DHQ: Digital Humanities Quarterly
2009
Volume 3 Number 3
Volume 3 Number 3
XML, Interoperability and the Social Construction of Markup Languages: The Library Example
Abstract
The past decade has seen XML widely adopted within a variety of communities, including the digital library community. While it now plays a critical role in the infrastructure of many digital library operations, XML's promise of interoperability of data across systems and organizations has not been fully realized within digital libraries. The reasons for this are not primarily technical in origin, but social, and relate to the cultures of XML's designers and XML language implementors, and a failure on the part of the digital library community to grapple with the sociotechnical nature of XML and its implementation. Possible strategies for addressing these issues of interoperability might include reduction of the flexibility afforded by specific XML-based markup languages used by the digital library community, and an increased focus on standardizing translations between various communities of practice use of such markup languages.
Introduction: Failures of Interoperability with XML
Eleven years after its endorsement by the World Wide Web Consortium, XML has been
widely adopted within numerous, disparate communities and in a vast range of
application domains, from standards for electronic filing of federal income tax [Internal Revenue Service 2007] to user interface design [Goodger et al. 2001]. The
digital library community has been an active and early adopter of XML, for use in
structuring both content and metadata. The reasons for this rapid uptake of XML
within the digital library community are familiar to anyone with experience in the
world of markup languages:
- XML helps ensure platform (and perhaps more critically vendor) independence, simplifying the migration of content between systems;
- XML provides the multilingual character support critical to the handling of library materials;
- XML's extensibility and modularity allow libraries to customize its application within their own operating environments;
- XML helps minimize software development costs by allowing libraries to leverage existing, open source development tools;
- XML, through virtue of being an open standard which enables descriptive markup, may assist in the long-term preservation of electronic materials; and perhaps most importantly
- XML provides a technological basis for interoperability of both content and metadata across library systems.
Despite its success, however, XML has not lived up to many librarians' expectations
within one area, that of interoperability. Efforts to exchange information employing
XML-based metadata standards such as Dublin Core have fallen prey to a number of
encoding and semantic inconsistencies [Shreeves et al. 2005]. Variations in
the use of namespacing (<date> vs. <dc:date>), in regional conventions
(08-11-2008 in the U.S. vs. 11-08-2008 in the U.K.) and language and culture (11
Août, 2008 vs. 8 Sha'aban 1429 A.H.) confound application developers trying to
process XML data.
Perhaps more surprising is the failure of XML to ensure interoperability at a
syntactic level.[1] Digital library developers have expected that shared use of a
XML standard for structuring content and metadata (what is commonly called
“structural metadata” within the digital library community) would ensure
content interoperability and promote the development of tools and services designed
to work with content encoded according to that standard [Hurley et al. 1999].
In practice, however, this goal has proved rather elusive. Experiments conducted by
participants in the Library of Congress National Digital Infrastructure for
Preservation Program (NDIIPP) to test the exchange of digital objects between
repositories failed even when participants were using the same XML-based encoding
format [DiLauro et al. 2005], [Shirky 2005].
While some of the failures experienced by the Library of Congress NDIIPP tests were
the result of incompatible repository infrastructure, others resulted from mismatched
expectations regarding the appropriate use of METS, one of the XML formats employed
for the test. DiLauro et al., discussing Johns Hopkins University's experience in the
NDIIPP tests, state,
As alluded to by [DiLauro et al. 2005], a critical difficulty for achieving interoperability using structural metadata standards such as METS is the level of flexibility they enable in structuring a description of an object. As [Nelson et al. 2005] note in their discussion of using the MPEG-21 Digital Item Declaration Language during the NDIIPP test, it is possible to map a single object into multiple different encodings using MPEG-21, depending on the level of granularity you wish to employ in the description. The same is true of METS and other, similar information packaging standards. They each provide a grammar to describe the structure of complex digital objects. To facilitate description of arbitrarily complex structures, these standards employ a relatively flexible grammar, and document authors can and do find a variety of different ways to describe the structure of a single object using one of those grammars.Stanford commented after their ingest of the JHU archive that they had expected one METS object for the entire archive. Because our approach resulted in many METS files – on the order of the number of items in the archive – the Stanford ingest programs experienced out-of-memory conditions. This situation may have been ameliorated had they used the reference code provided by JHU; however, this will be an area that we will look into for future archive ingest projects.
This matter points to a broader issue observed during the various import processes of this phase. Though three of the four non-LC participants (including JHU) used METS as part of their dissemination packages, each of our approaches was different. Clearly there would be some advantage to working toward at least some common elements for these processes. [DiLauro et al. 2005]
To date, the digital library community has treated these interoperability issues
surrounding structural metadata standards as a technical problem demanding a
technical solution. Most efforts to solve these interoperability problems have
focused on the use of a profiling mechanism to further constrain the creation of
instance documents conforming with a XML schema, sometimes in conjunction with a
validation mechanism (such as Schematron) to test instance documents conformance with
the additional requirements established in the profile [Littman 2006],
[Keith 2005]. In essence, profiles exist to limit authors'
flexibility in the use of a given XML language. If different institutions can agree
on using a particular profile of a language, they are far more likely to be able to
produce content objects which can be readily exchanged and interoperate with a
variety of different systems.
