The distributed nature of digital libraries presents both an opportunity and a challenge. The ability to compose, aggregate, and transform content from distributed heterogeneous sources makes it possible to present users with new and customized resources. However, providing access to these new resources, while maintaining the integrity of a traditional library environment, is often problematic. With the distribution of content, services and users, institutions face a formidable task in providing reliable service, protecting the privacy of users and rights of content providers, and ensuring the long-term preservation of digital content that may not be in their direct control.
The issue of integrity in digital libraries is the focus of our research at Cornell University in Project Prism, a part of Digital Libraries Initiative Phase 2. Prism is a four-year collaboration between the Cornell University Library, the Human-Computer Interaction Group, the Cornell Digital Library Research Group, and the Library of Congress to investigate the organizational and technical aspects of maintaining integrity when content and services are distributed.
Our initial experiments in Prism build on existing work in new architectures for digital objects and repositories. The FEDORA digital object architecture  permits the aggregation of heterogeneous content and the association of extensible behaviors with those aggregations. We have shown that this architecture is an effective tool for providing uniform access to a mixture of objects distributed among multiple repositories . Recently, we have been investigating the integration of policy enforcement into the FEDORA architecture, with applications to both access management and preservation .
Our research in providing integrity for distributed information environments has demonstrated a new application of the Fedora architecture. We originally conceived of Fedora as a tool for creating containers of content and associated metadata, in the manner of the Warwick Framework . Recently, we have begun to use Fedora as a vehicle for creating surrogates for distributed content. These Value-Added Surrogates can be used to enhance the functionality of the content and provide a framework for implementing integrity-enforcing mechanisms. Services such as data integration, access control, and preservation monitoring can be implemented in a uniform and interoperable manner, even though underlying data sources may be heterogeneous and highly distributed. By creating repositories of Value-Added Surrogates, institutions can create a virtual control zone to implement curatorial responsibilities over resources not in their direct control. In effect, the establishment of a virtual control zone allows institutions to manage selected Internet resources in a manner that approximates the care they can provide for resources contained within their physical boundaries.
From a technical perspective, the V-A Surrogate is a type of mediator, as described by Wiederhold . According to Wiederhold, mediators reside between raw data sources and end-user applications where they integrate and transform data to fit higher-level abstractions in support of real-world problem solving. Along the same lines, Paepke  discusses mediation for its role in facilitating interoperability for digital libraries. Mediation technologies such as "wrappers" and "proxies" can provide clients the "illusion of a highly integrated system" while underlying system components can maintain a high degree of autonomy in their implementation. Mediation has also been highlighted as a technique for converting and integrating semi-structured data on the Web . In the object-oriented realm, there is a design pattern known as a mediator that encapsulates and coordinates the interactions of multiple objects .
The Value-Added Surrogate extends the notion of a mediator beyond its typical roles -- e.g., data conversion, exchange, integration. Value-added surrogates can fulfill these functions, but also fulfill unique usage and integrity requirements of distributed content objects in digital libraries. Like mediators, V-A surrogates encapsulate service requests or queries upon one or more data sources. The V-A surrogate does not actually store the content, but obtains it as needed from the external source. The key advantage of the V-A Surrogate, however, is that it is able to provide specialized services not inherent in the original data source. In our research, we are particularly interested in using these augmented services to facilitate integrity in digital libraries.
In this paper, we describe our experiments with different types of V-A Surrogates. First, we introduce the notion of a virtual control zone for digital libraries and how V-A Surrogates can be used to define the boundaries and types of control for distributed objects. Then we describe our implementation of V-A Surrogates as Digital Objects using our Fedora architecture. In section 4, we describe the different roles that V-A Surrogates can play: the Booster, the Guard, the Caretaker, and the Auditor to address the basic challenges of distributed digital libraries. In section 5, we describe our experiments with V-A Surrogates for access control, reference linking, and preservation.
