The big idea: Cloud computing means renting computing from a provider over the internet instead of buying and running the machines yourself.
You pay for what you use, and the provider keeps the hardware working.
The one question that separates the three: How much of the stack do you still look after?
Every cloud service hands some layers to the provider and leaves the rest to you. Where that line falls is the whole topic.
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The same six layers every time. Step through the models to watch the you/provider line move up the stack.
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IaaS — you get the machine
- The provider runs the hardware and the virtualisation
- You install the operating system and everything above it
- Most control, most work
PaaS — a place to run code
- The provider also runs the OS and the runtime
- You upload your program and its data
- Less control, far less setup
SaaS — the finished program
- The provider runs every layer
- You sign in and use it
- Least control, no maintenance at all
A way to keep them straight: The same school website, three ways.
IaaS — rent a bare server and build the site on it. PaaS — upload your site's code and let the provider run it. SaaS — subscribe to a ready-made school website product.
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You cannot have both: Every layer you hand over saves you work and takes away a choice.
Give away the operating system and you no longer pick its version — but you also stop patching it at 2 a.m.
More control (towards IaaS)
- Choose the OS, the versions, the configuration
- Run software with unusual requirements
- You are responsible for updates and security patches
- Slower to set up, and needs staff who know how
More convenience (towards SaaS)
- Ready in minutes, no setup
- Provider handles updates, patching and backups
- You take whatever features the provider offers
- Hard to move elsewhere later
Availability and resources, briefly: Cloud providers can add resources on demand — more storage, more processing — so a service copes with a busy day without you buying hardware for it.
The trade is that you depend on their uptime and their network.
How this is tested — you must match a model to a scenario and justify it by how much control that user needs. It comes up two ways:
Paper 1 Section A
- Describe or distinguish SaaS, PaaS and IaaS, 2-4 marks
- Give an example of each
- Explain the control-against-convenience trade
Paper 1 Section B — case study
- Recommend a model for the case study organisation
- Justify it from their staff, budget and needs
The classic trap: Naming the model but not saying who manages what. "They should use SaaS" earns nothing on its own; "SaaS, because the provider maintains the software and the school has no IT staff" earns the mark.
A small charity has no technical staff and needs email and document editing. A software company needs to test its product on several operating system versions. Explain which cloud service model suits each.
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