Chapter 33 

Facets and Versioning

33.1 Introduction

Facets provide a general-purpose mechanism for non-intrusively extending the type system of an application. This is particularly useful for versioning an applica­tion. Section 33.2 introduces the facet concept and presents the relevant APIs. Section 33.3 presents a few traditional approaches to versioning and their prob­lems. Sections 33.4 to 33.6 show how to use facets to implement versioning, and Section 33.7 discusses design choices when adding versioning to a system.