2. Creating a Virtual Control Zone with V-A Surrogates
Ross Atkinson  describes the notion of the control zone to characterize the manner in which libraries define boundaries of responsibility. According to Atkinson, libraries have traditionally defined the control zone via the principle of physical containment -- custodial obligations are assumed for those objects within the walls of the institution.
The Internet has increasingly challenged this well-defined boundary. Wishing to facilitate access to rich Internet content, libraries have often responded by cataloging selected networked content. The creation and maintenance of these catalog surrogates effectively extends the library collection outside its traditional boundaries. Rather than acting solely as a container for content, the library becomes a portal to content controlled and managed by external organizations.
Although the cataloging surrogates are instrumental in extending the access zone of the library, they fail to extend or redefine the control zone. Simply cataloging Web resources essentially forsakes the integrity role that is so important to libraries. Our work with Value-Added surrogates is guided by the principle that a more powerful mechanism is necessary to create a virtual control zone. By controlling a repository of Value-Added Surrogates that map to external resources, an institution effectively defines the boundaries of its virtual control zone. The functionality of each surrogate provides a means of defining and implementing the level of responsibility that the institution assumes for each external resource. V-A Surrogates can be deployed with varying levels of granularity: they can represent individual resources, selected groups of resources, or external collections as a whole.
The functionality of surrogates and of the virtual control zone is highly correlated with the levels of cooperation that exist among institutions. As with many digital library solutions, automated mechanisms for asserting control in a distributed environment co-exist with human and organizational realities. As a result, V-A Surrogates may implement well-defined responsibilities shared between trusted partners, or they may represent best-effort attempts of an organization to assert some degree of influence or monitoring over external resources that are completely out of their physical control. This section describes a number of scenarios along this control and cooperation spectrum.
Cooperative agreements have historically enhanced the functionality of individual libraries. For example, agreement among libraries on cataloging standards and MARC encoding has facilitated widely practiced sharing of cataloging records. Initiatives such as the Committee on Institutional Cooperation (CIC) have made it possible for libraries to extend their catalogs beyond institutional boundaries. Recently, with the availability of an increasing number of digital resources, libraries have entered into new types of agreements for distributing responsibility for these resources. For example, an agreement between the Library of Congress and Bell and Howell Information and Learning Company (formally UMI) effectively cedes some of the Library's custodial responsibility for dissertations to Bell and Howell while stating a fail-safe provision to ensure their long-term preservation.
Value-Added surrogates enable and empower such cooperative relationships by creating a mechanism for uniformly automating provisions of such agreements. Some examples of the use of V-A surrogates for such cooperative arrangements are as follows.
Mitigation of Vulnerability
While Value-Added Surrogates can implement functionality defined through cooperative relationships, such cooperation is sometimes impractical or unavailable. In such cases, a V-A Surrogate will be limited in the level of control and integrity that it can enforce. Nevertheless, we can imagine some useful roles for V-A surrogates in such a context.
3. Digital Object Architecture for V-A Surrogates
V-A Surrogates require an architecture that supports (1) aggregation of distributed content accessed via a variety of protocols, (2) association of rich and extensible behaviors with that content, (3) modular mechanisms to provide mediation capabilities, and (4) an open interface to promote uniformity and interoperability. Although these requirements can be implemented in a number of ways, we have used our Fedora implementation as a basis for experimentation. Several years ago, we designed Fedora to enable Digital Objects and Repositories to meet the above requirements. Since then, we have implemented this model using object-oriented technology, and have demonstrated its flexibility and interoperability through collaborations with the Corporation for National Research Initiatives (CNRI) and others . The digital library research and development group at the University of Virginia has also implemented the Fedora model using a relational schema and web technologies. Virginia has created a testbed containing over 30,000 Digital Objects that aggregate metadata and content from several pre-existing EAD and image collections.
