Preservation metadata
Michael Day
UKOLN, University of Bath
Preservation metadataMichael Day
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Prepublication draft of chapter published in: G. E. Gorman and Daniel G. Dorner (eds.), Metadata applications and management, International Yearbook of Library and Information Management, 2003-2004, London: Facet Publishing, 2004, pp. 253-273. ISBN 1-85604-474-2 (Facet Publishing); ISBN 0-8108-4980-1 (Scarecrow Press).
Ensuring the long-term preservation of information in digital form will be one of the greatest challenges for the information professions in the twenty-first century. While there has been an awareness of digital preservation problems for some time, their importance have recently been magnified because of the increasing dependence of the world on computers and networks. For example, the recent rapid growth in the use of the internet in the past decade has demonstrated, how much it has become, in the words of Manuel Castells, "the fabric of our lives" (Castells, 2001, 1). Chen (2001, 24) has described the digital preservation problem as the "critical, cumulative weakness in our information infrastructure."
In response, things are now beginning to happen. For example, in 2001 the Digital Preservation Coalition was set up in the UK to form the basis of co-operation and already (April 2003) has ten full institutional members and thirteen associates. Also, in February 2003, the US Congress approved the Library of Congress's plan for a National Digital Information Infrastructure and Preservation Program (NDIIPP).
The other chapters in this volume demonstrate the wide range of roles that metadata can serve. Since the mid-1990s, however, there has been a growing awareness of the part that metadata can play in supporting the long-term preservation of digital objects (e.g., Day, 2001). Preservation is integral to some definitions of metadata. For example, Cunningham (2000, 9) defines it as "structured information that describes and/or allows us to find, manage, control, understand or preserve other information over time." This chapter will introduce some proposed digital preservation strategies, noting how metadata comprises a key component of them all. This will be followed by short introductions to the influential Open Archival Information System (OAIS) reference model and a number of other selected initiatives, based on projects originating from national and research libraries, digitisation projects and the archives community. The final sections will highlight some issues and look at what needs to be done in the future.
Digital preservation is both a technical and organisational challenge. The technical problems of relatively short media lifetimes and hardware and software obsolescence coincide with a realisation that the essential malleability of digital information, though useful in many ways, means that it can be difficult to have trust in its authenticity. Digital preservation, however, means much more than just providing solutions to these problems, but refer to the whole "series of actions that individuals and institutions take to ensure that a given resource will be accessible for use at some unknown time" (Smith, 2003, 2).
In practical terms, the successful preservation of digital information is dependent upon organisations identifying and implementing suitable preservation strategies. To date, there have been a number of attempts to characterize these. For example, Lee et al. (2002) have identified four distinct strategies, based on preserving technology, emulation, migration and encapsulation.
With the possible exception of technology preservation, all of these strategies will depend upon the capture, creation and maintenance of metadata, e.g. emulation strategies will depend upon metadata that link the preserved object with an emulator specification. Migration strategies will depend on metadata that record the intellectual and technological contexts of an object's creation and its migration history. Encapsulation techniques presuppose the linking of data object and metadata.
Preservation metadata is all of the various types of data that will allow the re-creation and interpretation of the structure and content of digital data over time (Ludäsher, Marciano and Moore, 2001). Defined in this way, it is clear that such metadata needs to support a number of distinct, but related, functions. Lynch (1999), for example, has written that within a digital repository, "metadata accompanies and makes reference to each digital object and provides associated descriptive, structural, administrative, rights management, and other kinds of information." The wide range of functions that preservation metadata is supposed to fulfil means that defining metadata standards is not a simple task and that most of the currently published schemas are either extremely complex or only attempt to define a basic framework. The situation is complicated further by the perception that different kinds of metadata will be required to support different digital preservation strategies or digital information types.
The Reference Model for an Open Archival Information System (OAIS) is an attempt to provide a high-level framework for the development and comparison of digital archives. Its development was co-ordinated by the Consultative Committee for Space Data Systems (CCSDS) as part of an ISO (International Organization for Standardization) initiative to develop standards that would support of the long-term preservation of satellite data, but it has been developed as a generic model, applicable in any preservation context.
The model aims to provide a common framework that can be used to help understand archival challenges, especially those that relate to digital information. This is the model's real value, providing a high-level common language that can facilitate discussion across the different communities interested in digital preservation. The standard defines a high-level reference model for an OAIS, defined as an organisation of people and systems that have "accepted the responsibility to preserve information and make it available for a Designated Community" (CCSDS, 2002, 1-11).