However, while these mechanisms may be successful in ensuring interoperability within
a narrowly defined local context, they are not in themselves any guarantee of
interoperability at the scale envisioned by digital library projects such as Aquifer
[Kott et al. 2006], which hope to promote the ready exchange and
interoperability of digital library content among a multitude of institutions. The
official METS profile registry already contains a variety of mutually incompatible
profiles for similar types of objects, with profiles varying in their choices of
descriptive metadata (Dublin Core vs. MODS), use of controlled vocabularies in
descriptive and administrative metadata sections, and their structure (e.g.,
requiring the use of a single <structMap> element in the case of the Oxford
Digital Library METS Profile and mandating the use of two <structMap> elements
in the case of the Indiana University METS Navigator profile).[2] While profiles may enable
localized interoperability, they do not necessarily lead to widespread agreement
regarding the best ways of describing objects' structure, and in fact, it is
conceivable that to a certain extent they reify differences between institutions.
Allowing the specification of local profiles of a XML language may help formalize the
problem of interoperability, but it does not solve it.
If we are to deal with the issues of interoperability that continually manifest
themselves in the realm of structural metadata standards for digital libraries, we
need to cease viewing this purely as a technical problem, and acknowledge that it is
the result of the interplay of technical and social factors. The XML
standards for structural metadata employed by the digital library community represent
cases of sociotechnical systems, and only when we have analyzed the social, as well
as the technical, components of these systems will we be able to suggest how they may
be optimized to achieve the goals of interoperability, usability and preservability
desired by librarians and their patrons.
XML from a Sociotechnical Perspective
One of the fundamental tenets of sociotechnical systems theory is that technological
design and technological evolution are not value neutral processes. Technological
design is both informed by, and seeks to inform, the social environment in which
technology is used, and the work of designers and engineers can be seen as being as
much social engineering as technical engineering. By providing a new means of
accomplishing a task, a technologist is also prescribing a new set of behaviors
centered on the new technology (and possibly proscribing others). This
conceptualization of technology was concisely summarized by [Akrich 1992], who argues that
All the existing and developing standards for structural metadata[3] within the digital library community are XML-based. Any sociotechnical examination of these standards therefore must start with at least some consideration of XML itself. Our questions concerning XML, then, are what world view have XML's authors inscribed within it and what influence has that had on XML's uptake and use within the digital library community....when technologists define the characteristics of their objects, they necessarily make hypotheses about the entities that make up the world into which the object is to be inserted. Designers thus define actors with specific tastes, competences, motives, aspirations, political prejudices, and the rest, and they assume that morality, technology, science, and economy will evolve in particular ways. A large part of the work of innovators is “ inscribing ” [emph. original] this vision of (or prediction about) the work in the technical content of the new object [Akrich 1992, 207–8]
We can learn a great deal about the viewpoints of a particular technology's designers
from the documents they author which define the goals for the technology (e.g., use
cases and user needs analysis), those which help implement the technology (e.g.,
specification documents), and those which attempt to explain or promote the new
technology to potential users. If we look at the original specification document for
XML, we find a relatively clear set of goals for the technology enumerated:
These goals convey some of the world view that XML's designers brought to bear in creating the technology. They saw XML as a transmission format for communications (hence the requirement that it be usable over the Internet). They believed that XML's success was contingent upon it being flexible enough to “support a wide variety of applications.” They also clearly believed that for XML to succeed it must be easy to use, but they also recognized that the meaning of “ease of use” is contingent upon the use one might make of the technology. Ease of use for a document author (“XML documents shall be easy to create,” “XML documents should be human-legible and reasonably clear”) is a good deal different from ease of use for a software developer (“It shall be easy to write programs which process XML Documents,” “The design of XML shall be formal and concise”).
- XML shall be straightforwardly usable over the Internet.
- XML shall support a wide variety of applications.
- XML shall be compatible with SGML.
- It shall be easy to write programs which process XML documents.
- The number of optional features in XML is to be kept to the absolute minimum, ideally zero.
- XML documents should be human-legible and reasonably clear.
- The XML design should be prepared quickly.
- The design of XML shall be formal and concise.
- XML documents shall be easy to create.
- Terseness in XML markup is of minimal importance
This relatively small set of goals for the XML language was further elaborated upon
by members of the original World Wide Web Consortium (W3C) XML Working Group in a
variety of papers they authored to introduce and clarify XML to its potential user
community. [Bosak 1998] further defines the goals of XML as supporting
the user needs of “extensibility, to define
new tags as needed,”
“structure, to model data to any level of
complexity,”
“validation, to check data for structural
correctness,”
“media independence, to publish content in
multiple formats,” and “vendor
and platform independence, to process any conforming document using standard
commercial software or even simple text tools.” The benefits adhering to
XML's providing a standardized format are identified as including “complete interoperability of both content and
style across applications and platforms; freedom of content creators from vendor
control of production tools; freedom of users to choose their own views into
content; easy construction of powerful tools for manipulating content on a large
scale; a level playing field for independent software developers; and true
international publishing across all media.” Emancipatory language is
invoked repeatedly here through the use of the terms freedom and
independence, particularly affording users the freedom “to define new tags” and in so doing
“choose their own views into
content.” In the designers' world view, a key benefit to XML is the freedom
it provides users to define their own structure for documents and data, using their
own semantics, and to escape restrictions that software vendors (through their own
inscriptions in their own products) might wish to impose on their users. Other
articles by members of the XML Working Group (see, for example, [Bosak 1997] and [Bosak & Bray 1999]) reiterate a vision of XML as
a technology allowing users to define their own structures while simultaneously
supporting interoperability of documents and data on a global scale.