A brief review of the Fedora model will illuminate our V-A Surrogate discussion. Fedora is a modular architecture for Digital Objects, built on the principle that interoperability and extensibility is best achieved by the clean separation of data, interfaces, and mechanisms. A Fedora Repository provides a general-purpose management layer for Digital Objects. In their simplest form, Digital Objects are containers that aggregate mime-typed streams of data (e.g., digital images, XML files, metadata), known as DataStreams. Clients can interact with Fedora Digital Objects through a set of generic methods, collectively known as the Primitive Disseminator. This provides a well-defined and open interface for all Digital Objects.
In addition to behaving in a generic manner, Digital Objects can provide content-specific functionality. For example, a natural behavior for a book would be "Get Table of Contents." Fedora allows the association of rich and extensible behaviors with Digital Objects by "plugging in" generic components known as Typed Disseminators. Each Disseminator aggregates references to: (1) a formally defined behavior interface that specifies a set of methods for running a particular type of digital library service (e.g., Lecture Browser interface, Image preservation interface), and (2) an executable mechanism that runs these methods. These interfaces and mechanisms are themselves disseminated by Digital Objects, laying the foundation for unlimited and persistent extensibility of the architecture. A major strength of the Fedora extensibility model is that clients can use the generic interface of a Digital Object to discover and invoke type-specific methods defined in Typed Disseminators. The Digital Object facilitates the invocation of these extended methods, returning customized disseminations of content to the client.
Figure 1 shows our use of Fedora Digital objects as V-A Surrogates. In this application, DataStreams are actually encapsulated service requests upon external sources. The requests can be upon other Fedora Digital Objects, or upon other data sources (e.g., web servers, databases, online catalogs). In Fedora, these encapsulated requests are known as ReferenceDataStreams. The referenced content can be manipulated, integrated, secured, or augmented by associating a Typed Disseminator with the container. Within the Disseminator are references to shared specifications and mechanisms to perform the mediation work. All communication with the V-A Surrogate is through the generic interface. In section 5, we will describe specific examples of V-A surrogates implemented in this manner.
Collections of V-A Surrogate objects are stored in Fedora Repositories. A Repository provides a management layer for Digital Objects. Repositories should be implemented as secured servers, so that they can enter into trusted relationships with source content servers. Figure 2 shows a collection of V-A Surrogates, each playing a different role. These roles -- the Booster, Guard, Caretaker, and Auditor -- represent the different functions that V-A Surrogates can fulfill. These functional abstractions are described in the next section.
4. Roles of Value-Added Surrogates
Value-Added Surrogates can address different functions, including behavior augmentation, access control, preservation, and auditing. We have created a set of abstractions that essentialize the roles surrogates can play in adding value to distributed content. These roles are not exhaustive, nor are they mutually exclusive. Any particular V-A Surrogate could exhibit aspects of one or more roles. Nevertheless, these basic abstractions help clarify the purpose(s) of particular surrogates in digital library implementations.
Booster (behavior-augmented surrogate)
A Booster surrogate is used to add or extend behavior for content aggregations. Also known as a behavior-augmented surrogate, it "boosts" the functionality of the base content to which the surrogate refers. Booster surrogates can be used to deliver new presentations of existing content, to provide integrated views of formerly unrelated content, or to create complex objects with new capabilities. Although all types of V-A Surrogates augment functionality in some manner, the Booster abstraction is used to distinguish functionality that improves access-oriented behaviors of content. As such, this new functionality will provide visible results to end-users.
A simple example of augmenting the behavior of content is providing on-the-fly conversion of images that are aggregated to form a journal article. A more complex example is a Booster surrogate that delivers multimedia views of lecture content. The basic underlying content of the lecture -- a digitized video and a set of related slides -- can be brought to life by adding behaviors to provide synchronized disseminations of the two content sources (e.g., the right slides with the right video segments), and ability to locate relevant segments using keyword searching. In section 5, we describe Booster surrogates that apply dynamic reference linking behavior to documents from multiple sources.
