Choosing the service model for a hosted service

Cloud services are classified typically as infrastructure-as-a-service (IaaS), platform-as-a-service (PaaS), and software-as-a service (SaaS). In the IaaS model, the core service provided is a virtual machine (VM) with a Guest OS. The customer is responsible for everything about the Guest OS, including hardening it and adding any required software. Amazon Elastic Cloud Compute (EC2) is the paradigm for IaaS. In the PaaS model, the core service provided is a VM with a hardened Guest OS and an application-hosting environment. The customer is responsible only for the service injected into that environment. In the SaaS model, a service is exposed over the Internet and the customer merely has to access it.

Windows Azure is the paradigm for PaaS. As such, Windows Azure provides a high-level, application-hosting environment modeled on services, roles, and instances. The specification of the roles in the service is referred to as the service model.

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