Multi-tenancy is the key common attribute of both public and private clouds, and it applies to all three layers of a cloud: Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS) and Software-as-a-Service (SaaS).
Most people point to the IaaS layer alone when they talk about clouds. Even so, architecturally, both public and private IaaSes go beyond tactical features such as virtualization, and head towards implementing the concept of IT-as-a-Service (ITaaS) through billing -- or chargeback in the case of private clouds -- based on metered usage. An IaaS also features improved accountability using service-level-agreements (SLAs), identity management for secured access, fault tolerance, disaster recovery, dynamic procurement and other key properties.
By incorporating these shared services at the infrastructure layer, all clouds automatically become multi-tenant, to a degree. But multi-tenancy in clouds has to go beyond the IaaS layer, to include the PaaS layer (application servers, Java Virtual Machines, etc.) and ultimately to the SaaS or application layer (database, business logic, work flow and user interface). Only then can tenants can enjoy the full spectrum of common services from a cloud -- starting at the hardware layer and going all the way up to the user-interface layer, depending on the degree of multi-tenancy offered by the cloud.
The exact degree of multi-tenancy, as it's commonly defined, is based on how much of the core application, or SaaS, layer is designed to be shared across tenants. The highest degree of multi-tenancy allows the database schema to be shared and supports customization of the business logic, workflow and user-interface layers. In other words, all the sub-layers of SaaS offer multi-tenancy in this degree.
In the lowest degree, multi-tenancy is limited to the IaaS and PaaS layers, with dedicated SaaS layers for each tenant.
And in the middle degree of multi-tenancy are clusters of homogenous tenants that share database schemas (or schemae) and other application layers. In the middle level, each cluster of users has its own version of database schema and the application itself.
We can sum up the discussion on the degree of multi-tenancy as follows:
For example, Salesforce.com, at the relatively high end of the multi-tenancy spectrum, has 72,500 customers who are supported by 8 to 12 multi-tenant instances (meaning IaaS/PaaS instances) in a 1:5000 ratio. In other words, each multi-tenant instance supports 5,000 tenants who share the same database schema. Intacct, a financial systems SaaS provider in the middle of the spectrum, has more than 2,500 customers who share 10 instances in a 1:250 ratio.
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