An adaptive, standardized service layer is a key feature of a multi-tenant on-demand enterprise application. Custom business logic and data need to be exposed via services that are tailored to the respective customer organization. Ideally, this layer of web services can be automatically derived from the underlying domain model. This paper aims to describe means to design and implement such a service layer by following a lean, model-driven approach based on the runtime interpretation of metadata. Finally, the implementation will be validated against a real-world show case.