The XML 1.0 Recommendation bears the inscription of its designers' ideological stance
towards appropriate mechanisms for data and document structuring as well as
appropriate relationships between document creators and software and platform
vendors. The effort to promote a metalanguage over any specific markup language, the
adoption of Unicode as a basic character set, and the elimination of SGML features
which proved difficult to implement (including CONCUR, OMITTAG and SUBDOC) are some
of the technological means through which XML's designers sought to normalize and
reify a particular set of social and technological relationships. Nor did this
process stop with the release of the XML 1.0 recommendation in 1998. The period
between February 1998 and October 2001 saw the development and release of a plethora
of additional XML specifications, including XML Namespaces, XSLT, XPath, XML Schema,
XLink/XBase, XML Information Set and XSL-FO, as well as a variety of XML software
tools including parsers, editors and stylesheet engines. All of these various
technological objects presented their own opportunities for their designers to
further refine the ideological inscription carried within the XML 1.0 Recommendation.
One of these objects in particular, the Namespaces in XML Recommendation, deserves
further examination due to its significant affect on structural metadata standards
developed by the digital library community.
[World Wide Web Consortium 1999a] provides the following justification for the introduction
of a formal namespace mechanism into XML:
A strong motivating force for the “Namespaces in XML” recommendation, then, was a desire to promote modularity in the design of markup languages. Interoperability was also cited as a motivating factor by the World Wide Web Consortium in the introduction of “Namespaces in XML” [World Wide Web Consortium, 1999b]. Fundamentally, the authors of the “Namespaces in XML” recommendation wanted to simplify XML document authors' lives by ensuring that they did not need to reinvent markup languages which already existed, and that they could readily mix elements and attributes conforming to disparate schemas within a single document instance without worrying about collisions between element and attribute names. Again, modularity and flexibility in design of markup languages would give users the freedom they need while also insuring interoperability.We envision applications of Extensible Markup Language (XML) where a single XML document may contain elements and attributes (here referred to as a “markup vocabulary”) that are defined for and used by multiple software modules. One motivation for this is modularity; if such a markup vocabulary exists which is well understood and for which there is useful software available, it is better to re-use this markup rather than re-invent it.
Such documents, containing multiple markup vocabularies, pose problems of recognition and collision. Software modules need to be able to recognize the tags and attributes which they are designed to process, even in the face of “collisions” occurring when markup intended for some other software package uses the same element type or attribute name.[World Wide Web Consortium, 1999b]
XML's designers have inscribed two overarching messages within the technology they
have created. The first is that XML is about establishing a new social
relationship between content creators and software vendors. By putting control of
data formats into the hands of the content creation community via an open standard,
XML provides that community with signicant political leverage. They can avoid the
proprietary data formats that software vendors have used to lock them into continuing
use of a particular software package. XML thus represents the path to freedom. The
second message is that XML enables easy communication and
interoperability. XML will not only allow you to control your content, it will make
it easier for others to use your content. Freedom and interoperability are the two
underlying themes running through the complete set of XML specifications, with
modular design embraced as the means for achieving these ends.
Structural Metadata Standards and the Digital Library Community
Libraries' exploration of the use of markup languages for encoding of library data
predates the origin of XML by several years. Collaborations with the digital
humanities community on the development of the TEI Guidelines, the development of
Encoded Archival Description, and early efforts to apply SGML to bibliographic
data[4] provided
the library community with experience in the use of markup languages and
demonstration of the benefits they could provide. When the XML 1.0 Recommendation was
released, many digital library projects were already using SGML, and libraries were
quick to embrace XML. XML's simpler design meant that software tools for processing
XML data were readily available, and the new capabilities for data typing introduced
by XML schema languages made XML even more attractive for certain uses than its
predecessor, SGML. Early projects which employed XML, such as the Making of America
II project [U.C. Berkeley Library 1997], were rapidly followed by a number of
XML-based markup languages intended for use in the library community. A significant
focus of much of the library community's work with XML has been developing languages
which can serve to structure all the metadata and data comprising a digital library
object into a coherent whole. Examples of languages developed and explored for this
purpose in the library community include the Metadata Encoding and Transmission
Standard (METS), the Fedora Object XML (FOXML) language, MPEG-21 Digital Item
Declaration Language, and the new Open Archives Initiative Object Reuse and Exchange
(OAI-ORE) specification.
Most of these languages employ a similar pattern for structuring content and
metadata. They provide an encoding mechanism which allows the author to record a
hierarchical structure defining the object, and then associate both content and
metadata with various nodes within that structure. Figure
1 depicts a very simple version of such a structure, a book with a single
chapter; metadata and content files (and metadata for the content files)
are associated with appropriate nodes.
