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Abstract

Defining metadata as "data about data" provokes more questions than it answers. What are the forms of the data and metadata? Can we be more specific about the manner in which the metadata is "about" the data? Are data and metadata distinguished only in the context of their relationship? Is the nature of the relationship between the datasets declarative or procedural? Can the metadata itself be described by other data? Over the past several years, we have been engaged in a number of efforts examining the role, format, composition, and architecture of metadata for networked resources. During this time, we have noticed the tendency to be led astray by comfortable, but somewhat inappropriate, models in the non-digital information environment. Rather than pursuing familiar models, there is the need for a new model that fully exploits the unique combination of computation and connectivity that characterizes the digital library. In this paper, we describe an extension of the Warwick Framework that we call Distributed Active Relationships (DARs). DARs provide a powerful model for representing data and metadata in digital library objects. They explicitly express the relationships between networked resources, and even allow those relationships to be dynamically downloadable and executable. The DAR model is based on the following principles, which our examination of the "data about data" definition has led us to regard as axiomatic: * There is no essential distinction between data and metadata. We can only make such a distinction in terms of a particular "about" relationship. As a result, what is metadata in the context of one "about" relationship may be data in another. * There is no single "about" relationship. There are many different and important relationships between data resources. * Resources can be related without regard for their location. The connectivity in networked information architectures makes it possible to have data in one repository describe data in another repository. * The computational power of the networked information environment makes it possible to consider active or dynamic relationships between data sets. This adds considerable power to the "data about data" definition. First, data about another data set may not physically exist, but may be automatically derived. Second, the "about" relationship may be an executable object -- in a sense interpretable metadata. As will be shown, this provides useful mechanisms for handling complex metadata problems such as rights management of digital objects. The remainder of this paper describes the development and consequences of the DAR model. Section 2 reviews the Warwick Framework, which is the basis for the model described in this paper. Section 3 examines the concept of the Warwick Framework Catalog, which provides a mechanism for expressing the relationships between the packages in a Warwick Framework container. With that background established, section 4 generalizes the Warwick Framework by removing the restriction that it only contains "metadata". This allows us to consider digital library objects that are aggregations of (possibly distributed) data sets, with the relationships between the data sets expressed using a Warwick Framework Catalog. Section 5 further extends the model by describing Distributed Active Relationships (DARs). DARs are the explicit relationships that have the potential to be executable, as alluded to earlier. Finally, section 6 describes two possible implementations of these concepts.

Type

a

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Abstract

For several years the Digital Library Research Group at Cornell University and the Corporation for National Research Initiatives (CNRI) have been engaged in research focused on the design and development of infrastructures for open architecture, confederated digital libraries. The goal of this effort is to achieve interoperability and extensibility of digital library systems through the definition of key digital library services and their open interfaces, allowing flexible interaction of existing services and augmentation of the infrastructure with new services. Some aspects of this research have included the development and deployment of the Dienst software, the Handle System®, and the architecture of digital objects and repositories. In this paper, we describe the joint effort by Cornell and CNRI to prototype a rich and deployable architecture for interoperable digital objects and repositories. This effort has challenged us to move theories of interoperability closer to practice. The Cornell/CNRI collaboration builds on two existing projects focusing on the development of interoperable digital libraries. Details relating to the technology of these projects are described elsewhere. Both projects were strongly influenced by the fundamental abstractions of repositories and digital objects as articulated by Kahn and Wilensky in A Framework for Distributed Digital Object Services. Furthermore, both programs were influenced by the container architecture described in the Warwick Framework, and by the notions of distributed dynamic objects presented by Lagoze and Daniel in their Distributed Active Relationship work. With these common roots, one would expect that the CNRI and Cornell repositories would be at least theoretically interoperable. However, the actual test would be the extent to which our independently developed repositories were practically interoperable. This paper focuses on the definition of interoperability in the joint Cornell/CNRI work and the set of experiments conducted to formally test it. Our motivation for this work is the eventual deployment of formally tested reference implementations of the repository architecture for experimentation and development by fellow digital library researchers. In Section 2, we summarize the digital object and repository approach that was the focus of our interoperability experiments. In Section 3, we describe the set of experiments that progressively tested interoperability at increasing levels of functionality. In Section 4, we discuss general conclusions, and in Section 5, we give a preview of our future work, including our plans to evolve our experimentation to the point of defining a set of formal metrics for measuring interoperability for repositories and digital objects. This is still a work in progress that is expected to undergo additional refinements during its development.

Type

a