Figure 1: OAIS Functional Entities. Source: CCSDS, 2002, Fig. 4-1
The OAIS model has a much wider scope than metadata. It defines both a functional model and an information model. The functional model outlines the range of functions that would need to be undertaken by a repository, and defines in more detail those functions described within the OAIS specification as access, administration, archival storage, data management, ingest and preservation planning (Fig. 1). The information model defines the broad types of information (or metadata) that would be required in order to preserve and access the information stored in a repository. However, it is important to realize that the OAIS standard is a reference model, not a blueprint for an implementation. All of the many different communities interested in digital preservation will have to apply the model (including the information model) in their own particular contexts, both organisational and technical.
The OAIS information model defines a number of different Information Objects that cover the various types of information required for long-term preservation. A basic assumption of the model is that all Information Objects are composed of a Data Object -which would typically be a sequence of bits for digital data - and the Representation Information that would permit the full interpretation of this data into meaningful information (CCSDS, 2002, 4-19). The OAIS model then defines four distinct Information Objects.
The OAIS information model sub-divides the PDI into four distinct groups based on categories discussed in the 1996 report of the Task Force on Archiving of Digital Information commissioned by the Commission on Preservation and Access and the Research Libraries Group (CCSDS, 2002, 4-28). The task force wrote that "in the digital environment, the features that determine information integrity and deserve special attention for archival purposes include the following: content, fixity, reference, provenance and context" (Garrett and Waters, 1996). Having already defined Content Information, the OAIS information model divided PDI into the four remaining categories. These are defined as follows:
The OAIS model also defines a conceptual structure for Information Packages. This is viewed as a container that logically encapsulates Content Information and its associated PDI within a single Data Object. Information Packages are defined for submission (SIP), archival storage (AIP) and dissemination (DIP). Of these, the Archival Information Package (AIP) is the most important for digital preservation, as it contains, in principle, "all the qualities needed for permanent, or indefinite, Long Term Preservation of a designated Information Object" (CCSDS, 2002, 4-33). The OAIS information model has influenced the development of a number of preservation metadata element sets and has informed the development of others. In the next section we will introduce some of these initiatives.
To date, the majority of preservation metadata initiatives have originated from three contexts, namely to deal with the preservation concerns of national and research libraries, digitisation initiatives and archives. This section will look at each of these in turn and introduce some of the most prominent initiatives. At this point, however, it should be emphasized that many other metadata standards will contain things that have relevance to digital preservation. For example, the MPEG-7 standard (ISO/IEC 15938) is intended to support the management of audio-visual content, and its description schemes can store information about compression methods, data size, access conditions, etc. (Chang, Sikora and Puri, 2001, 690). The IEEE Learning Object Metadata (LOM) standard (IEEE 1484.12.1-2002) includes elements that describe technical requirements and remarks on installation. It clear that one of the most important challenges of preservation metadata initiatives will be to make best use of all the relevant metadata that exists in other forms.
National libraries and some research libraries have had a traditional interest in preservation. In particular, national libraries are motivated by their statutory obligation to collect and retain copies of certain types of publication in perpetuity. Many have already adapted legal deposit legislation to enable the collection of digital materials. This has led to a growing awareness of the digital preservation problem, including the need for preservation metadata.
One of the first to address these issues was the National Library of Australia (NLA), who from the 1990s onwards developed a practical response to the collection and preservation of Australia's published digital content. Chief amongst these has been the creation of its PANDORA (Preserving and Accessing Networked Documentary Resources of Australia) Archive of online publications. In 1999, the NLA issued an exposure draft of its Preservation Metadata for Digital Collections. This defined 25 high level elements (some with sub-elements) that a digital storage system would need to generate in order to facilitate the preservation management of digital information (Phillips, et al., 1999). The main focus of the NLA element set was pragmatic, based on the library's experiences with PANDORA and other initiatives. While its development had been informed by the OAIS model, Webb (2000, 182) commented that the library's "focus on building and managing an existing archive, rather than developing an universal model for archiving, ... produced some differences in language as well as approach."
Shortly afterwards, another draft element set was published by the UK Cedars (CURL Exemplars in Digital Archives) project (Russell, et al., 2000). Unlike the practical focus of the NLA element set, the Cedars specification was developed to support the development of the project's demonstrator services and as a contribution to the international standardisation of preservation metadata. The element set was described as an 'outline specification' because it only defined the highest levels of the metadata that would be required for a full implementation. Unlike the NLA element set, it arranged elements explicitly using the OAIS information model as a framework. It also attempted not to make too many assumptions about the actual form of the digital objects being preserved or about the 'granularity' of specific objects. It was hoped that the specification would be applicable at any level of granularity, but the authors recognized that the specifics of implementation would need to be the responsibility of individual repositories. The specification also made no assumptions about which particular preservation strategy would be used, although it was understood that this would have an impact on which particular elements would be required. The project team were aware that the proposed metadata element set would not necessarily support all of the wider roles identified in the OAIS functional model, e.g. the administration or data management functions. However, it was recognized that some of the information would be able to be used elsewhere within a preservation system, e.g. parts of the Provenance Information could help support administrative functions like rights management. In fact, the Provenance Information defined in the Cedars outline specification contained a number of elements specific to rights management that went well beyond the assumption of the OAIS model that provenance is primarily concerned with supporting the integrity of a given Data Object. This reflected the difficulty of defining metadata schemas where the same information can be used by functionally different parts of a system.
