How to cite this paper
Some Assembly Required
Reflections on XML Semantics, Digital Preservation and the Construction of Knowledge
Balisage: The Markup Conference 2013
August 6 - 9, 2013
"You keep using that word. I do not think it means what you think it means."
— Inigo Montoya, The Princess Bride
The issue of semantics has been of perenniel interest to those who study markup languages. From Eliot Kimber's presentation on HyTime semantic groves Kimber, 1996 to Steven Newcomb's work on semantic integration Newcomb, 2003 through more recent work such as Dubin's and Birnbaum's Dubin & Birnbaum, 2008 comparison of markup technologies for knowledge representation and Karen Wickett's Wickett, 2010 discussion of contextual aspects of semantic interoperability of markup langauges, discussions of what exactly tagging structures and markup mean and the implications of semantics for dealing with issues of interoperability and knowledge representation have consumed the time and energy of markup enthusiasts for decades. In some sense, the entire literature on markup languages can be viewed as a continuing conversation about semantics and the best means for their technical and syntactical implementation in the realm of text.
For the past six years, participants in the Preserving Virtual Worlds projects have been investigating the preservation of computer games and interactive fiction. A great deal of that work has focused on how to best employ XML and semantic technologies in the packaging of content and metadata for preservation. XML metadata standards such as METS McDonough, 2006, PREMIS PREMIS Editorial Committee, 2012 and Dublin Core Weibel, Kunze, Lagoze & Wolf, 1998, along with new ontologies created in OWL-DL, have been employed in conjunction with data models such as FRBR IFLA Study Group, 1997 and the Open Archival Information System (OAIS) Reference Model CCSDS, 2012 to try to develop packaging mechanisms for long-term preservation of game software as well as knowledge bases regarding file formats necessary for digital preservation within the OAIS model. Our team has benefited from earlier work on XML semantics, but as we endeavored to create XML mechanisms suitable for long-term preservation, there has seemed to us to be a component missing in much of the discussions of semantics and markup.
Ultimately, much of the discussion of XML semantics has focused on the question of
How is the meaning of a document augmented or otherwise affected by the presence of markup?Marcoux, Sperberg-McQueen & Huitfeldt, 2009 This is a valid and important question, but in focusing at the document level, I am afraid we may be focusing on the trees (and the groves) to the neglect of the larger forest. By employing a mental model of markup semantics which focuses on meaning as emerging from the interaction between a text, its markup and a reader, we may be missing other influences on meaning that operate at a larger scale than a single document, or even a collection. In many situations, such influences may be irrelevant and safely ignored, but within the realm of digital preservation and, I suspect, many other enterprises worried about large scale knowledge representation, focusing on meaning at the document level may be problematic. In the remainder of this paper, I will describe some of the problems we encountered in attempting to preserve computer games, XML's and OWL's role in our solutions to those problems, and discuss a possible alternate formulation of markup semantics based on social constructionist theory.
The Problem of Meaning in Digital Preservation
The Preserving Virtual World project McDonough et al., 2010 was an investigation into the preservation of computer and video games and interactive fiction. As part of this project, the research team investigated the application of various XML technologies to the problem of packaging these complex media objects for long-term preservation. Of critical importance was trying to develop a means of packaging games which complied with the OAIS Reference Model.
Compliance with the OAIS Reference Model imposed some difficult requirements on the project team. Of particular note is one aspect of the information model employed by the OAIS Reference model shown in Figure 1.
Figure 1: OAIS Archival Information Package Model
The information model depicted here sets out what OAIS calls the Archival Information Package, the body of information which an archive needs to preserve to insure the longevity of the data object, which is the original target of preservation. As shown, an archival information package for a preserved digital object should contain not only the object itself, but a variety of additional information, of which one of the most important types is Representation Information
, the information that allows
someone to take the sequences of bits which comprise the data object and
interpret them as meaningful information. Representation information can be either
structural or semantic in nature. Structure information maps bit sequences into basic
data types (e.g., character data, integers, floats); the Unicode standard Unicode Consortium, 2012
, for example, allows one to map a series of zeros and ones into
a particular character set, while IEEE Standard 754-1985 IEEE, 1985
defines a mapping from bit sequences into floating point numbers. Semantic information,
on the other hand, provides someone who is using the data with additional information
they need to fully interpret it. Knowing that a particular string of 32 bits represents
two short integers requires structure information; knowing that those integers represent the X and Y map coordinates of an object in a DOOM game map requires semantic
As can also be seen in Figure 1, representation information can be recursive in nature. If structure or semantic representation information is itself in digital form, the archive should additionally store the representation information needed to decode the other representation information. Ultimately, this recursive chain can only be broken by storing representation information in a form that does not require the technological assistance of computer to display that information in readable form to a human being (e.g., on paper).