Guard (policy-augmented surrogate)
Another variation of the V-A Surrogate is the Guard, also known as a policy-augmented surrogate. Guard surrogates enable the enforcement of fine-grained access control policies customized to meet content-specific requirements. Like all V-A Surrogates, the Guard aggregates references to distributed content and associates a set of behaviors with the aggregation. In addition, it associates a security policy and an enforcement mechanism with the content to restrict access via the defined object behaviors.
Access control mechanisms in digital libraries must be scalable, flexible, and extensible accommodating a wide range of objects and usage scenarios. Standard operating system based access controls do not provide the basis for highly expressive policy enforcement because they support a relatively fixed set of abstractions (e.g., files, nodes) and actions (e.g., reading, writing, etc.).
Our earlier example of a license agreement between a library and a 3rd party information provider offers an example where traditional approaches to access control may be insufficient. In such arrangements, libraries are permitted to provide access to a remote resource via their digital library portals, given that these portals are equipped to restrict access to the licensed resource by unauthorized users. A typical means of fulfilling this requirement is IP checking, which essentially approximates the identity of users based on incoming network addresses. This is a rather coarse policy that does not easily scale to handle other scenarios pertinent to particular applications and complex objects. For example, it is difficult to enforce policies that require a distinction between students and faculty, or policies that restrict access to sub-components of a source (e.g., table of contents is available to anyone, but full-text is restricted).
V-A Surrogates can be used to enforce access control policies that are fine-grained and tailored to the customized functionality of complex objects -- in a uniform and modular manner. In section 5, we describe how V-A Surrogates implemented in the Guard role can be used to enforce more expressive policies over distributed content, while also promoting architectural uniformity and interoperability.
Caretaker (preservation-augmented surrogate)
The Caretaker is a preservation-augmented surrogate that can contribute to the long- term management and care of distributed content. The Caretaker abstraction is designed to support the notion that objects can participate in their own preservation. V-A Surrogates in this role can take on responsibilities such as (1) disseminating information relevant to preserving content, (2) detecting undesirable conditions that could affect the longevity of content, or (3) initiating action to protect content.
For example, a Caretaker can perform diagnostic tests on distributed content to assess its health. At a minimum, the surrogate can disseminate metadata about the state of the distributed content to which it refers. More significantly, a Caretaker can take action to protect its underlying content. Depending on the level of trust and cooperation that exists between a V-A Surrogate repository and an external source, a Caretaker can initiate migration or conversion tasks upon source content.
A Caretaker surrogate can work independently, or it can collaborate with system-wide services that manage large distributed collections. For example, in our Prism Project we are designing a preservation service that interacts with Caretaker surrogates in two ways. First, it uses them to collect data about the state of select resources. Then it uses them as mechanisms for initiating preservation actions such as creating mirrors or format migration.
Auditor (inspection-augmented surrogate)
A close relative to the Guard and the Caretaker, the Auditor is a V-A Surrogate whose purpose is to apply a degree of scrutiny over content that is not in one's direct control. While the Guard and Caretaker assume an active role in security and preservation of content, the Auditor is more passive in its duties. The Auditor abstraction acknowledges the reality that our control over distributed content may be quite limited, or our trust in source providers may not be strong. When all else fails, we can at least monitor resources and track events that may be leading indicators of potential problems, or indicative of new trends (i.e., in usage, performance, availability).
The Auditor is the least mechanically robust of the V-A Surrogates. Its main role is to alert us to problems or trends, and to help in decision making. These V-A Surrogates can be used to gather the requisite information to prompt human intervention. Auditors can monitor resources in many ways, such as tracking the frequency of "404" errors on web resources, testing the volatility of resources by comparing periodic checksums of data, or logging usage statistics. The information collected can be the impetus for action, the creation of new agreements, the definition of new policies, or the institution of new practices.
5. Selected Experiments
V-A Surrogates for Security and Access Control
We have implemented the Guard abstraction to apply access controls to distributed content in our Prism testbed. Using Fedora, we have created V-A Surrogates that enforce security policies. Our test source is the Cornell Computer Science Lecture Archive, which stores multi-media representations of courses taught by the department. Content includes digitized videos, slides, and XML-encoded metadata.
