Amazon EC2 is at one end of the spectrum. An EC2 instance looks much like physical hardware, and users cancontrol nearly the entire software stack, from the kernel upwards. This low level makes it inherently difficult forAmazon to offer automatic scalability and failover, because the semantics associated with replication and other statemanagement issues are highly application-dependent. At the other extreme of the spectrum are application domainspecific platforms such as Google AppEngine. AppEngine is targeted exclusively at traditional web applications,enforcing an application structure of clean separation between a stateless computation tier and a stateful storage tier.AppEngine’s impressive automatic scaling and high-availability mechanisms, and the proprietary MegaStore datastorage available to AppEngine applications, all rely on these constraints. Applications for Microsoft’s Azure arewritten using the .NET libraries, and compiled to the Common Language Runtime, a language-independent managedenvironment. Thus, Azure is intermediate between application frameworks like AppEngine and hardware virtualmachines like EC2.
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