A METS encoding for such an object can be seen in Example 1. The hierarchical structure for the object is defined within the
<structMap> element as a set of recursive <div> elements. Subsidiary
<fptr> elements within a <div> are used to associate that
<div> element with content files described in separate <file>
elements, and ID/IDREF linking attributes are used to associate the root <div>
element with a descriptive metadata record, and the individual <file> elements
with an administrative metadata record. A TYPE attribute on the <div> elements
allows the METS document author to indicate the type of subobject represented by each
node in the structural hierarchy.
Figure 1.
A simple digital object <?xml version="1.0" encoding="UTF-8"?> <mets
xmlns="http://www.loc.gov/METS/"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xlink="http://www.w3.org/1999/xlink"
xsi:schemaLocation="http://www.loc.gov/METS/
http://www.loc.gov/standards/mets/mets.xsd"> <dmdSec ID="DM1" >
<mdWrap MDTYPE="OTHER">
<xmlData><meta>Descriptive Metadata for
Book</meta></xmlData> </mdWrap> </dmdSec>
<amdSec> <techMD ID="AM1"> <mdWrap
MDTYPE="OTHER"> <xmlData>
<meta>Administrative metadata applicable to TIFF
files</meta> </xmlData> </mdWrap>
</techMD> </amdSec> <fileSec> <fileGrp>
<file ID="F1" ADMID="AM1"><FLocat LOCTYPE="URN"
xlink:href="urn:x-mets:Location_of_Page_One_TIFF_Image"/></file>
<file ID="F2" ADMID="AM1"><FLocat LOCTYPE="URN"
xlink:href="urn:x-mets:Location_of_Page_Two_TIFF_Image"/></file>
<file ID="F3" ADMID="AM1"><FLocat LOCTYPE="URN"
xlink:href="urn:x-mets:Location_of_Page_Three_TIFF_Image"/></file>
</fileGrp> </fileSec> <structMap> <div
TYPE="book" DMDID="DM1"> <div TYPE="chapter">
<fptr FILEID="F1" /> <fptr FILEID="F2" />
<fptr FILEID="F3" /> </div> </div>
</structMap> </mets> example 1.
The goal of representing the structure of a work as a hierarchy of nested
<div> elements with TYPE attributes was to have a relatively simple,
abstract hierarchical structure that could be readily applied to a variety of
materials. This was intended to promote the adoption of the standard (a single,
simple standard is more likely to be adopted than a variety of complex ones), which
in turn was seen as promoting interoperability. Having all of the digital library
community using a single standard for structuring content and metadata was seen by
METS' designers as preferable to the community adopting a disparate set of
standards.
It should be noted that this move towards abstraction was a relatively significant
break from the SGML design practices that many research libraries had been using to
date. While it is true that the notion of using nested <div> elements was
derived from the TEI text apparatus, TEI does not rely on pure abstraction; one does
not expect to encounter encoding such as <div type="figure"> in a TEI
document, when a <figure> element is available to use. Just as XML itself, the
METS schema carries an inscription of its designers' world view, that it was
preferable to develop a single, simple, generalizable, highly abstract model and
encoding mechanisms to structure content and metadata for digital library objects,
rather than to pursue the development of a variety of highly specific schemas (one
for photographs, one for journals, etc.), or a grand encompassing schema that
contained elements appropriate to different genres that could be combined as needed
(e.g., the TEI model). Despite its use of the abstract <div> element with a
TYPE attribute to represent the structural components of a digital library object,
however, the METS schema insisted on the use of more specific concrete elements to
identify different forms of metadata, with the <dmdSec> element used for
descriptive metadata and the <amdSec> element used for administrative
metadata, along with a series of subelements for different forms of administrative
metadata (technical, rights, provenance and source). This typification of different
forms of metadata was itself an effort to promote both modularity in further metadata
schema development and the creation of certain types of metadata schema. By
identifying specific subclasses of metadata within the METS schema, METS' designers
hoped to encourage XML developers in the digital library world to create discrete,
specialized metadata standards that would align with those subclasses, and that those
creating digital library objects could then select from a set of such modular XML
metadata standards in composing a particular object. Through METS' design, its
implementers consciously sought to encourage the adoption of modular schema design
practices within the digital library community.
Other XML-based markup languages adopted by the digital library community have taken
a similar approach. The MPEG-21 Digital Item Declaration Language also employs a
rather abstract hierarchical structural mechanism for ordering content and metadata.
It differs inasmuch as non-structural metadata (<Descriptor> elements in
MPEG-21 parlance) are not typed, and structural metadata elements, while still rather
abstract, are of three different types: <Container>, <Item> and
<Component>. The Open Archives Initiative Object Reuse and Exchange
specification is perhaps the most abstract of all the structural metadata standards
adopted within the digital library community; while it has multiple serialization
syntaxes, all of them employ a single mechanism to link an abstract aggregation with
a set of aggregated resources (although the specific linking mechanism varies
according to the serialization syntax employed). Those aggregated resources may in
turn be aggregations, and any aggregation may be associated with a variety of
additional metadata.