A typical way to implement a guard surrogate is to represent a delegation of responsibility pattern between trusted partners. For this scheme to work in a secure manner, a source provider must grant a V-A Surrogate repository access rights to its content but prevent direct access by other users and applications. The source trusts that the surrogate repository will implement access control according to agreed upon terms. The surrogate repository expects that the source is secure so that users can't sneak in a back door and avert all access controls. A benefit of this approach is that institutions can manage surrogate repositories to tailor policies to its own requirements. Different institutions can apply different policies to the same shared, distributed content. Also, an institution can create fine-grained policies that define different access conditions for different parts of a source.
Figure 3 depicts a V-A Surrogate for a lecture. There are multiple DataStreams in the surrogate that reference particular pieces of external content from the lecture archive: a low-resolution video of a lecture (Video-L); a high-resolution version of the video (Video-H); a set of accompanying slides (Slide-1, Slide-2, etc.); and an XML file that encodes both synchronization and descriptive metadata. Although it is not apparent in the diagram, the Disseminator (labeled Guarded Lecture Mechanism) not only references a secured mechanism, but also a formal definition of access methods for this type of object. As suggested by the watchdogs in the diagram, the V-A Surrogate acts as a Guard, overseeing the execution of all methods that can be invoked to gain access to underlying content.
What we have achieved with these V-A Surrogates is the ability to enforce fine-grained policies that are not easily accommodated by more general-purpose access control schemes that treat resources in a non-specific manner. Each V-A Surrogate stores or references an encoded policy that is customized to the particular requirements of the object. If a policy happens to be applicable to all lectures, it can be stored centrally and applied by default to all lecture surrogates. In our example, a policy is stored as Datastream in the surrogate object. It specifies that, for this lecture, the high-resolution video is accessible only by students, but that other users can view the low-resolution video for a fee. Furthermore, some of the slides are proprietary, so the policy states that only students can view these (e.g., slide #21 to #25). Any user can view the other slides.
From a technical perspective, our Guard functionality is implemented in the form of In-line Reference Monitors (IRMs) using Cornell's PoET security software [3, 4]. A discussion of the application of IRM technology to Fedora Digital Objects, and the implications for digital library security, can be found elsewhere . Briefly, PoET is a toolkit that provides a policy definition language and a policy enforcement mechanism based on Schneider's security automata theory . In PoET, Java applications are converted to secured applications by a code rewriter that embeds checks into Java Machine Language programs (bytecode). When the programs run, the policy is enforced. An easy way to understand this is to imagine PoET "baking" program executables so that policies becomes ingrained within them. Using this technology, we dynamically and securely apply access control policies to V-A Surrogate mechanisms at run time.
V-A Surrogates for Reference Linking
In the The Open Citation Project  we are experimenting with an application of the Booster abstraction to facilitate linking among distributed objects. In order to interlink heterogeneous distributed archives we have been exploring the use of link-friendly surrogates. As shown in Figure 4, these Value-Added Surrogates point to both an original document, available from an external document server, and a CiteRef database that is typically constructed via prior analysis of a collection of documents. The V-A Surrogate adds reference-linking functionality to the source document (e.g., a report from the Los Alamos arXiv repository) via a mechanism that integrates documents and metadata.
This work is built on the architectural presumption that document inter-linking has distinctly separate data and consumption layers. In the data layer, documents should supply structured information about references (outbound links) and citations (inbound links). Our work with V-A Surrogates has been in defining and implementing the architecture of the data layer. This first involves the definition of a linking API for documents, which includes the following service requests:
These service requests will return relevant data in an XML format. Information disseminated from the surrogates (i.e., data layer) can be consumed in a variety of ways. Within the consumption layer of the linking architecture, we have defined two general classes of clients: presentation clients and analysis clients. Presentation clients render the linking data into human usable forms including simple presentation of references as hyperlinks, displays of metadata about a referenced document, or even vocalization of link information for sight-impaired users. Analysis clients consume linking information from a set of documents for varying types of statistical analysis.
