runs the buildings and hardware. You install and manage the operating system and everything above it. IaaS gives the most control, which helps when an old app needs one exact OS version.

Sam: The old warehouse app only runs on one OS version. IaaS lets me pick it.

Platform as a Service (PaaS)

With Platform as a Service (PaaS), the provider runs the servers, the operating system and the tools, and gives developers a ready-made platform. Developers just upload their code and the provider runs it. Nobody on your team patches an OS. You trade some control for a lot less work.

Theo: That's what the online store needs. I upload code; somebody else patches servers.

Sam: Music to my ears.

Software as a Service (SaaS)

With Software as a Service (SaaS), the provider runs a finished app, usually in a web browser, and handles the servers, the software and the updates. You just sign in and use it, like a web email app or online payroll. SaaS gives the least control and the least work.

Mr. Barkley: Our email in the browser. I've never seen its server.

Sam: Exactly. That's SaaS.

Exam tip Most control: IaaS (you run the OS). Middle: PaaS (you bring code). Least: SaaS (you just use the app).

On premises, cloud or hybrid

Deployment models describe where systems run. On premises means on your own hardware in your own building. It keeps working on the local network when the internet is down, and the data never leaves. Cloud means at a provider, reached over the internet. Hybrid mixes the two, working together: some systems on premises and some in the cloud.

Mr. Barkley: Payroll stays here where I can see it. The online store goes to the cloud.

Sam: Then Kibble & Co. is hybrid.

Exam tip Deployment model = where it runs. Service model (IaaS, PaaS, SaaS) = how much the provider manages.

Hypervisors and guest operating systems

Virtualization lets one physical computer run several virtual machines. Each virtual machine acts like a separate computer with its own operating system. The hypervisor is the software that creates and runs the virtual machines and shares the real hardware among them. The OS inside a virtual machine is the guest operating system. Sam can run four test servers as four virtual machines on one powerful server instead of buying four separate computers.

Exam tip Linux running in a virtual machine on Sam's Windows laptop: Linux is the guest OS. The hypervisor is what runs it.

IaaS, PaaS and SaaS

Cloud service models describe what the provider manages for you:

• IaaS (Infrastructure as a Service): rented virtual servers, storage and networks. You install and control the OS. Good when an old app needs one exact OS version.
• PaaS (Platform as a Service): a ready platform. You upload your own code; the provider runs the servers and OS.
• SaaS (Software as a Service): a finished app, like web email. You just sign in and use it.

Exam tip The more 'as a Service' you rent, the less you manage. Your data and accounts are always yours to look after.

On premises, cloud or hybrid

Deployment models describe where systems run:

• On premises: the company's own hardware in its own building. Keeps working on the local network when the internet is down, and the data stays inside.
• Cloud: a provider's data centers, reached over the internet. Easy to grow for holiday traffic.
• Hybrid: some of each, working together, like payroll on HQ servers and the web store in the cloud.

Exam tip Don't mix the lists: IaaS, PaaS and SaaS are service models. On premises, cloud and hybrid are deployment models.

Match the clue to the answer

Cloud questions usually hide one clue:

• 'Just sign in and use it in a browser': SaaS.
• 'Upload our code; don't make us manage servers': PaaS.
• 'We must install and control one exact OS version': IaaS.
• 'Must keep working when the internet is down, data stays in the building': on premises.
• 'Payroll stays at HQ, the store runs in the cloud': hybrid.
• 'Four test servers, one physical box': virtualization with a hypervisor.