Conformance with this information model means that a digital archive will store not only the original data object, but also the associated standards and documentation defining the data format in which the object is written. Within the realm of space science data preservation for which the OAIS Reference Model was originally intended, this is a relatively straightforward and unproblematic endeavor. Standards for codebook documentation of scientific data sets such as the Data Documentation Initiative Codebook Data Documentation Initiative, 2012 and the Data Format Definition Language (DFDL) Powell, Beckerle & Hanson, 2011 provide relatively concise definitions of data formats used for scientific data. While they have some degree on reliance on additional standards for the definition of fundamental data types (e.g., DFDL assumes the use of W3C XML Schema data type definitions), the complete set of representation information required for data such as a social science survey is small and fairly well-bounded.
This proved not to be the case for the vast majority of the computer games that the Preserving Virtual Worlds project examined. There were a number of problems involved in identifying and collecting a set of representation information documenting the data formats used in a particular game, but one stood out as being particularly problematic. Unlike scientific data, which is often quite intentionally recorded in a manner to facilitate its exchange and use across a variety of computing platforms using standards such as NetCDF or HDF5, executable software is tightly bound to a particular platform. The representation information set for a computer game thus often becomes equivalent to the information necessary to completely document its operating platform.
Consider a game like Blizzard Entertainment's Warcraft III: Reign of Chaos for OS X, and let us ignore the large number of different files and file formats employed by the game and focus solely on its main executable. This is a file in the Mach-O format. Structure information needed to interpret the Mach-O format begins with the OS X ABI Mach-O File Format Reference Apple, Inc., 2009. However, this work, while providing the basic format definition for Mach-O, is not a complete specification of the format in that it makes specific reference to another document, Mach-O Programming Topics. This document in turn references other documentation necessary to understand its content, including System V Application Binary Interface AMD64 Architecture Processor Supplement, the OS X ABI Dynamic Loader Reference, Plug-in Programming Topics, etc. Moreover, the ability to understand the OS X ABI Mach-O File Format Reference involves more than simply having the set of additional reference documentation available. A section of text such as this:
is readily interpreted by anyone with experience in the C programming language, but a good deal less so if you lack that experience. It is easy enough to include documentation on the C programming language in the representation information set for the Mach-O format but understanding that it is actually a C language data structure (and hence that the representation information set should include documentation on C) requires a relatively deep knowledge of computer software design and implementation.
This problem is exacerbated when we move beyond considering structure information to considering the semantic information necessary to interpret a Mach-O binary executable. Binary executables contain platform-specific machine code; in the case of the version of Warcraft III on my desktop, the executing platform is based on the Intel 64 architecture. We should, therefore, add the seven volume Intel® 64 and IA-32 Architectures Software Developer's Manual documenting the machine code for Intel systems to our representation information, plus the set of additional documents referenced by those volumes as well as the additional technical library necessary to achieve the level of knowledge necessary to comprehend the Intel® 64 and IA-32 Architectures Software Developer's Manual. In short, understanding a single file requires the equivalent of a small, specialized engineering library.
We found this problem recurring in our examination of computer game files. Even something that on its face would seem relatively unproblematic, Adobe's Portable Document Format specification, proved to have a truly remarkable representation information set, for it references a large number of additional standards documents from ISO, IEEE, the World Wide Web Consortium and others. Understanding the specification also requires knowledge of the mathematics of Bézier curves, color theory, audio encoding techniques, and principles of font design. For any number of file formats employed by computer games, representation information sets are both large and poorly defined.
