Chapter 30 

Facets and Versioning

30.1 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 30.2 introduces the facet concept and presents the relevant APIs. Section 30.3 presents a few traditional approaches to versioning and their problems. Sections 30.4 to 30.6 show how to use facets to implement versioning, and Section 30.7 discusses design choices when adding versioning to a system.