The NEDLIB (Networked European Deposit Library) project developed a deposit system for electronic libraries (DSEP) based on the OAIS model and as part of this tried to define the minimum metadata that would be necessary for preservation management (Lupovici and Masanès, 2000). Like the Cedars specification, the NEDLIB element set explicitly adopted the terminology and structure of the OAIS information model. The element set was much smaller than that proposed by Cedars (18 elements with 38 sub-elements), partly because it was focused only on the identification of mandatory elements, but also because it was only concerned with defining the metadata that would address the problem of technological obsolescence, not with data that would be used for descriptive, administrative or legal purposes.
By 2000, there was a need to find a way of bringing these three schemas together into a single framework. In response to this challenge, OCLC Online Computer Library Center and the Research Libraries Group (RLG) convened an international working group to consider the further development of preservation metadata. The group first produced a state-of-the-art report. This described the OAIS reference model in some detail - noting its importance as a conceptual framework - and provided a comparison and mapping between the NLA, Cedars and NEDLIB element sets (Working Group on Preservation Metadata, 2001). The working group next set to work on producing a fresh metadata framework. This resulted in two proposals for Content Information and Preservation Description Information (PDI) that were collected together and published in June 2002 as: A metadata framework to support the preservation of digital objects (Working Group on Preservation Metadata, 2002).
The framework produced by the working group effectively supersedes the element sets developed by the older initiatives, and represents a good starting point for future practical implementations of preservation metadata. Like the Cedars and NEDLIB element sets, the OCLC/RLG metadata specification explicitly uses the OAIS information model as its framework. The recommendation for Content Information includes the Content Data Object (bit streams) and as Representation Information, both elements that relate to the object itself (e.g., file descriptions, significant properties) or its hardware and software environment (e.g., operating systems). The PDI recommendation defines elements according to the OAIS headings of reference, context, provenance and fixity. The Provenance Information is organized on event-based model, defining generic elements associated with processes that might be carried out on the Content Digital Object, e.g. transformations undertaken at ingest, format migrations, etc. It was not envisaged that the whole metadata framework would be utilized for each and every object within a preservation system, but that metadata would be implemented at varying levels of specificity. The working group noted that the elements were not necessarily atomic and that it was "easy to imagine cases where the needs and characteristics of particular digital archiving systems ... [would] require deconstruction of these elements into still more precise components." In 2003, a new group called PREMIS (Preservation Metadata: Implementation Strategies) was formed by the same sponsoring organisations to look at the metadata framework and investigate in more detail the practical aspects of implementing preservation metadata in digital preservation systems.
A step in the same direction has also taken by the National Library of New Zealand (NLNZ), which published its own preservation metadata specification in November 2002. The metadata is intended to accompany Preservation Masters kept by the library and refer to four entities, the objects themselves, the processes undertaken on them, individual files and administrative metadata (National Library of New Zealand, 2002). The framework specification also includes a mapping of the NLNZ element set to the OCLC/RLG framework, the OAIS information model and the draft NISO metadata standard for digital still images.
Some of the first projects and initiatives to consider the need for preservation metadata were those involved in digitisation. A heavy financial investment in digitisation meant that there was a need to consider the long-term management of digitized materials. As part of this, there was an awareness that some important information could only be captured at the time of digitisation (e.g., Kenney & Rieger, 2000). In response, the RLG constituted a working group on the Preservation Issues of Metadata in 1997 to help identify the kinds of information that would be required to manage a digital image master file over time. The final report of the working group defined sixteen metadata elements (RLG, 1998). A more complex metadata scheme was developed by the Making of America II (MOA2) testbed project (Hurley, et al., 1999), the general framework of which has recently been taken up by the METS initiative.