The OAIS Reference Model recognizes this as a potential problem for those administering digital archives, and states that an archive may limit the amount of representation information it stores to the minimum which the community it serves requires in order to decipher a particular format. If your archive serves a community of computer scientists, you presumably do not need to store a copy of Kernighan & Ritchie's C Programming Language to allow them to understand the Mach-O format; similarly, if you serve a community of Middle English scholars, your representation information set for William Langland's Piers Plowman need not include a Middle English dictionary and grammar. Unfortunately, our project was focused on preserving games within the context of academic research libraries, where detailed technical knowledge cannot be assumed for all users, and where even those users who possess advanced techical knowledge of existing platforms may have little knowledge of or experience with previous platforms. I would expect any computer scientist today to have a reasonable working knowledge of modern disk operating systems and disk formatting operations, but the exact form of an Apple II disk image file or the instruction set for the Apple II's MOS 6502 microprocessor is something for which I would expect many would need to consult a manual.
This has meant that for computer games, identifying and collecting representation information is not simply an issue of documenting a particular file format. It is an exercise in knowledge representation and management. If highly complex, multimedia objects such as game software are going to survive in the long-term, archivists will need to collect, organize and preserve a large body of additional information necessary to interpret those objects. Or perhaps more precisely, they need to collect, organize and preserve the information necessary to allow an individual to acquire the knowledge to interpret those objects. The problem the game preservationist confronts is that they do not need to merely preserve the game; they need to preserve people's ability to be able to understand the game, from the level of individual 0's and 1's on up.
This problem has dimensions other than the purely technical one we have been discussing. On the technical side, if we are to preserve the seminal interactive fiction game Adventure in its original form, we need to preserve the ability to understand DEC's version of Fortran IV from the mid-1970s. If we wish to preserve people's ability to understand the importance of Adventure in computing history, however, there is other information that we might wish to preserve, documentation such as Dennis Jerz's historical examination of the game's development Jerz, 2007. The OAIS reference model recognizes that all data objects have this form of contextualizing information, and recommends that an archival information package include information on "why the Content Information was created and hot it relates to other Content Information Objects."CCSDS, 2012 For scientific data collections, this will typically involve documentation on the reasons why a study was conducted, links to associated data sets, and in some cases links to publications based on the scientific data in question (metadata standards such as the previously mentioned Data Documentation Initiative provide XML facilities for recording these links). Attempting to apply the OAIS reference model outside the realm of scientific data quickly led our research team to the conclusion that digital preservation of computer games was more complicated. A cultural artifact such as a game has links to a much wider range of related materials. Documentation for the MIT game SpaceWar!, for example, would not really be complete without including reference to it being a tribute to E.E. "Doc" Smith's Lensmen novels as well as its information on its impact on later vector graphic games such as Atari's Tempest and Star Wars arcade games.
Ultimately, digital preservation is devoted to insuring users' ability to interpret the preserved material in both a specific technical and more general historical sense. Essentially, the goal of a preservationist is to allow users of libraries and archives to make sense of the information in a collection, whether their interests are technological, historical or cultural. In the next section, I will discuss the role that XML and OWL played in trying to pursue these goals, and then discuss the implications of this for larger discussions of XML semantics.
Problems in Using XML for Preservation: Trying to Make Sense of Sense-Making
Recognizing the need to incorporate significant amounts of representation information and contextual information about a game as part of a package used to preserve a game in the long term, and recognizing that this in turn meant preserving a great deal of information about the relationships between the digital files we would include in an archival package for a game, the Preserving Virtual Worlds team turned to XML and OWL as mechanisms to tackle this problem in knowledge representation and management. OWL-DL was used to create an ontology in which the major entities from the FRBR entity-relationship model were defined as OWL classes, and relationships from FRBR were defined as properties. Relationships between various information entities in the OAIS reference model were further defined as properties in the ontology relating different FRBR classes. A partial representation of the OWL class and property relationships in the Preserving Virtual Worlds ontology is shown in Figure 2.
Figure 2: Preserving Virtual Worlds Ontology
By using the OWL ontology in conjunction with XML standards from the digital library community intended for packaging bodies of content and metadata together (e.g., METS and OAI-ORE), we could organize the various materials we need to preserve a game over time. An OAI-ORE file, for example, could provide URLs for the files comprising a game along with all of the files containing representation information and contextual information. The OAI-ORE file could also be used to indicate that files represented instances of FRBR classes (e.g., file 'advf4.77-03-31' is a FRBR Item) and that different entities participated in relationships defined within the OAIS Reference Model (e.g., file 'advf4.77-03-31' (a Fortran IV source code file) has representation information in the form of the FRBR Expression of DEC's Programmer's Reference Manual for Fortran IV, which in turn has a physical manifestation and an item exemplifying that manifestation in the file 'DEC_FORTRANIV_1968.pdf').
