1. INTRODUCTION The trend towards increasingly open, flexible architec- tures can be traced in the development of digital library protocols [1]. The highly successful Open Archives Initiative Protocol for Metadata Harvesting (OAI-PMH) has provided a simple base-line for metadata access, and subsequent work strives to base component-based, modular protocols upon it [10]. In a complementary realm, the emerging METS docu- ment framework [6] provides an open, extensible system for representing documents in digital repositories. Set against the move towards standard protocols, today’s digital library systems must confront an increasing range of document formats and media, architectural designs for browsing and classification, indexing requirements, and user interface techniques. This drives the demand for open ar- chitectures, but makes it hard to provide well-formed and supportive infrastructures that maximize reuse of individ- ual components and the reliability of the system as a whole.