If we examine these other standards to determine what inscriptions their designers
have placed within them regarding their use, we find messages very similar to that of
METS. Structural metadata should be highly abstract, so that a very small set of
elements can be employed to structure widely disparate content genres. While METS was
perhaps more vocal in trying to push the message that further development of metadata
schemas should try to create small, focused and modular metadata sets that could be
drawn upon as needed to encode a particular object, the other standards convey the
same message (through the use <Descriptor> elements to associate metadata with
other elements in the case of MPEG-21, and through RDF mechanisms in the case of
OAI-ORE). Other structural metadata standards of interest to the digital library
community employ similar mechanisms. The XFDU standard for data archiving uses
hierarchies of <ContentUnit> elements that may be associated with
<dataObjects> and <metadataObjects>. The IMS Content Packaging
standard for learning objects uses hierarchies of <item> elements that may be
associated with <resources> and <metadata>. While implementations
differ in details, the pattern is similar and widespread across the various
structural metadata standards of interest to the digital library community. Again and
again we see designers seeking to achieve wide adoption of their standard in order to
promote interoperability between differing institutions; to secure this goal, they
favor a highly abstract structural mechanism which can be applied to a wide variety
of content, and mechanisms to allow a variety of additional metadata schemas to
“plug and play” within the larger structural framework.
While of perhaps some limited interest to social researchers of technology, none of
the preceding seems particularly surprising or problematic. That the designers of XML
itself, and the designers of encoding standards for digital library metadata and
content, should favor flexibility, extensibility, modularity and the use of
abstraction to support the generalizability of their standard, and hence promote its
widespread adoption to help achieve interoperability, would not be a great shock to
anyone who has spent more than five minutes in the company of computer scientists.
These are all considered almost innate goods among software engineers in general and
markup language enthusiasts in particular. Yet the NDIIPP tests cited previously
would seem to indicate that flexibility, extensibility, modularity and abstraction
are not in and of themselves sufficient to achieve interoperability. So what,
specifically, is the problem that METS and other structural metadata standards are
encountering?
Defining the Problem, or, Why is XML like a rope?
One of the earliest discussion points in the development of the METS standard was
which of the various elements within the schema should be declared mandatory and
which optional. After some discussion among the members of the working group that
established METS' original design, it was decided that the <structMap>
element, which records the basic tree structure on to which content files and
metadata are mapped in METS, would be the only required element. METS, in the group's
opinion, was fundamentally a structural metadata standard; it existed to
provide a framework into which other descriptive and administrative metadata, as well
as content, could be placed. The <structMap> element provided the tree upon
which all the other structural components of METS where hung, where the logical and
physical structure of a work could be delineated, and so was really the only section
that needed to be mandatory. As the <structMap> was the only mandatory portion
of a METS file, it was also expected that any structural description of a work should
reside there; software that would process a METS file would expect to find logical or
physical descriptions of the structure of a work residing within a structural map,
and not elsewhere in the METS file.
It was a matter of some surprise for many in the METS community, then, when the
Library of Congress, which serves as the maintenance agency for the METS standard,
began to produce METS files for digital versions of certain kinds of audio recordings
which placed the logical structure of the works in MODS records contained within the
METS descriptive metadata section (<dmdSec>) rather than in the structural
map, and registered a profile of METS establishing this as their formal internal
practice for “recorded events”
[Library of Congress 2006]. The MODS record within a METS file would provide a logical
structure for the work using a hierarchical arrangement of the MODS
<relatedItem> element, while the METS <structMap> would contain the
physical structure, with ID/IDREF links used to draw connections between the two
structural descriptions. A recorded concert, for example, might have a MODS record
containing a hierarchy such as this:[5]
<mods:mods xmlns:mods="http://www.loc.gov/mods/v3"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.loc.gov/mods/v3
http://www.loc.gov/standards/mods/mods.xsd"> <mods:titleInfo>
<mods:nonSort>The </mods:nonSort> <mods:title>1946
Library of Congress recital</mods:title> </mods:titleInfo>
<mods:relatedItem type="constituent" ID="DMD_disc01_tr001">
<mods:titleInfo type="uniform"><mods:partName>Chaconne von
Vitali</mods:partName></mods:titleInfo>
</mods:relatedItem> <mods:relatedItem type="constituent"
ID="DMD_disc01_tr002_005"> <mods:titleInfo><mods:title>Sonata
in G minor, BWV 1001</mods:title></mods:titleInfo>
<mods:relatedItem type="constituent" ID="DMD_disc01_tr002">
<mods:titleInfo><mods:partName>Adagio</mods:partName></mods:titleInfo>
</mods:relatedItem> <mods:relatedItem type="constituent"
ID="DMD_disc01_tr003"> <mods:titleInfo><mods:partName>Fuga
: allegro</mods:partName></mods:titleInfo>
</mods:relatedItem> <mods:relatedItem type="constituent"
ID="DMD_disc01_tr004">
<mods:titleInfo><mods:partName>Siciliano</mods:partName></mods:titleInfo>
</mods:relatedItem> <mods:relatedItem type="constituent"
ID="DMD_disc01_tr005">
<mods:titleInfo><mods:partName>Presto</mods:partName></mods:titleInfo>
</mods:relatedItem> </mods:relatedItem>
<mods:relatedItem type="constituent" ID="DMD_disc01_tr006">
<mods:titleInfo><mods:title>Paganiniana :
variations</mods:title></mods:titleInfo> </mods:relatedItem>
<mods:identifier type="lccn">99594334</mods:identifier>
</mods:mods> example 2.