METS, the Metadata Encoding & Transmission Standard (www.loc.gov/standards/mets/), is an attempt to provide an XML-based document format for encoding metadata to aid the management and exchange of digital library objects. The initiative has adapted the XML Document Type Definition developed by the MOA2 project to create an XML schema. This schema separates metadata into four sections. These are 'descriptive metadata,' 'administrative metadata,' 'file groups' and 'structural maps,' the last two of which are intended to group together all of the files that make up a particular digital object and to link content and metadata to a particular structure. The administrative metadata section is intended to store technical information about the file, as well as information about intellectual property rights held in the resource, the source material, and provenance metadata that records relationships between files and migrations. Broadly speaking, the METS schema provides an XML-based container that could be used to store much of the metadata defined in preservation metadata specifications like that published by the Cedars project. Also, a document fully encoded in METS could easily be viewed as a conceptual Information Package, as defined by the OAIS model.
As part of a separate initiative, a specification of Technical metadata for digital still images is currently under review as a NISO (National Information Standards Organization) draft standard for trial use (NISO Z39.87-2002 AIIM 20-2002). The development of this standard first grew out of an 'Image Metadata Workshop' held in 1999, sponsored by NISO, the Council for Library and Information Resources (CLIR) and the RLG. The draft standard is not intended to duplicate work on descriptive metadata schemas, but to help define a standardized way of recording the technical attributes of digital images and the production techniques associated with them. The data dictionary includes elements that will record detailed information about images themselves (e.g., formats, compression, etc.), the image creation process, some quality metrics, and any change history (e.g., migrations). No particular encoding of the elements is recommended, although the Network Development and MARC Standards Office of the Library of Congress maintains an XML schema implementation of it called MIX (www.loc.gov/standards/mix/). Development of the standard is particularly based on the experiences of digitisation centres. If and when it is adopted as a standard, it will be of particular use for helping to support the long-term preservation of the products of digital imaging projects.
The archives and records professions have also been investigating what metadata might be required to support the long-term preservation of digital objects. As might be expected, their primary focus is on records, defined by ISO 15489 as "information created, received, and maintained as evidence and information by an organization or person, in pursuance of legal obligations or in the transaction of business" (Healy, 2001, 138). Recordkeeping metadata specifications, therefore, tend to have a strong emphasis on the development of systems that ensure the authenticity and integrity of electronic records.
There have been a number of initiatives that have attempted to identify and define the basic requirements for recordkeeping metadata. One of the first was based on the Business Acceptable Communications (BAC) model developed by the Functional Requirements for Evidence in Recordkeeping project (the Pittsburgh Project). This proposed a metadata structure that would contain a 'handle layer' for basic discovery data while other layers would store information on terms and conditions of use, data structures, provenance, content and the use of the record since its creation (Bearman and Sochats, 1996). It was envisaged that much of this information would be automatically generated a the time of creation, would be directly linked to each record, and would be able to describe the content and context of the record as well as enabling its decoding for future use (Bearman & Duff, 1996). Duff (2001, 292) has since commented that a large proportion (44%) of the elements defined in the BAC model related to structural information, e.g. the technical information that a computer would use to render a file.
The Pittsburgh Project inspired the development of a whole new series of recordkeeping metadata initiatives, especially in Australia. For example, in 1999 the National Archives of Australia (NAA) published a Recordkeeping Metadata Standard that defined the metadata that the NAA recommended should be captured by the recordkeeping systems used by Australian government agencies (NAA, 1999). Another significant development was the development of the Victorian Electronic Records Strategy (VERS). This defined a self-documenting exchange format (the VERS Encapsulated Object) that permitted the transfer of record content (and metadata) over time (Public Record Office Victoria, 2000). Waugh et al. (2000, 179) describe the function of encapsulation in VERS as wrapping the information that needs preservation with the metadata that describes aspects of this information. At the current time, the VERS Encapsulated Object is implemented as an XML object, chosen because this can be read using basic text editing tools.
All of these developments have been informed by the development of a framework known as the Australian Recordkeeping Metadata Schema (RKMS) by a research project that was led by Monash University. The project, amongst other things, attempted to specify and standardize the whole range of recordkeeping metadata that would be required to manage records in digital environments (McKemmish, et al., 1999). The RKMS also was concerned with supporting interoperability with more generic metadata standards like the Dublin Core and relevant resource discovery schemas like the AGLS Metadata Standard. The RKMS defined a highly structured set of metadata elements conforming to a data model based on that developed for the Resource Description Framework (RDF). The schema was designed to be extensible and to be able to inherit metadata elements from other schemas.
The emphasis of the RKMS on interoperability reminds us that there is a need for recordkeeping metadata standards (as with all preservation metadata) to be able to interoperate with a wide variety of other schemas. Hedstrom (2001, p.247) reminds us that metadata are expensive to create, capture and manage, noting that there may be a need to identify which aspects of existing metadata standards could (possibly with adaptation) be used to support recordkeeping requirements.
