IaaS, PaaS, and SaaS describe how much of a cloud system the provider operates and how much remains with the customer. Choose the model from the workload, required control, and the team’s capacity to maintain it.
The NIST definition of cloud computing identifies these three service models. They are distinct from deployment models such as public, private, and hybrid cloud. A business can use more than one service model at the same time.
Compare the management boundaries
| Model | Provider supplies | Customer typically manages |
|---|---|---|
| IaaS: infrastructure as a service | Computing, storage, networking, and underlying infrastructure | Guest operating system, installed software, application, configuration, data and access |
| PaaS: platform as a service | Infrastructure and a managed application platform | Application code, dependencies within its scope, configuration, data and access |
| SaaS: software as a service | A hosted application and its underlying platform | Users, permissions, configuration, data use, integrations and endpoint practices |
This is a planning summary. Read the exact service contract and responsibility documentation before assuming a backup, patch, security control, or recovery task belongs to the provider.
When IaaS fits
IaaS is useful when a workload needs control over operating-system configuration, installed software, or network behavior that a managed platform cannot provide. A legacy application with a specific runtime may fit this pattern.
You avoid operating the physical infrastructure, but still need skills for patching, hardening, deployment, capacity, monitoring, and recovery inside your scope. Provisioning a virtual machine does not automatically make the application resilient or scalable.
Budget for idle resources, storage, traffic, backup, support, and staff time as well as compute. The cloud bill can exceed an initial estimate if resources remain provisioned or data-transfer assumptions are wrong.
When PaaS fits
PaaS can reduce infrastructure work for a custom application when the supported runtimes, deployment model, and service limits fit. The team can concentrate more on application behavior while the provider manages more of the platform.
Check supported language and runtime versions, background-job behavior, storage persistence, network access, scaling limits, and deployment rollback. Do not assume every PaaS supports every language or that automatic scaling requires no configuration.
The tradeoff is reduced low-level control and possible dependence on platform-specific APIs or deployment behavior. Test the important integration and a realistic workload before choosing it for the whole product.
When SaaS fits
SaaS is a candidate when an existing application meets the business need with manageable configuration and integration. Email, collaboration, CRM, and standard support workflows often justify evaluating a hosted product before building one.
The provider handles application operation within its service scope. Your business still needs account lifecycle management, access reviews, configuration, data handling, integration ownership, and a plan for service interruption.
Updates may arrive on the provider’s schedule and can affect workflows or integrations. Offline access, exports, retention, and release controls vary by product and plan. Verify them rather than assuming any browser-based subscription has the same behavior.
Security is shared
AWS’s shared-responsibility model distinguishes provider infrastructure responsibilities from the customer’s responsibilities, which vary with the service. Use the same explicit mapping when evaluating any supplier.
Neither “the provider handles all security” nor “the SaaS provider has no data-security responsibility” is a reliable general rule. Write down who configures access, patches each layer, reviews logs, manages secrets, handles incidents, and restores data. Confirm the service-specific evidence before relying on a certification or a broad cloud claim.
Choose from the workload
| Workload | Starting option to evaluate | What would change the decision? |
|---|---|---|
| Standard business process | SaaS | Essential requirements cannot be configured or integrated |
| New custom web application | PaaS | Required runtime, networking, or operational behavior is unsupported |
| Legacy application needing OS control | IaaS | A tested modernization can remove those dependencies |
| Narrow integration between existing tools | Managed integration service or small application | Complex rules, ownership, or reliability require more control |
These are starting options, not rankings. Compare the actual product and service rather than a whole provider: a cloud vendor may sell IaaS, PaaS, and SaaS offerings.
Model costs consistently
For a hypothetical approval workflow, compare a SaaS subscription plus configuration and integration with a PaaS-hosted custom app plus development and support. An IaaS option also needs operating-system and runtime maintenance in its estimate.
Use the same users, volume, service hours, retention, recovery needs, and planning horizon. Include migration, training, administration, incident work, and exit costs. Lower subscription spend does not establish lower total cost if the team must build and operate missing functionality.
Test portability and recovery
Before committing, export a representative dataset and confirm that another system can interpret its fields, relationships, and attachments. Record proprietary dependencies and the work required to replace them.
Test a service interruption and restoration with the actual operational owners. Agree recovery expectations, communication, and any manual fallback. Redundancy or provider availability commitments do not automatically recover deleted records or a faulty application change.
Make the decision reviewable
Document the workload, selected service, management boundary, cost assumptions, unresolved risks, acceptance tests, and named operating owner. Revisit the choice when the workload or team changes.
For a broader build-or-buy decision, see enterprise application development. The best cloud model is the one that meets the requirements with responsibilities the team can actually fulfill.
Use these operating boundaries when comparing software engagement options, so implementation and ongoing service ownership are explicit.
Further questions
What are IaaS, PaaS, and SaaS?
IaaS supplies computing infrastructure, PaaS supplies a managed platform for deploying applications, and SaaS supplies an application users consume. The exact management boundary depends on the service.
Which cloud model should a business choose?
Start with the workload and operating capacity. Buy suitable application functionality with SaaS, consider PaaS for a custom app, and use IaaS when lower-level control is necessary and the team can manage it.
