XPoints: Extension Interfaces for Multilayered Applications

Extensibility is a key requirement in modern software applications. In the context of business applications it is one of the major selection criteria from the customer perspective. However, there are some challenges concerning the specification and enforcement of extension interfaces. Extension interfaces define the resources of the base applications that are allowed to be extended, where and when the extension code will run, and what resources of the base application an extension is allowed to access. While concepts for such interfaces are still a hot research topic for "traditional" software constructed using a single programming language, they are completely missing for complex systems consisting of several abstraction layers. In addition, state-of-the-art approaches do not support providing different extension interfaces for different stakeholders. This paper attempts to fill this gap by introducing XPoints, an approach and a language for specifying and enforcing extension interfaces in multilayered applications. An extension interface in XPoints defines the available extension points on the different abstraction layers, controls the access and visibility of the core application to the extension, and constrains the interplay between extension points possibly from different abstraction layers. Several extension interfaces can be overlaid over the same core application, hence, enabling multiple extender views to co-exist. Using an XPoints interface, a software provider can automatically generate the extensibility code infrastructure to provide the extension interface for the core application.