Meanwhile, the structural map would record the physical structure of the work as
follows:
<div TYPE="cd:compactDiscObject" DMDID="MODS1"> <div
TYPE="cd:disc"> <div DMDID="DMD_disc01_tr001"
TYPE="cd:track"></div> <div DMDID="DMD_disc01_tr002"
TYPE="cd:track"></div> <div DMDID="DMD_disc01_tr003"
TYPE="cd:track"></div> <div DMDID="DMD_disc01_tr004"
TYPE="cd:track"></div> <div DMDID="DMD_disc01_tr005"
TYPE="cd:track"></div> <div DMDID="DMD_disc01_tr006"
TYPE="cd:track"></div> </div> </div> example 3.
This represents a valid use of METS (in the technical XML sense), but is
a departure from expected practice, which would be to include both the logical and
physical structural information within one or more <structMap> elements.
This example demonstrates two problems that have impeded the development of
interoperable content within the digital library community. The first is that the
implementation of highly abstract elements for the definition of structure provides a
tremendous amount of flexibility to document encoders; there are a vast number of
potential encodings of any given object in METS, with variations possible in depth of
structure (do I limit my structure to musical movements or do I provide structural
information down to the measure level?), labeling (you say TYPE="book",
I say TYPE="monograph"), and arrangement (should the Lord of Rings film
trilogy be encoded as a single METS file? Three METS files? Three METS files for the
individual films and a fourth representing the abstract notion of the Trilogy?). This
can lead to significant variation in encoding practices, even between two
institutions dealing with remarkably similar material and using the same metadata
standards, as noted by [DiLauro et al. 2005].
The second problem is what we might call the problem of standards independence. Many
of the XML metadata schemas that have been developed with the help of the digital
library community have been created with the understanding that ensuring their
usefulness in a variety of application environments requires that they not contain
inherent dependencies on other schemas; they need to be able to express all the
relevant information within their particular domain on their own. In many of these
XML standards, the designers recognized a need to be able to account for
relationships between various content objects being described, whether the
description being applied was the more traditional form of intellectual description
you would expect in a library catalog, or a technical description of the composition
of a TIFF image. The result has been that a number of common metadata schemas within
the digital library field contain elements for expressing structural metadata, even
schemas that are not primarily intended for recording structural metadata. Dublin
Core has its <relation> element, MODS has its <RelatedItem> element,
the PREMIS schema for preservation metadata has a <relation> element, even the
MIX standard for still image technical metadata contains an element for referencing
previous image metadata. As the standards' developers felt they should not make their
efforts dependent on structural metadata mechanisms in other standards, they
implemented their own. Unfortunately, with the addition of each new metadata standard
containing structural metadata capabilities, the potential for difficulties with our
first problem increases. Every new metadata standard created within the digital
library community seems to add another mechanism for describing the structural
relationships between content objects, and hence greater potential for variation in
object encoding practices.
The irony is that both these problems derive from the flexibility, extensibility,
modularity and use of abstraction to create structural metadata elements that the
designers of the metadata schemas hoped to promote. The potential range of variation
in encoding structural metadata is the result of each of these factors. The use of
abstraction in METS (i.e., the <div> element) was an attempt to make the
standard flexible in application; however, it opens up a tremendous degree of play in
encoding practice. If you ask two different individuals how many page breaks there
are in a text, the likelihood that they will give the same answer is a good deal
greater than if you ask them how many divisions there are in a text. The use of
abstraction opens up encoding to a much greater degree of personal interpretation,
and hence variation. The extensibility of METS, and the hope to promote a modular
system of metadata schema reuse that its authors inscribed within it, opens up the
possibility of using other metadata schemas to encode structural metadata. And it was
this same desire for flexibility and modularity that has led other metadata schema
designers to include structural metadata components in their own schemas; they wanted
to ensure that their own efforts were flexible enough to be applied in a variety of
settings, and with a variety of others. But having to design their own schemas
without knowing the specific supporting capabilities to be found in other schemas
with which their own might be used, they are inevitably forced to create structural
metadata capacities of their own within their schemas. The designers of metadata
schemas (structural or otherwise) within the digital library community have sought to
adhere to a particular set of design practices, seeking to create flexible,
extensible, modular and generalized tools, and to promote like practice in others
through inscription of their view of appropriate XML design within their
technological artifacts. Unfortunately, promoting such good practices has been a
death blow to one of the principle reasons for adopting XML in the first place: to
ensure interoperability of digital library materials across systems. Wide-scale
interoperability requires wide-scale adoption, but the design practices of schema
implementers intended to promote wide-scale adoption run directly counter to wide
spread interoperability.
Hence XML's similarity to a rope. Like a rope, it is extraordinarily flexible;
unfortunately, just as with rope, that flexibility makes it all too easy to hang
yourself.
Strategies for Interoperability in a World of Multilingual Markup
The digital library community seems to face a dilemma at this point. Through its
pursuit of design goals of flexibility, extensibility, modularity and abstraction,
and its promulgation of those goals as common practice through their inscription in
XML metadata standards, it has managed to substantially impede progress towards
another commonly held goal, interoperability of digital library content across a
range of systems. How then, should the community respond?
One possible response to this situation would be to say that perhaps our community
cares less about interoperability than we thought. Despite projects intended to
promote interoperability, such as the Digital Library Federation's Aquifer, it may be
that interoperability is actually a lower priority for the digital library community
than it likes to believe, and the adoption of metadata standards that impede
interoperability is merely a reflection of that underlying reality, and not a major
problem to resolve. There is at least some reason to suspect this may be the case.