While the specific terms employed by our project as an effort in digital preservation differ from those employed elsewhere in the digital library community, this use of XML for packaging digital library materials is hardly a novel application of markup technologies (see, for example, Morrissey et al., 2010 and Bekaert, Hochstenbach and Van de Sompel, 2003). We were able to employ our ontology with a variety of XML packaging languages for digital library objects and capture the requisite content and metadata we believed formed the backbone of a digital preservation effort for game software. But examining our work more closely in retrospect, certain aspects of our ontology and its application strike me as troubling.
The first problem we encountered was that the OAIS relationships between information objects that appeared relatively clear semantically as described in the Reference Model turned out be somewhat more polysemous in actual application. If we examine the full network of representation information necessary to interpret a file in Adobe PDF format, for example, we find a great deal of subtle variation in the roles performed by representation information necessary to interpret the Adobe PDF specification itself. IEEE-754 obviously constitutes representation information for the Adobe PDF specification, and of a type clearly defined within the OAIS Reference Model: structure information. IEEE-754 defines the underlying bit patterns used to represent floating point numbers in Adobe PDF. A specification like CIE L*a*b* ISO, 2008 also constitutes representation information necessary for a complete interpretation of PDF, as L*a*b* is one of the standard color spaces referenced within the PDF specification and the specification is necessary for understanding the semantics of color information embedded within some PDF documents. But other documents referenced within the Adobe PDF specification, while of assistance in interpreting information in a PDF file, are not representation in the same way that IEEE-754 or CIE L*a*b* are, which is to say, the Adobe PDF specification does not have a technical dependency on those specific documents. Graphics Gems II Arvo, 1994 and Graphics Gems III Kirk, 1994 are both valuable texts for achieving an understanding of the Bézier curves used within the PDF specification, but no more so than a variety of other texts on that topic. They do provide semantic information that allows someone to make sense of a PDF document, and hence qualify as representation information, but their relationship to the PDF specification is not the same as the standards documents identified as normative by Adobe. The Graphics Gems series books could be replaced by substitutes and still leave the PDF specification interpretable in a way that is not true if IEEE-754 vanished from the Earth and all knowledge of it was lost. All of these texts, however, are at least referenced by the PDF specification itself. Their status as representation information is therefore at least subtly different than, say,Visual Color and Color Mixture Cohen, 2001, which while it provides the semantic information necessary to understand the matrix algebra operations for color space transformation employed within the PDF specification, is not referenced by that document at all. All of these ancillary texts might be considered representation information for the PDF specification; but their status as such is varied, and the application of a common ontological predicate (http://people.lis.uiuc.edu/~jmcdonou/PVW2.owl#hasRepresentationInformation) to link them all to the PDF specfication hides a great deal of semantic nuance.
Also somewhat disturbing in our examination of the XML preservation packages we were developing was the realization of the situationally-determined nature of the truth of assertions regarding representation information and context information. Consider a PDF instruction manual that comes with a game; while the PDF specification allows for embedding of JPEG image data within a PDF file, this particular file does not use JPEG data for its images. If we ask, 'does this particular file have the ISO/IEC 10918-1:1994 ISO, 1994 standard which defines the JPEG data format as part of its representation information network,' the answer is clearly 'no.' You do not need ISO/IEC 10918-1:1994 to decode the manual. However, if we ask instead, does the representation information network for the PDF specification include ISO/IEC 10918-1:1994,' the answer is 'yes.' ISO/IEC 10918-1:1994 is cited as a normative reference by the PDF specification, "indispensable" for the application of the PDF specification itself.