Compared with other areas of metadata deployment, preservation metadata standards are still very much in the early stages of their development. The first generation of library-based element sets were published less than five years ago and there is still not enough feedback from practical experience to see whether they can solve the problems that they claim to address. The metadata framework published by the OCLC/RLG working group represents a good attempt at consensus, but needs considerable work before a version could be implemented for particular formats or repositories. It will be interesting to see how the PREMIS working group will address the implementation issue. Some recent criticism of the OCLC/RLG metadata framework suggests that there may be a need to revisit first principles. For example, Hofman (2002, 16) notes that the metadata framework, as with the OAIS information model, omits information on preservation strategies, policies and methods. There may be, therefore, a need to link the elements defined in the framework with the preservation processes that they intend to support.
The multiplicity of initiatives and specifications available makes for a good deal of confusion. There will be an ongoing need to analyse how new standards like the National Library of New Zealand standards framework, METS, NISO Z39.87 or the XML DTD for e-journals proposed by Harvard University Library (Inera, 2001) fit into the rapidly evolving 'landscape' of preservation metadata.
One key concern will be interoperability, e.g. standards that will permit the easy exchange of preservation metadata (or information packages) between repositories. At the present time, it seems unlikely that it would be possible to develop a single preservation metadata schema, but some kind of agreement on a baseline exchange format would be useful, possibly based on a schema like METS.
In addition, there will be a need for some kind of interoperability with the range of metadata formats developed for other purposes, but which would provide useful information in a preservation system, e.g., descriptive data from MARC21 or Dublin Core databases, rights metadata, etc. It is interesting that both the Cedars specification and the OCLC/RLG framework defined an 'existing metadata' element, indicating the importance of capturing what metadata already exists, wherever this is possible.
The complexity and highly technical nature of preservation metadata suggest that it will be expensive, especially where interventions in the creation and maintenance processes are required. There may be ways, however, of reducing some of these costs. One way, for example, would be to learn from the experiences of library cataloguing and to try to minimize the duplication of effort through co-operation.
In a digital preservation context, minimising duplication will in the first place depend upon timely information being available about which resources digital repositories have attempted to preserve. Secondly, a repository might also be able to reduce costs by partially automating the creation of metadata, wherever this is possible. So, for example, it would be useful if the systems that will need to be developed to facilitate the ingest or migration of digital objects can automatically output metadata about the processes being carried out, and the people and organisations that have authorized them. Thirdly, some thought should also be given to how best digital repositories should deal with any metadata that already exists. In the longer term, it may be useful to open a dialogue with the creators and distributors of digital objects concerning the nature of the metadata they create. If they were able to adopt metadata strategies conforming to the best practice for preservation metadata, there would be potential cost-savings for repositories. It is also worth noting that any significant time delay between the creation of a digital object and its ingest into a repository may have adverse cost implications, as there is a possibility that some significant information will be lost.
Generating and maintaining preservation metadata is likely to be expensive but is, however, a prerequisite of ensuring successful digital preservation. The difficulty of preserving digital objects without metadata may mean that it is ultimately a cheaper and more effective option than the alternative. Chen (2001, 26-27) has written, "although more semantics in metadata will increase costs, it will minimize human intervention in accessing data; seamless support, transition of stewardship and lifetime maintenance will improve."
Future work on preservation metadata will need to focus on several key issues. Firstly, as suggested earlier, there is an urgent need for more practical experience of undertaking digital preservation strategies. Until now, many preservation metadata initiatives have largely been based on theoretical considerations or high-level models like the OAIS. This is not in itself a bad thing, but it is now time to begin to build metadata into the design of working systems that can test the viability of digital preservation strategies in a variety of contexts. This process has already begun in initiatives like VERS and the 'self-validating knowledge-based archives' researched by the San Diego Supercomputer Center (Ludäsher, Marciano and Moore, 2001). Hopefully, the work of the new PREMIS working group will also help move the OCLC/RLG metadata framework towards implementation.
A second need is for increased co-operation between the many metadata initiatives that have an interest in digital preservation. This may include the further comparison and harmonisation of various metadata specifications, where this is possible. The OCLC/RLG working group is an example of how this has been taken forward within a particular domain. There is a need for additional co-operation with recordkeeping metadata specialists, computing scientists and others in the metadata research community.
Thirdly, there is a need for more detailed research into how metadata will interact with different formats, preservation strategies and communities of users. This may include some analysis of what metadata could be automatically captured as part of the ingest process, an investigation of the role of content creators in metadata provision, and the production of user requirements.
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