Cloud computing: IaaS, PaaS and SaaS
Start in your own server room, then climb the ladder — infrastructure, platform, software and serverless — with a real product and a clear owner at every rung.
Cloud computing
Cloud computing is renting somebody else's computers, but the interesting question is how much of the work you rent along with them. The answer splits into a few named rungs, and those names are on almost every technology job advert written today.
The nine layer stack
Running any application means somebody handles nine things. The building, the power, the servers, the storage, the network, the virtual machines, the operating system, the runtime and the application itself. Your data sits on top of all nine, and it is always yours.
On premises
Before the cloud, all nine of them were yours. You bought a rack, air conditioned a room, and ordered machines twelve weeks ahead based on a guess about next year's traffic. Guess low and you crash; guess high and you have bought idle metal.
You manage all nine
The hidden cost was never the hardware, it was the people. Somebody had to patch operating systems at two in the morning and swap failed disks, and a typical company server sat idle at under fifteen percent of its capacity.
Infrastructure as a service
Infrastructure as a service hands you the bottom layers. You rent virtual machines, disks and networking by the hour, and you still install and patch the operating system yourself. Maximum control, and maximum homework to go with it.
Elastic Compute Cloud
Amazon's Elastic Compute Cloud is the archetype, alongside Azure Virtual Machines and Google Compute Engine. A server that once took three months to purchase now takes about forty seconds to start, and you can delete it just as quickly.
Platform as a service
Platform as a service takes the operating system away as well. You hand over code, and the provider handles the machines, the patching and the scaling. The trade is convenience for control, and you build the way the platform prefers.
Heroku and App Engine
Heroku made this famous with a single command that pushed a web application live. Google App Engine, Azure App Service and Vercel do the same job today, and most developers using them never learn which machine their code is running on.
Software as a service
Software as a service is the top of the ladder. There is nothing to install and nothing to operate. You open a browser tab and use finished software, usually paying a fee per person per month and nothing else.
Gmail and Salesforce
Gmail, Salesforce, Slack and Dropbox all live here. You manage exactly one layer, your own data, and that happens to be the layer worth thinking hardest about, because getting your data back out again is often the difficult part.
Serverless functions
Serverless goes one rung further. You upload a single function and it runs only when something calls it, scaling from zero to thousands of copies and back down again within seconds. Sitting idle costs you nothing at all.
Lambda and Cloud Run
Amazon's Lambda bills in slices of a single millisecond, so a function called a hundred times a day can cost less over a year than one cup of coffee. The catch is the cold start, the small pause while a sleeping function wakes up.
Shared responsibility
Every provider publishes a shared responsibility model, and it repays reading carefully. They secure the cloud itself; you secure whatever you put inside it. Many of the loudest cloud leaks were storage buckets left open to the public internet, not broken data centres.
Pay for what you use
The deeper change is the billing clock. A large capital purchase became a running operating cost, so three people in a room can start a hundred servers for one afternoon experiment and hand them all back before dinner.
The pizza test
The classic way to remember the ladder is pizza. Cooking at home is on premises, a take and bake tray is infrastructure, delivery to your door is platform, and sitting down in the restaurant is software as a service.
Choosing your rung
Higher up means less work and less control, and the right answer is almost never one rung for everything. Sensible teams buy finished software for email, use a platform for the web app, and rent raw machines only where something genuinely unusual lives.
Watch it explain itself
Every step above is narrated aloud. Play it, or open it in the editor and make it yours — no account needed.