The OAIS Reference Model discussion of representation information seems to imply that representation information networks are specific to a data file, so for PDF files as a class, there can be no a priori determination of the representation network necessary for decoding a file. Identification of a representation information network has to be conducted on a file-by-file basis, significantly increasing the knowledge management burden on digital preservationists. Given that tools for automating the identification of representation networks for a file at this level of detail are non-existent, this is an unhappy realization for a digital preservationist. Equally unhappy, however, was the realization that the semantic interpretation we had been applying to our "hasRepresentationInformation" term was somewhat at odds with how the OAIS Reference Model describes representation information as a concept. Our understanding aligned with asking the question of "in the abstract, does Document A have Document B as representation information" and not "in the case of this specific file, does Document A require Document B as representation information." This is clearly problematic at a variety of levels, not the least of which is 'how is a user in 100 years, seeing our ontology applied in a preservation environment, going to know that our interpretation of representation information is at variance with the original standards document which proposed the concept?'
One final problematic arose in applying our ontology in packaging of computer games, with respect to the idea of context information in the OAIS Reference Model. The OAIS Reference Model defines context information as "the information that documents the relationships of the Content Information to its environment. This includes why the Content Information was created and how it relates to other Content Information objects." The problem with this definition with respect to games is that the task of documenting a game's relationship to its environment and identifying how it relates to any and all other content information objects is a nigh-infinite task. If I take a game like the original DOOM, do I need to document it's relationship to predecessor and successor games that employed similar themes (e.g., MindScape's The Colony and Valve's Half-Life), to alternative media presentations of the story (ab Hugh and Linaweaver, 1995 and Bartkowiak, 2005)), to other games from its manufacturer, id Software, to legal documents in a lawsuit filed by relatives of the survivors of the Columbine shootings against id Software? To archives of speedrun competitions using the game? To fan websites? To the approximately 4,000 journal articles retrieved from Google Scholar in response to a search for "doom 'computer game'"? Completely specifying context is an impossible task, and for any given user, much of that context will be irrelevant.
This leads to a problem similar to the representation information problem. A given user will consider some documents as valuable context information and others as irrelevant, but those judgements will vary from user to user. Given this, any record I make as preservationist of context information is going to have an implied semantic connotation beyond simply 'this is context information' of 'this is what we believe is the important context information.' And implicit in that relative value judgment will be a number of assumptions about what sort of scholarly activity is important (and what is necessary to support that scholarly activity). Scholars will interpret the application of a 'hasContextInformation' predicate to information they need for their work as an endorsement of the significance of their field of study; similarly, finding that information they need doesn't receive recognition as context information (and information they consider irrelevant has) will also speak to the significance (or lack thereof) of their work. Languages, including ontological languages are not written or read in a vacuum, and a seemingly simple predicate such as 'hasContextInformation' can easily acquire additional layers of meaning in use.
A common thread running through all of these problems is the relationship between terms' meanings and the contexts in which they're applied. Each application of an ontological term such as 'hasRepresentationInformation' or 'hasContextInformation' can reinforce or redefine that term's meaning. Our research team had lengthy discussions on issues such as whether something was really representation information or whether it was something else. Should documentation of the architecture of an Intel processor-based personal computer be considered representation information for a binary executable (for surely apprehension of the full system architecture is necessary to completely understand how an op code will be interpreted) or is it contextual information (for surely historians of tomorrow will want to understand the complex interplays between game design and computer architectures)? Or is it both? Or neither? These discussions were the beginning of a process by which the meaning of our ontology's terms were defined and refined by our somewhat localized community of practice, but they were not the end of that process. Like any other language development process, the application of our ontology will be a process in which our community negotiates to achieve a shared, if imperfect, common understanding of the ontology's terms' meaning.
Viewing the semantics of an OWL ontology as an example of language activity in which meaning of terms is socially constructed and variable is fine if you are a linguist interested in pragmatics, but somewhat problematic from the point of view of a preservationist; if the meaning of ontological terms is socially constructed, I cannot assume that this process will magically end when the ontology is published; it is more likely that the meaning derived from these terms by humans will continue to be redefined and reinterpreted. What if the meaning of the ontological terms I have created suffer linguistic drift over the next 100 or 500 years? If I'm employing an ontological language to convey to users what documents they need to obtain to decipher a bit stream, how can I help insure that they understand the language I'm using to express those thoughts today, when I cannot be assured they will be interpreting the terms in the same way I am?
