Phil Wainewright writes astutely today on the many degrees of multi-tenant SaaS architecture, highlighting "true" vs. "everything else." Considering that customers and end-users have little to no idea what's running at SaaS companies it's a bit ironic that the technology powering these companies is interesting--I suppose it's only so to technical people and other vendors.
Salesforce.com: First-degree multi-tenancy. In this model, all customers are served from a single infrastructure in which every component is shared, all the way down to the tables in the database.
Intacct: Second-degree multi-tenancy. Like many SaaS pureplays, Intacct uses replication much more broadly than Salesforce.com to distribute its shared-schema instances across large numbers of server clusters.
Oracle and others: Lesser-degree multi-tenancy. There are a lot of terms floating around for these lower levels of multi-tenancy, including isolated tenancy, mega-tenancy or hybrid tenancy.Link: Many degrees of multi-tenancy