6.2 IaaS / PaaS / SaaS
IaaS gives you virtual hardware; PaaS gives you a runtime; SaaS gives you a finished product. The shift up the stack means more value, less control, and different cost models.
6.2 Cloud Service Models
The stack of responsibility
Layer IaaS PaaS SaaS Hardware provider provider provider Network provider provider provider OS YOU provider provider Runtime YOU provider provider Application YOU YOU provider Data YOU YOU YOU (often) Configuration YOU YOU YOU Identity YOU YOU YOU
Examples by model
Model Examples IaaS AWS EC2, Azure VMs, GCP Compute Engine, vSphere PaaS AWS App Runner, Azure App Service, GCP Cloud Run, Heroku SaaS Microsoft 365, Google Workspace, Salesforce, Slack, Atlassian CaaS Managed Kubernetes; ECS, AKS, GKE FaaS Lambda, Azure Functions, Cloud Run Functions
Cost characteristics
Model Pricing Ops effort Lock-in IaaS On-demand / reserved / spot High Low (mostly) PaaS Per-app, per-deploy Medium Medium-high SaaS Per-seat or per-tenant Low High
Worked example - internal portal for 50 staff
Model Monthly cost (illustrative) Ops effort Notes IaaS PHP 18,000 High Full VM + ops time PaaS PHP 8,000 Medium Cloud Run / App Svc SaaS PHP 6,000 Low SharePoint / Notion
When to pick what
Choose IaaS when legacy software, full kernel control, hybrid sync Choose PaaS when greenfield apps, small ops team, fast iteration Choose SaaS when commodity function, integration beats customisation Choose CaaS when polyglot containerised workloads, portability Choose FaaS when event-driven, bursty, sub-second cold-start OK
Common pitfalls
IaaS migration that is just on-prem with bigger bills.
PaaS lock-in (proprietary triggers, runtime-specific extensions).
SaaS sprawl; 40 SaaS subscriptions, no SSO catalogue.
FaaS where steady traffic would be cheaper on PaaS.
Where this lives in EA
The service-model policy is Technology Architecture. Pair it with Application Architecture rules: 'new apps default to PaaS, customise only after measurement.'
Mentor’s tip: IaaS for control; PaaS for speed; SaaS for commodity work. Default new internal apps to PaaS. SaaS sprawl is the real cost trap - catalogue every subscription and govern under SSO.
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