Maintenance of V-A Surrogates to provide the functionality of the data layer of the reference-linking scheme provides a number of benefits, including:
V-A Surrogates for Preservation
We are in the early phases of our research of V-A Surrogates that bring preservation-oriented functionality to digital content. The motivation for this work is the belief that digital objects should be able to participate in their own preservation. We are not proposing self-preserving objects; instead we are designing preservation-augmented surrogates that will be participants in a broader preservation service architecture.
Figure 5 depicts our preliminary design for the Caretaker surrogate and its intended interaction points with a preservation service node. Initially, these surrogates will provide information about the status of the its underlying digital content. Ultimately they will be capable of multiple functions, including the preparation of customized views of content change history and the issuing of problem alerts.
Figure 5: V-A Surrogate (Caretaker) helps images participate in their preservation
As part of our Prism research, we are investigating metadata models that are capable of expressing significant events in the life course of a digital object. This metadata will form the basis of monitoring functionality for V-A Surrogates, and the broader preservation service. V-A Surrogates will disseminate views of this metadata (XML-encoded) for a preservation service to interpret. Additionally, some V-A Surrogates will be equipped with mechanisms to provide direct interpretations of this event metadata. As part of our research, we are examining conditions under which this analysis functionality should reside in surrogates vs. external services that leverage surrogates.
Ultimately, automated interpretations of event metadata will yield preservation action. This could include anything from sending a report to assist in human decision making, copying content to a caching server, converting content to new formats, or moving content to a certified archive. We intend to explore the notion of preservation policy enforcement by extending the work we have done in security and access control. Using similar notions of policy and mechanism, we will experiment with mechanisms to enable surrogates to detect unacceptable or risky transitions in underlying content, with the goal of initiating a preservation response -- either automated action or alerts with information to promote human intervention.
We have designed a form of digital object known as the Value-Added Surrogate that can enhance the functionality of digital content that is not in one's direct control. The Fedora Digital Object model is the basis for the V-A Surrogate design. Initially, Fedora was designed to aggregate distributed content and associate a set of behaviors with that content. The flexibility of the Fedora model has enabled its use as a mediation architecture for improving access and integrity for distributed content. We have discussed four types of V-A Surrogates for augmenting content functionality: (1) the Booster for adding new access-oriented behaviors, (2) the Guard for applying access control, (3) the Caretaker for helping to preserve content, and (4) the Auditor for implementing monitoring capabilities. A major benefit of the V-A Surrogate architecture is that it allows for significant flexibility in adding functionality to content, while also providing architectural uniformity.
An institution can create repositories of V-A Surrogates to create a virtual control zone for valuable content that is not in its direct control. Unlike other surrogates, such as catalog records for internet resources, the V-A Surrogate lets institutions implement specific usage and integrity requirements upon the referenced content. Although the institution may not directly manage the source content, the V-A Surrogates for that content are under its direct stewardship.
Institutions can increase the power of their V-A Surrogates through cooperative agreements with source content providers. In such agreements, providers may delegate or share responsibilities. By establishing a trusted relationship between a source server and a V-A repository, the source provider can grant institutions the right to develop customized views of data, apply context-specific access controls, or even take other action to ensure integrity. Even when such agreements are not possible, an institution can still implement V-A Surrogates to apply a limited amount of customization to content, including monitoring and inspection of content.
We acknowledge the following people for their contributions: Naomi Dushay for her development work on the Fedora reference implementation; Donna Bergmark for surrogate designs and an API for reference linking; Anne Kenney and Oya Rieger for their intellectual contributions to the digital preservation aspects of this work; Fred Schneider for security automata theory and PoET software; Ulfar Erlingsson for PoET software; Sugata Mukhopadhyay and Jill Newman for work on the Lecture archive experiments. We would also like to thank Christophe Blanchi and Edward Overly (CNRI) for their collaboration in developing interoperable architectures for digital objects and repositories. Also, Thorny Staples and Ross Wayland (University of Virginia) for evaluating Fedora in the context of their library collections, and for an alternative technical implementation of the Fedora design. Work described here was supported by NSF Grant No. IIS-9817416 and NSF Grant# IIS-9907892.
