In an example embodiment, a definition is loaded into a client application for execution on a user device. Then, one or more classes defined in a central repository database of a server are loaded into the client application. It may then be determined that one or more custom classes not defined in the central repository database are contained in the definition. The one or more custom classes may be validated by ensuring that they contain one or more preset functions. The one or more custom classes may then be loaded into the client application, thereby overriding or extending at least one of the one or more classes defined in the central repository database.