Software as a Service

SaaS services are full, multitenant applications, with all the architectural complexities of any large software platform. Many SaaS providers build on top of IaaS and PaaS due to the increased agility, resilience, and (potential) economic benefits.
Most modern cloud applications (SaaS or otherwise) use a combination of IaaS and PaaS, sometimes across different cloud providers. Many also tend to offer public APIs for some (or all) functionality. They often need these to support a variety of clients, especially web browsers and mobile applications.
Thus all SaaS tends to have an application/logic layer and data storage, with an API on top. Then there are one or more presentation layers, often including web browsers, mobile applications, and public API access.
The simplified architecture diagram below is taken from a real SaaS platform, but generalized to remove references to the specific products in use:
These reference and architectural models should not be construed as being canonical. They are included to provide security professionals a deeper understanding of how cloud computing works and is constructed, but there are far too many different approaches in the real world to include them all.

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