 S. Abiteboul, P. Buneman, and D. Suciu, Data on the web: from relations to semistructured data and XML. San Francisco: Morgan Kaufmann, 2000.
 R. Atkinson, "Library Functions, Scholarly Communication, and the Foundation of the Digital Library: Laying Claim to the Control Zone," The Library Quarterly (July), 1996.
 U. Erlingsson and F. B. Schneider, "IRM Enforcement of Java Stack Inspection," Computer Science Technical Report TR2000-1786, http://cs-tr.cs.cornell.edu:80/Dienst/UI/1.0/Display/ncstrl.cornell/TR2000-1786, February 19 2000.
 U. Erlingsson and F. B. Schneider, "SASI Enforcement of Security Policies: A Retrospective," Cornell University, Computer Science Technical Report TR99-1758, http://cs-tr.cs.cornell.edu:80/Dienst/UI/1.0/Display/ncstrl.cornell/TR99-1758, July 19 1999.
 E. Gamma, Design patterns: elements of reusable object-oriented software. Reading, Mass.: Addison-Wesley, 1995.
 S. Hitchcock, L. Carr, Z. Jiao, D. Bergmark, W. Hall, C. Lagoze, and S. Harnad, "Developing services for open eprint archives: globalisation, integration and the impact of links," presented at ACM DL2000, San Antonio, 2000.
 C. Lagoze, C. A. Lynch, and R. Daniel, Jr., "The Warwick Framework: A Container Architecture for Aggregating Sets of Metadata," Cornell University Computer Science, Technical Report TR96-1593, http://cs-tr.cs.cornell.edu:80/Dienst/UI/2.0/Describe/ncstrl.cornell/TR96-1593?abstract=, June 1996.
 A. Paepcke, C.-C. Chang, H. Garcia-Molina, and T. Winograd, "Interoperability for Digital Libraries Worldwide," Communications of the ACM, 41 (4), 1998.
 S. Payette, C. Blanchi, C. Lagoze, and E. Overly, "Interoperability for Digital Objects and Repositories: The Cornell/CNRI Experiments," D-Lib Magazine, http://www.dlib.org/dlib/may99/payette/05payette.html, May1999.
 S. Payette and C. Lagoze, "Flexible and Extensible Digital Object and Repository Architecture (FEDORA)," presented at Second European Conference on Research and Advanced Technology for Digital Libraries, Heraklion, Crete, 1998.
 S. Payette and C. Lagoze, "Policy-Enforcing, Policy-Carrying Digital Objects," submitted to Fourth European Conference on Research and Advanced Technology for Digital Libraries, Lisbon, 2000.
 F. B. Schneider, "Enforceable Security Policies," Cornell University, Department of Computer Science, Computer Science Technical Report TR98-1664, http://cs-tr.cs.cornell.edu:80/Dienst/UI/1.0/Display/ncstrl.cornell/TR98-1664, 1998.
 H. Van de Sompel and P. Hochstenbach, "Reference Linking in a Hybrid Library Environment, Part 2: SFX, a Generic Linking Solution," D-Lib Magazine, http://www.dlib.org/dlib/april99/van_de_sompel/04van_de_sompel-pt2.html, April, 1999.
 G. Wiederhold, "Mediators in the Architecture of Future Information Systems," IEEE Computer (March), pp. 38-49, 1992.
Copyright © 2000 Sandra Payette and Carl Lagoze
|Top | Contents
Search | Author Index | Title Index | Monthly Issues
Letters | Next Story
Home | E-mail the Editor
D-Lib Magazine Access Terms and Conditions