Research libraries typically have a clearly defined local clientele, and while voices
within the digital library community have been calling for some time for the
liberation of content from local silos to enable their use by a larger community [Seaman 2003], libraries' primary responsibility will always be to their
local communities. The first sentence in the mission statement for the University
Library at the University of Illinois at Chicago exemplifies the priorities present
at most research libraries: “The
University of Illinois at Chicago (UIC) Library strives to meet the information
needs of UIC students, faculty, and staff.”
[6] Prioritizing service to the local community is endemic to the social
structure of library systems, and if systems developed to deliver digital library
content to that community are successful in that context, and if the costs associated
with achieving much more widespread interoperability are high, then many libraries
may decide that interoperability, while desirable, is a goal which may have to
wait.
If libraries do wish to make progress on the issue of interoperability of structural
metadata, they will need to recognize that, as [Renear & Golovchinksky 2001] observed,
“every significant information
processing standardization effort must skillfully negotiate competing and
apparently irreconcilable objectives, [and] serve a wide variety of stakeholders
with many different interests.” In the case of structural metadata, the
particular competing objectives that the digital library community does not seem to
have successfully reconciled to date are what [Kendall 2007], in a
discussion of blogging practices, has labeled the problem of “control vs. connection.” The structural metadata
standards which have been developed to date, with their emphasis on flexibility,
extensibility and modularity have sought to afford local institutions the greatest
degree of control possible in their encoding practices. The standards are designed to
allow any given institution to do what it wants. This has clear benefits in terms of
easing adoption of the standard in any given context, and as a result insuring the
standard's widespread adoption (obviously a good thing in a standard). However,
increasing the amount of local control over the ways in which a language is used and
developed is fundamentally at odds with a language's ability to serve as a means for
connection with others outside the local context. It is, in essence, promoting the
development of regional dialects at the expense of mutual intelligibility. The
particular case of structural metadata standards reveals that sufficient local
variation in syntax, the ways in which people structure their objects using a markup
language, can be as fatal to communication as variation in semantics.
Given this fundamental tradeoff between internal control and external connection,
libraries wishing to promote interoperability of digital library content have two
possible strategies. The first, and most obvious, is to attempt to alter the balance
currently struck between connection and control to more strongly favor connection.
There are several mechanisms which the library community might employ in pursuit of
this strategy, including the design and use of schemas which more significantly
restrict both the means for recording the structure of objects and the ability to
employ arbitrary additional schemas within instance documents (or developing profiles
of existing schemas to achieve the same ends), establishing formal rules of
structural description (equivalent to rules of description used in cataloging for
creating bibliographic records) dictating aspects of object encoding not susceptible
to enforcement through XML's validation mechanisms, and mandating the use of
particular controlled vocabularies and ontologies within document instances to record
information such as a <div> element's TYPE attribute in METS.
Decreasing the possibility for local variation in encoding of structural metadata
will certainly help improve digital libraries' capability to interoperate with each
other. However, removing local capacity for variation will also tend to reduce the
number of institutions who are willing to use such a markup language. If the digital
library community, for instance, was to revise the METS standard to forbid any use of
a <relation> or <RelatedItem> element in a descriptive metadata section
to express the logical structure of a work, it would assist in insuring
interoperability of digital content, but it might also very well mean losing the
Library of Congress's support for the standard. More importantly, however, such an
approach overlooks one of the fundamental realities of the web environment:
communities of practice no longer operate in isolation from each other (if indeed,
they ever did). Even if libraries could agree on a structural metadata standard that
enabled a significantly greater degree of support for interoperability than we find
with today's standards, libraries must now interact with a variety of other
communities (publishers, museums, archives, educational technology companies, etc.)
that are also creating their own structural metadata standards. This is not to say
that pursuit of this strategy is futile or even inappropriate in many instances;
libraries' previous experience with standard efforts such as MARC demonstrate that
with sufficient time and effort a particular community of practice can achieve
widespread interoperability of metadata. However, the library community's
interactions with other communities clearly indicates that this strategy by itself is
insufficient to resolve the interoperability problems that libraries confront
today.
To deal with these wider issues of interoperability, the library community must adopt
a second strategy based on accepting that the need for community control over
encoding practices is a valid one, that community “dialects” of markup languages
are inevitable, and that we must find ways to facilitate information exchange across
the boundaries of different communities' markup vernacular. However, this will
require a significant shift in the digital library community's relationship to the
notion of standards. Specifically, the library community needs to shift from its
current singular focus on schema development to a dual focus on both schema
development and translation between schemas.
This is certainly not the case today, as can be seen if we examine the work of some
of the major agencies involved in metadata standardization in libraries such as the
Library of Congress. The Library of Congress currently serves as maintenance agency
for a variety of XML standards developed within the library community; if you examine
the list of standards that they are maintaining [Library of Congress 2008], however,
you will find that while there are several metadata standards listed, standardized
stylesheets to enable conversion between formats are not listed here. Such
stylesheets do exist in some cases. The Library of Congress has, for example,
provided stylesheets to enable conversion of MODS descriptive metadata records into
MARC/XML format and back. These efforts to formalize prior work that established
crosswalks between different descriptive metadata standards are not, however, seen by
the community as having the status and importance of standards, as exhibited by their
omission from the “Standards at the Library of Congress”
web page. If the digital library community wishes to support interoperability while
simultaneously affording institutions localized control over encoding practices, that
situation needs to change. We can no longer view the creation of translations between
standard formats as an ancillary activity; instead, we must regard it as a form of
standards activity in its own right, as important, if not more important, than the
creation of schemas for metadata sets.
