SaaS vs IaaS: who manages the stack?

SaaS, or Software as a Service, gives you a fully built application you log into and use immediately, like Gmail or Microsoft 365. IaaS, or Infrastructure as a Service, hands you virtual machines, storage, and networking, but you must install and manage the operating system and any software yourself. The core difference is management responsibility: with SaaS, the vendor handles everything from hardware to app updates; with IaaS, you control the operating system and above, while the vendor manages the physical data centre.

To avoid mix-ups, picture a pizza: SaaS is a delivered, ready-to-eat pizza; IaaS is just the dough and sauce you have to top and bake yourself. If a question describes a team that needs to install custom software or configure an OS, that points to IaaS, not SaaS. Another trick: if the service is accessed via a web browser with no installation, it is likely SaaS. If you get a virtual machine login screen, it is IaaS. The exam often tests this by listing tasks like 'patch the operating system' — that is your job on IaaS, but the vendor's on SaaS.

A quick way to lock this in: remember the acronym 'SaaS stops at the app, IaaS starts at the infrastructure.' For a memory test, ask yourself: 'Can I use this without touching a server?' If yes, it is SaaS. If I need to set up a server first, it is IaaS. This one distinction will catch most exam questions on this topic.