Digital libraries and archives stand to benefit greatly from the Semantic Web, which may provide a basis for novel end-user functions targeted at research and teaching. The project “Image Preservation, Information Systems, Access and Research” seeks to develop an adaptable digital library application based on a back-end of semantically modeled data. By “adaptable” we mean able to adapt to diverse library and archive scenarios, especially involving the integration of different types of material (photographic prints, negatives, drawings, periodicals, books, etc.) in a single system. This requires “mixing and matching” standard and non-standard catalogue record formats and ontologies to model them. A central problem we have encountered is: how to structure application logic in this context in a way that follows best-practice principles. In this paper we discuss the problem and propose, as a tentative solution, a Semantic Component Architecture, which would provide an integrated, encapsulating way of selecting vocabularies and establishing inference rules, recurrent path patterns, graph constraints, catalogue record templates and arbitrary logic. We also review the related issue of encapsulation of low-level graph structures.
Digital libraries and archives stand to benefit greatly from the Semantic Web (SW). Semantically modeled catalogues should provide a basis for new functions to help users sift through large and diverse repositories, discover patterns, explore associations among objects, find relevant information, and create and share descriptions of objects in a structured, flexible manner. This is the promise the SW holds for knowledge repositories, and one can hardly underestimate its potential impact in History and other Social Sciences: archives are primary sources— essential deposits of partially processed information, used for research in these disciplines—and despite the high degree of interrelation among data in different archives, catalogues are often isolated and employ divergent record formats that are hard to align using standard information technology.1
This issue is one of the reasons the project Image Preservation, Information Systems, Access and Research (IPISAR) set out to build a SW-based digital library application. The project investigates the dissemination, study and management of heritage resources, and attempts to provide solutions to common problems in these areas.
The application being built, called “Pescador”, will store catalogue data in a persistent triple store (whose function will be similar to that of a relational database in traditional systems). The requirements for the application include the ability to integrate data in various catalogue formats and adapt to the cataloguing needs of diverse archives. In this paper, the terms “catalogue format” and “record format” refer to the selection, organization and meaning of fields used to describe objects in an archive or library catalogue, as well as other conventions related to catalogue creation. Since Pescador will use the SW to model catalogues, each record format will correspond to a distinct kind of graph structure, often requiring specialized vocabulary and rules, and related to specialized application logic.
The application will have three main types of users: (1) regular users (or
“patrons”) who will consult the material provided by the digital library, (2)
cataloguers, who will provide and manage the library’s materials and metadata, and
(3) catalogue designers/modelers/programmers, who will select or create the
catalogue record formats and corresponding ontologies, and adapt the system to the
needs of a given scenario. Pescador will provide a Web interface for the first two
kinds of users; on this level, numerous functions targeted at research, teaching
and cataloguing are planned [
The third type of user, the catalogue designer/modeler/programmer, will use
a programming interface. The problem considered in this paper is: how to design
an interface that allows users of this type to adapt the application to their needs,
in a way that follows best-practice information engineering principles such as the
encapsulation of complexity, the separation of concerns and the non-repetition
of declarations. To achieve this we propose a Semantic Component Architecture
(SCA), that is, an adaptation of component architecture principles to a SW
application context, in which data structure rules and application logic are closely
linked. In addition, we propose a mechanism for encapsulating low-level graph
structures, which should also help catalogue designers/modelers/programmers
follow best-practice principles.
1 The situation of historical archives varies greatly from one archive to another. Other
recurring difficulties include access restrictions and insufficient funding; the first of
these is also a major focus of the project described in this article. See [
To date, two incomplete versions Pescador have been created. Both are
currently used for Web sites that offer simple consultation functions for on-line
archives (available at [
All versions of Pescador are provided under the terms of the free GNU GPL license. 2
Since before work on Pescador began, it was clear that to integrate data in
various formats and adapt to diverse scenarios, the system would have to offer ways
of selecting vocabulary, establishing graph constraints2 and configuring the
algorithms used to transform information as it passed between the model and the UI.