A heightened emphasis on standardizing translation between markup languages will mean
further work on formalizing translations between markup languages using XSLT, and
treating those with the level of attention and care that the community has lavished
on metadata schemas. However, it might also be worth considering whether the notion
of formal rules of structural description mentioned earlier might be of benefit in
trying to achieve greater translatability between different markup languages. As an
example of what this might mean, consider the example of the 1:1 principle in Dublin
Core [Hillmann 2005], that a single Dublin Core record should describe
one and only one resource. The 1:1 principle provides guidance on the relationship
between a metadata record and a described resource that is applicable outside the
realm of Dublin Core; in fact, several other descriptive metadata standards developed
since Dublin Core refer to the 1:1 principle as a guide to usage. We could easily
envisage similar principles being developed for structural metadata that could guide
usage of a variety of different structural metadata standards, and by working to
insure similar use practices, would help insure ease of translation between different
structural markup languages. We might, for instance, take as a working principle that
any given structural metadata document should never contain more than two levels of
structural hierarchy. Our METS example above passes muster with this rule; if,
however, we modified it so that a third level of <div> elements was needed (of
TYPE “subchapter,” for example), then we would be in violation of this
principle. To fix this problem, we could employ METS' <mptr> element to allow
the <div> elements for each chapter to reference separate METS files
containing the structural descriptions for the individual chapters. Through the
establishment of common principles of structural encoding and standardized
stylesheets for translation, we might be able to improve our ability to interoperate
while simultaneously retaining some flexibility for local encoding practice (although
obviously adoption of common principles of structural encoding may impede local
control in favor of connection to some degree).
The rise of the network information society is presenting libraries with a variety of
new challenges. Perhaps the most significant of these is the heightened degree of
interaction with communities of practice that do not share libraries' standards,
practices or values. If libraries are to survive and thrive in this new information
society, they must alter their own value structure to prioritize communication with
other communities to an equal, if not greater, extent than internal communication
between libraries. If they pursue this course, they may find that issues of internal
interoperability of library systems are more tractable than they have appeared to
date.
Notes
[1] As a reviewer for this article noted, the meaning of
interoperability is vague and its interpretation highly variable
depending on the community and context in which it is used. Within the discussions
among the members of the digital library community upon which this article is
based, interoperability is interpreted primarily as the ability to reuse data and
metadata outside of the technological system in which they were originally
instantiated as well as outside of the community they were originally intended to
serve. Interoperability thus has both a technological and a social
component.
[2] See http://www.loc.gov/standards/mets/mets-registered-profiles.html for the
set of formally registered METS profiles.
[3] I am
including within this set the Metadata Encoding & Transmission Standard (METS,
see http://www.loc.gov/standards/mets/), the MPEG-21 Digital Item
Declaration Language (DIDL, see http://www.chiariglione.org/mpeg/standards/mpeg-21/mpeg-21.htm), the
Open Archives Initiative Object Reuse & Exchange standard (OAI-ORE, see http://www.openarchives.org/ore/) and the Fedora Object XML
specification (FOXML, see http://www.fedora-commons.org/documentation/2.2.2/userdocs/digitalobjects/introFOXML.html).
There are a variety of other structural metadata standards that are of particular
interest to the digital library community, although not being developed within it,
including standards for data archiving such as the XML Formatted Data Unit
specification (XFDU, see http://sindbad.gsfc.nasa.gov/xfdu/) being developed by the Consultative
Committee on Space Data Systems and standards for structuring content for
e-learning systems such as the IMS Content Packaging specification (IMS-CP, see
http://www.imsglobal.org/content/packaging/) developed by the IMS Global
Learning Consortium and the Shareable Content Object Reference Model (SCORM, see
http://www.adlnet.gov/Technologies/scorm/default.aspx) developed by the
United States Department of Defense.
[4] See http://www.loc.gov/marc/marcsgmlarchive.html for information regarding
the development of an SGML version of the MARC 21 record format.
[5] Complete MODS and METS records for this
example can be found at the Library of Congress webpage for the 1946 Library of
Congress recital at http://lcweb2.loc.gov/diglib/ihas/loc.natlib.ihas.200003790/default.html
[6] For the complete mission state of the University of Illinois at Chicago
Library, see http://www.uiuc.edu/lib/about/libmission.shtml
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URL: http://www.digitalhumanities.org/dhqdev/vol/3/3/000064/000064.html
Last updated:
Comments: dhqinfo@digitalhumanities.org
Published by: The Alliance of Digital Humanities Organizations
Affiliated with: Literary and Linguistic Computing
Copyright 2005 -
This work is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.
Last updated:
Comments: dhqinfo@digitalhumanities.org
Published by: The Alliance of Digital Humanities Organizations
Affiliated with: Literary and Linguistic Computing
Copyright 2005 -
This work is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.