IaaS, PaaS & SaaS

IaaS, PaaS, and SaaS are the three levels at which you can consume the cloud: IaaS (Infrastructure as a Service) rents raw building blocks, virtual servers, storage, networking; PaaS (Platform as a Service) runs your code on a managed platform; SaaS (Software as a Service) delivers finished applications you simply log into.

In plain terms

Think of running a company payroll system. With SaaS, you subscribe to Gusto and never see a server. With PaaS, you deploy your own payroll app to Heroku or AWS Elastic Beanstalk, you own the code, the platform handles servers, patching, and scaling. With IaaS, you rent virtual machines on EC2, Azure VMs, or Compute Engine and install everything yourself, just like your old server room, minus the hardware.

Each step down the list means more control and more operational responsibility; each step up means less of both.

Why it matters when you migrate

• It sets your migration strategy. Moving VMs to IaaS as-is is a lift-and-shift; adopting PaaS means replatforming; sometimes the right move is retiring an app for a SaaS product entirely. See lift-and-shift vs replatform vs refactor.
• It sets your cost and staffing profile. IaaS is cheapest per unit but you keep patching and on-call; PaaS and SaaS cost more per unit and eliminate work.
• It defines the security split. The higher the level, the more the provider secures for you, and the less you can misconfigure.