Clouds today come in two basic flavors: the private cloud (wholly behind the firewall) and the public cloud, which runs on remote hosts. The private cloud currently enjoys IT management’s attention, because taking lots of individual servers, putting them into a pool, and parceling out their capabilities as needed has tremendous advantages for the data center. Of these, none is more prominent that the ability to scale up resources when projects demand and back down again when the need declines.
Private clouds require little changes in programming. Instances of virtual machines are spun up from an administrative console and the application is migrated to this home in the clouds from which, by and large, it works as expected.
The public cloud — whose leading hosts include Amazon, Google, Microsoft — is a different thing altogether. Code cannot be migrated simply to these hosts and expected to work correctly. It won’t. Google’s App Engine, for example, allows only a select list of core Java classes to run on its platform. If your code relies on a proscribed class, your app will not run.
Moreover, each platform uses its own unique datastore, which does not run at all like traditional relational DBMSs. (Microsoft Azure does offer a "cloudified" version of its SQL Server database product as an option.) So, if you plan to run applications in the public cloud, you’ll either have to invest considerable effort porting existing code or writing new apps from scratch. Doing so will reveal a second problem: No two platforms use the same API. So, from the get go, you are coding to a proprietary platform &mdash with all the constraints that implies.
This problem is widely acknowledged, but efforts to provide a universal API, such as the Simple Cloud API, have garnered little enthusiasm from the cloud hosts. This situation puts IT organizations in a bind. If considering using the public cloud, therefore, run extensive pilots before committing to a platform. Know your platform’s limitations and costs intimately before making it the basis of an important app. You’re likely to be residing there a long time.
— Andrew Binstock is the Executive Editor of Dr. Dobb's. He can be reached at firstname.lastname@example.org.