A platform (referred to herein as Extensible Application Platform, or XAP) for developing extensible business applications. In one set of embodiments, XAP can include an Extensible Markup Language (XML) based application model that is designed to support a high degree of application extensibility. For example, the XAP application model can support declarative definition of application semantics, separation of application data and metadata, and a flexible type system that allows constraints and calculations (e.g., XQuery constraints and calculations) for a data object instance (e.g., an XML document) to change over time. In certain embodiments, the XAP application model can be implemented via a server-side database/application engine that is configured to interoperate with a spreadsheet-like, client-side user interface. Using the spreadsheet-like UI, a non-programmer can build, customize, and run an application workflow that operates on top of a shared database like a traditional business application.