It's like glue. Platform as a Service (PaaS) may be the linchpin of cloud computing. It's the application development, integration and management layer between cloud infrastructure offerings like storage and SaaS applications. Microsoft's Azure, Salesforce's \nForce.com and Google's AppEngine promise to help developers write applications for a virtualized operating system that draws on computing resources distributed across many servers. \n\nYou can test it now. In early 2010, Kelley Blue Book will pilot Azure to manage overflow capacity for its website, says Andy Lapin, Kelley's director of architecture. PaaS's benefits come from the vendor maintaining large pools of reserve computing capacity, taking the guesswork out of technology development and deployment and eliminating the need to buy excess capacity as a buffer. "This is, 100 percent, a way to cut operational costs," Lapin said.\n\nStart small. Try PaaS first on applications with few external dependencies. "It's an option, but it's not always a good fit," says Dave Linthicum, author of Cloud Computing and SOA Convergence in Your Enterprise. He advises clients to find a low-risk application they can use to gain experience with this model. An application that has many dependencies and connections with other enterprise applications wouldn't be an ideal fit to move to a PaaS environment.\n\nScalability isn't guaranteed. "That's one of the great myths\u2014that as soon as you go to the cloud, you scale," says Steve Bobrowski, a consultant. A cloud platform allows you to add server capacity quickly, \nbut your application still needs to be smart about distributing workload. Lapin agrees, pointing out that scaling from 10 servers to 10,000 is a lot \ndifferent than going from 10 to 20, and each scenario \nleads to different decisions. \n\nPlatforms aren't Portable. Force.com applications are written in a language specific to that platform, Apex. "It's something people can sign up with to build their app and get it up and running in almost no time," Bobrowski said, but "you cannot move off Force.com." Other PaaS offerings include support for more standardized languages and frameworks, such as Java or Ruby on Rails, but may still require applications to be tailored for a specific cloud environment.