At the one-year mark for its Azure cloud service Feb. 1, Microsoft said it has 31,000 Azure customers, 55% more than the number listed at the mid-year mark last July. That's a respectable number, for sure, but Azure's real test is yet to come.

Microsoft should have a larger lead than it does, since the open source alternatives in cloud computing needed to designed and engineered from scratch, while Microsoft had a programmer's warehouse full of technologies to begin with. The open source options are rapidly multiplying, and enterprises are in the process of deciding how they want to build out their private clouds. As they do so, that will determine which public cloud they are most compatible with.

Microsoft is doing such a great job of backtracking on all those nasty things it once said about open source code that it's almost as much fun to watch as Larry Ellison's backtracking on all those nasty things he once said about cloud computing.

On a more solemn note, Microsoft recently pointed out how its SQL Server plays nicely with the Drupal 7 open source content management, both on premises and in the Azure cloud. It's true that it's now willing to go the extra mile to work with select open source projects.

But that still doesn't solve the problem of being compatible overall with the next big thing that the enterprise wants to build out, especially if it chooses a lot of open source to do so. The private cloud, like the public cloud, is likely to have an open source infrastructure. For the private cloud to be compatible with Azure, it will need to be based on the proprietary technologies that Azure is designed to be compatible with -- such as Visual Studio, SQL Server, and Silverlight. What if the majority of enterprises don't want to do that?

Their choices abound. They have the NASA Nebula and Rackspace OpenStack project; Eucalyptus Systems with its EC2-compatible APIs; Zend's Simple API project, which holds out the prospect of cloud interoperability; the Nimbula Director cloud operating system; WSO2; or Cloud.com's CloudStack. Don't forget the Red Hat-originated Deltacloud project inside the Apache Software Foundation's incubator, where it will quietly gather strength for a few more months, or the cloud assists that both Red Hat and Ubuntu package up with the KVM open source hypervisor.