In retrospect, we can say that we have consistently underestimated the
complexity of the mechanisms required for these and related processes. The first version
of Pescador (version 0.1) allowed catalogue designers/modelers/programmers to
select RDF schemas and set presentation information, which was modeled using
an augmented version of the Fresnel Display Vocabulary [
The SCA we envision would coordinate pluggable “components” or “bundles” 3
that would wrap interrelated elements of any of the following types: schemas,
constraints, inference rules, ontologies, path definitions, executable code,
display specifications, Abox configuration information, and links to external data
sources. As in other component frameworks, bundles would be able to provide
hooks of various kinds and use the hooks exposed by other bundles. These hooks
would be the bundle’s means of controlling and simplifying access to the elements
it wraps. A bundle could also “depend on” other bundles, and would thereby know
which external hooks are available to it. The standard definition of a component
as “a software artifact consisting of three parts: a service interface, a client
interface and an implementation”[
An SCA would help solve issues of encapsulation and re-usability. Consider, for example, an archive that includes books, photographic prints, negatives and handwritten manuscripts. Catalogue record formats for each type of object would certainly vary, though they would also have elements in common. For example, all objects would probably be associated with an author and a creation date. But only the negatives, photographic prints and books could be associated in a chain of image reproductions (as in, a negative could be reproduced on a print, and the print could be touched up and cropped, then reproduced in a book). Many objects might have a place of creation, but the precise meaning of this concept would not be the same for a book (place of publication) and for a negative (place the photo was taken). Of course, the SW excels in the alignment of data structures like these. However, setting up a digital archive to integrate records of such objects involves much more than establishing data structures and their intersections; bits of logic have to be defined, and it makes sense to define them alongside related data structure information in an encapsulated way—in other words, to “keep together what goes together”.
Thus, in an SCA, the logic needed to transform the SW model of a book’s metadata into a standard bibliographic citation might be stored in the same bundle as the vocabulary, graph constraints, and remaining display information for books. Similarly, a component could be created for chains of image reproductions (like the one described above). The component could enclose vocabulary, constraints, path definitions, and code for generating visual representations of these chains. Other components—such as those for books and photographs— could depend on it, interfacing with the hooks provided and thus employing in a simplified manner the enclosed complexity. 3 Herein the terms “bundle” and “component” are used as synonyms.
The advantages of component architectures, and of the modularity and reusability they offer, have long been recognized, and countless frameworks and related technologies are used in applications of all shapes and sizes.4 Component architectures are increasingly viewed as a programming paradigm in their own right.
Our experience shows that a SW-based system that relies, as Pescador 0.1 did, on partially overlapping RDF data sets coupled with fragments of related logic scattered throughout the application, ends up grossly violating best-practice principles in many respects, to the detriment of flexibility and scalability. An SCA would address these problems in a thorough manner, and provide a basis for the re-use of ontologies coupled with application logic. 5
In both versions of Pescador, the algorithms that extract information from the
graph frequently traverse paths from one resource to another. These paths vary
greatly in length and complexity. To allow for encapsulation and separation of
concerns as proposed, an SCA must include a means of defining path patterns
that supports these principles. (We first ran into path-related encapsulation
issues in version 0.1 of Pescador, which used SPARQL to extract information from
the graph. In that version, we found ourselves writing SPARQL queries that
repeated information already set out in several other parts of the application—
quite the opposite of a maintainable application structure.) The solution we
suggest is to adapt an existing path definition mechanism (of which there are
many [
Let us illustrate this proposal with reference to the hypothetical archive described in 4 Justification. Consider: one bundle might define paths from any photographic image to its original negative, along a chain of image reproductions; another might define paths from cities to the continents on which they are located. In such a scenario, it should be possible, in a third bundle, to aggregate paths and define a route from any photographic positive to the continent on which the negative that created the photo was snapped. Fig. 1 presents this example.
A preliminary survey of existing path definition mechanisms indicates that
the SPARQLeR extension to SPARQL may be the best candidate for adaptation
to our SCA and Pescador. The authors of SPARQLeR, working in the life
sciences domain, found they needed a means of detecting “semantic associations”—
undirected paths “that connect two entities in the knowledge base using named
relationships” [
This section describes a mechanism called “the SW graph abstraction layer”, which seeks to apply, in low-level graph operations, the same principles that underlie the SCA. The abstraction layer will offer simplified access to and correct processing of very basic graph structures that occur repeatedly in a wide variety of models that we have worked with. It will be integrated directly with the Pescador’s triple store; graph extraction and modification operations will access it in a transparent manner. The abstraction layer will be logically above a SWconformant graph. Both the abstraction layer and the SW-conformant graph will represent exactly the same information; each will simply provide a different view of that information. For interchange with other SW applications, the Pescador will be able to export the SW-conformant view of the contents of its triple store in standard SW serialization formats (like RDF/XML).
