With the announcement on Tuesday that Microsoft will open its datacenters for hosting of its Dynamics CRM solution, the obvious conclusion might be that Salesforce.com will have a major competitor to contend with.

However, Salesforce still holds one very important ace up its sleeve, according to one industry analyst: Salesforce offers the benefits of a true multi-tenant architecture while Microsoft appears to be tap dancing around the multi-tenant architecture issue.

“I can hear Fats Waller playing the piano right now,” said Denis Pombriant, principal at Beagle Research, explaining that Microsoft is indeed marching, if not dancing, to a different drummer.

By its own definition, Microsoft is offering is a different architecture than what Salesforce.com is offering. Bill Patterson, director of product management at Microsoft Dynamic CRM Online says that SQL Server is designed to be "one database server that runs lots of database instances."

In terms of multi-tenancy this means Microsoft has one architecture that handles everything from customer requests to report generation and business processes.

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But while Patterson says it is all done through one common architecture, he adds, "the only area where we physically isolate the data is at the database level," Patterson said.

Enter the tap dance, says Pombriant, who says this design will cause nothing short of vendor lock-in. The reason, Pombriant claims, is because each customer has its own version of the database while in a true multi-tenant model there is only one version of the software running, one application to patch, one application to upgrade.

As a consequence, any vendor delivering a multi-tenant architecture as Microsoft defines it, will have a hard time keeping all of its customers -- and especially partners -- in synch with the same version. In order to work in this situation, the partner will have to keep up with all the versions.

"The danger is that much of the software development organization will have to support multiple versions," said Pombriant.