Knowledge Scaffolding: XML as a Tool in the Social Construction of Knowledge
Dervin, 2003 proposed that information design "is, in effect, metadesign: design about design, design to assist people to make and unmake their own informations, their own sense." Our efforts to employ XML and OWL technologies as the basis for long-term knowledge preservation and management conform to Dervin's definition of information design. We are trying to assist others in the future make sense of bit streams from the present day, both at the basic technical level of learning how to decode a particular bit stream and at the higher intellectual level of understanding a game's role and value within our culture. Enabling this, however, requires that those in the future be able to make sense of our knowledge management framework. How do we approach the problem of enabling people to make sense of digital information when the frameworks we use for managing that information involve terminology that are problematic to interpret even for a contemporary user, with meanings that appear to be far less stable than we might hope?
I cannot claim to have the solution to these problems, but the nature of the problems we experienced with Preserving Virtual Worlds I believe points to at least a few parameters that need to be part of our solution. The first is to develop an account of meaning in XML and other markup technologies that more adequately handle the issue of meaning in context. Work on relevance theory (see, for example, Wilson and Dan Sperber, 2004 and various other theoretical takes on pragmatics and sociolinguistics (Barron and Schneider, 2009, Linell, 2006) offer what may be a more productive account of semantics for several reasons:
they abandon the simplistic encode/transmit/decode model of communication set forth by Shannon and Weaver,1949 in favor of a model that allows for linguistic inputs as one of multiple inputs to a message's recipient;
in so doing, they explicitly account for an individual's context (social and otherwise) as part of the process of inferring meaning from linguistic inputs; and
they are founded on a model in which, as Linell observes, "knowledge of language is emergent [emph. original] from practice" and practice is inherently social, which is to say meaning is constructed through a variety of communicative interplays in society, and is not innate or static.
These characteristics of much modern work on pragmatics align nicely with our observations of ontology use in the Preserving Virtual Worlds project. Specific ontological terms in our project originated within the Open Archival Information System Reference Model, but as we applied our ontology and packaging standards, we found that our practice was somewhat at odds with the original formal definition, which is to say, our team constructed a somewhat different meaning for the terms than they originally had. Linguistic theory which accounts for and explains these linguistic practices in a manner more sophisticated manner than labeling them 'noise' or 'transmission error' is necessary to plan for preservation systems which mitigate misunderstandings caused by linguistic change over time.
For some, this may sound perilously close to an endorsement of tag abuse. I would argue that tag abuse is simply another name for language praxis, and that if XML systems are to be useful in information systems designed for extended use, we need to account for that activity in knowledge management systems design. Failure to do so will lead to brittle systems which do not perform well outside of a very limited setting.
An account of pragmatics sufficient to at least begin thinking about the concept of a pragmatics of XML is, then, part of the solution we need. Another part is a better understanding of how to take that knowledge and apply it in information systems design. Alan Rector's piece on problems in the development of medical terminology provides an interesting example of failure analysis in this domain, and similar interesting work on classification systems has been done by Bowker and Star, 1999 and others, but we need more work on how to account for terminological fluidity in information systems design.
Within the domain of digital preservation, I believe our experience with OWL and XML packaging does make one thing clear: the users of the future cannot and will not be able to decipher the meaning of our ontological terms separate from seeing their application in practice, and if semantic variation and change over time is going to be made visible and explicit to our users, then a fundamental aspect of digital preservation systems must become preserving information about the preservation system itself, including the XML languages used within the system. And given the social nature of language use, such preservation cannot take the form of simply saving versions of specific vocabularies as they are changed over time. We will need to preserve records of the actual use of markup languages over time so that if necessary users can examine the historical record of our markup languages' development.
Those of us working in digital preservation need to remember that our job is to support individuals in making sense of the historical record of digital materials, that such sense-making is fundamentally social in nature and involves drawing upon a number of sources, and that making sense of the historical record requires making sense of what preservationists have done with the historical record. What I am arguing for here is an effort to preserve information about our preservation systems that extends well beyond traditional archival notions of provenance. We need a record of not only the specific actions we have taken with a historical object, but documentation of the larger socio-technical systems in which an object has been housed, categorized, discussed, reformatted, linked and disseminated. Making sense of the past will require making sense of the preservation systems, and without a record of those systems and their use, including markup technologies, such a sense-making project is inconceivable.
The research described in this paper was made possible by the Institute of Museum & Library Services' Grant LG-06-10-0160-10.
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