Below are some examples of simple graph structures as seen by the abstraction layer and as reflected in the underlying SW specifications-compliant graph. The first two examples are presented in detail and the rest are summarized.
Note that version 0.2 of Pescador already implements many of the features described here. Unlike the SCA, the abstraction layer has demonstrated its usefulness in a functioning implementation, and can be considered a relatively mature proposal. Nonetheless, substantial work remains to be done to develop a theoretical grounding for this solution, analyze its implications in formal terms, and design a new implementation.
Multilingual Values A common idiom in SW graphs is the multilingual value: conceptually a single value with alternate versions for different languages. It is often modeled with a blank node that is an instance of rdf:Alt. The problem we have encountered is the recurring need to select an appropriate version of a multilingual value on the basis of information available at runtime. Fig. 2 shows the abstraction layer’s view of this idiom, together with the underlying standards-compliant model.
This type of structure is created only when specifically requested for a given property and a given subject resource. The query system takes the abstraction layer view of the model; queries are executed in a given language context, and only one of the “switchable” triples is available during a given operation. This makes for cleaner path definitions, which need only specify the main property to be traversed (in this example, sys:name). Without this feature, the sub-pattern for selecting the correct language alternative would have to be repeated in many path definitions.
Ordered Multiple Values In catalogue records, it is often necessary to present several values of the same field in a specific order—for examples, the authors of a publication must be listed in the same order in which they appear on the publication itself. Similarly, if several topics from a thesaurus are associated with a given entity, the most relevant topics are listed first. These fixed orders must be reflected in the model. Since in RDF multiple, identical properties on a single subject are unordered, when a fixed order is needed, multiple values may be grouped using an rdf:Seq. Fig. 3 shows how the abstraction layer understands this low-level structure.
As in the case of multilingual values, this structure is only created when it is requested for a given property and a given subject. The query system traverses from the main subject (photo2) directly to the objects proper (topic21, topic42 and topic25) and extracts them in the correct order. Again, no special sub-pattern need be repeatedly included in the path definitions that traverse this type of structure.
Missing Value Explanations It is frequently useful to include in a catalogue an explanation for a value’s absence (common reasons are “unknown” or “field does not apply”). The abstraction layer provides a special mechanism for modeling such explanations. When a path query does not produce any results, it may expose a pertinent missing value explanation, if one is expressed in the model.
N-ary Relations As explained in [
Structured Values The RDF specification establishes an idiom called a “structured value”, which consists of a blank node that is at once the object of a property and the subject of additional triples used to model the aforementioned property’s value. The abstraction layer builds on this concept by offering a mechanism for explicitly identifying nodes that are structured values, thus allowing the implementation of special features for constructs of this type. 7
Among the numerous issues facing Pescador development, several relate to the SCA and the system’s adaptability, including: Model generation, concurrency and validation To develop a Pescador Web interface for creating, deleting and modifying catalogue items requires the study of algorithms for changing the model in a multi-user environment. Input validation logic and graph constraint checking must also be considered and integrated with the SCA. Our intention to offer an “undo” mechanism further complicates the matter.
Inference rules In many situations it is desirable to establish custom inference rules for a model. Yet again, the details of how to do this must be established, and whatever solution we choose must integrate with the SCA.
Networking A proposed feature of the system is to allow meta-searches in the repositories of multiple Pescador Web servers linked over a peer-to-peer network. To make this a reality we must study, among other things, model alignment and the sharing of SCA bundles over a network. 8
In this paper we have considered some issues encountered in the development of an adaptable SW-based digital library application. Specifically, the issues discussed relate to modularization, the structuring of application logic and a programming interface for catalogue designers/modelers/programmers that allows the use of best-practice information engineering principles. We have also outlined solutions to these difficulties. Although the implementation and detailed design of these solutions is far from complete, our proposals as they stand establish research directions and provide starting points for further work towards the creation of the application we envisage.