Much of the value proposition for Windows Azure -- the star of the show Tuesday at PDC 2009 in Los Angeles -- has been its ability to open up new business avenues for customers who had not been able to envision hosting high-intensity data center operations before. Azure could give these customers a leg up, a new and more affordable way to get off the ground.

But once they're off the ground, the question becomes, why stay up in the air? What's to keep those customers grounded -- to mix metaphors like an old editor of mine -- in the cloud? The surprise answer to that question is coming from a senior product manager for Windows Server, not Azure. Scott Ottaway told Betanews today that provisions are being planned for customers to move their deployed applications back off the Azure cloud, onto on-premises data center servers.

"Our goal is to provide a common application platform, just the way that we support ASP.NET in both places, PHP in both places. We want to supply a common platform so customers don't have to make the hard choice up front about where they're going to run something."

During the October 2008 PDC conference when Windows Azure was first introduced, it wasn't exactly clear who Microsoft was targeting as its customer. Thirteen months later, we have a much clearer picture of cloud services customers comprising three discrete classes: one that is made up of SMB businesses investing in affordable data center architecture for the first time, and building entirely new cloud applications that have never been tried; another made up of applications and services hosts that are simply mirroring their existing apps to the cloud space for affordability and scalability; and a third class in-between comprised of businesses of all sizes, who aren't looking to the cloud as a migration platform, but as a way to backup, complement, or augment their existing customer services as necessary.

The common factor between each of those classes is the need for a bridge -- perhaps now, perhaps later -- between their on-premise and off-premise cloud platforms. That's the reason for Microsoft's latest brand, announced Tuesday morning.

"If you write your code for Windows Server AppFabric, it should run on Windows Azure," said Ottaway, referring to the new mix-and-match composite applications system for the IIS platform. "What we are delivering in 2010 is a CTP [community technology preview] of AppFabric, called Windows Azure AppFabric, where you should be able to take the exact same code that you wrote for Windows Server AppFabric, and with zero or minimal refactoring, be able to put it up on Windows Azure and run it." AppFabric for now appears to include a methodology for customers to rapidly deploy applications and services based on common components. But for many of these components, there will be analogs between the on-Earth and off-Earth versions, if you will, such that all or part of these apps may be translated between locales as necessary.

"Right now, if I have an ASP.NET app running on Windows Azure, and it's using SQL Azure, I can pull that off with minimal refactoring, in most cases, and run it on premises on Windows Server, because Windows Server supports ASP.NET via IIS, and our .NET Framework, and it supports SQL Server," remarked Ottaway. "So I can move a certain class of apps that I have up there, off -- not all." Applications that utilize binary large object (BLOB) storage, and other Azure-specific features, would still require significant refactoring. "But if it's just an ASP.NET app through SQL Azure, you should be able to pull it back on-premises without much effort at all."

Some of the more recently deployed, non-Microsoft language platforms, including PHP and MySQL, will also aid customers in that transition.

"Whether there's a total cost-of-ownership advantage to being off-premises or on-premises, depends on so many factors. Are you highly virtualized? Do you have really good management tools? Do you have affordable staff? Regulatory concerns? Privacy, security concerns?" remarked the Windows Server senior product manager. "There's many, many factors that may indicate to you that...you want to keep it on-premises, or that it's okay to go all the way off-premises. But it's probably going to be more of a non-enterprise decision; it's going to be project by project and app-by-app."

Next: Virtual machines migrate to Azure...