In the following paragraphs an architectural approach to the digital librarywill be developed, which is based on taking the fundamental capabilities,introduced above, as the fundamental requirements the architecture mustsatisfy. I begin with the following notional architecture for digitallibraries:

Figure 4.1 illustrates these fundamental distinctions between datamanagement and metadata management, and between ingestion and utilization ofcontent.

Operational Architecture

Operational architecture is an information management system represented interms of the business processes it supports, and how information related toconduct of the business processes passes through the system's components.

The example shown in Figure 4.2 is an enterprise that conducts training byutilizing an extensive computer-based simulation system. The operational(business) processes, most obvious in the example, depend on the timely andwell-organized capture of training information as it happens, and bothcontemporaneous and retrospective search and retrieval of information from atraining event. Although the information is generated in several differententerprise domains (eight in the example), effective utilization of informationoften depends on cross-domain searches and retrievals. Therefore, digitallibrary services must provide information interoperability in middleware.

Fig. 4.1. Notational architecture--building blocks of information to enhanceexisting functions and enable new operational capabilities.

Fig. 4.2. Operational architecture.

Technical Architecture

A technical architecture breaks down operational (business) processes intofunctional components and capabilities (Figure 4.3). Hardware and softwareimplementations are still not resolved.

The utilization of digital library materials depends on the existence ofmetadata to give an efficient and accurate view of content. Metadata must becreated as content is added to the digital library. Metadata and data must bebound together logically, and there must be a robust underlying technology tomanage the logical connection through time, across platforms, and overgeographical separations, all on a networked, distributed system.

Fig. 4.3. Technical architecture.

Systems Architecture

A systems architecture shows the technology enablers and theirinter-relationships. In Figure 4.4, the digital library is a centralizedsubsystem that interacts with a variety of data producers and consumers withina complex distributed system.

Fig. 4.4. Systems architecture.

A fully detailed systems architecture resolves into software and hardwaresystems. Desirable systems properties such as scalability and extensibility canbe taken into account at the systems architecture level. The systemsarchitecture is rationalized relative to the operational and technicalarchitectures.

Published: February 1999; WTECHyper-Librarian