2. Methods
2.2. General DNA manipulation
T
he purpose of this chapter is to help those who may be responsible for preserving digital heritage materials understand the basic nature, objectives and strategies of digitalpreservation. These are important understandings for managers as well as those designing and implementing programmes.
7.2 In a nutshell
Digital preservation consists of the processes aimed at ensuring the continued accessibility of digital materials. To do this involves finding ways to re-present what was originally presented to users by a combination of software and hardware tools acting on data. To achieve this requires digital objects to be understood and managed at four levels: as physical phenomena; as logical encodings; as conceptual objects that have meaning to humans; and as sets of essential elements that must be preserved in order to offer future users the essence of the object.
M ANAGEMENT PERSPECTIVE
7.3 Digital preservation
Digital preservation can be seen as all those processes aimed at ensuring the continuity of digital heritage materials for as long as they are needed. The most significant threats to digital continuity concern loss of the means of access. Digital materials cannot be said to be preserved if the means of access have been lost and access becomes impossible. The purpose of preserving digital materials is to maintain accessibility: the ability to access their essential, authentic message or purpose.
7.4 A „performance‟ approach to digital preservation
There is an underlying similarity in the way digital objects are accessed in the present, and how they will be accessed in any future use. In both cases, access can be seen as a
performance. 1 Digital objects are made accessible by applying software and hardware tools to data in orderto create a presentation or performance that has meaning to a user. It may be the presentation
1 This concept is well discussed in Heslop H, Davis S (2002) (unpublished). An Approach to the Preservation of Digital Records. National Archives of Australia, Canberra
of a word processing document, or a piece of recorded audio, or a Web page, or results from a database query, or any other kind of digital object depending on the way the data is encoded and on the actions the tools are programmed to perform. We expect that if we apply the same tools to the same data we will get a repeat performance each time.
Digital preservation must work in the same way, somehow re-presenting what are judged to be the essential elements of the original performance when required at some later time.
Conceptualised in this way, digital preservation can be seen as straightforward. Indeed, copying data from carrier to carrier, and providing the right tools to recreate the intended performance will preserve continuity of access to most digital objects. However, this simple model encompasses great complexities: it may be hard to define the performance that must be re-presented; it is usually difficult to work out what tools are needed once the original ones have been lost; the tools themselves typically rely on other tools that also may have been superseded; and it may be difficult to find tools that will create the required performance in a reliable, cost-effective and timely way, especially in the context of many thousands, millions or more of digital objects. Despite such underlying complexities, the performance model helps in recognising what digital preservation programmes must aim for: the best means of re-presenting what users needs to access.
7.5 Understanding the materials being preserved
Preservation programmes must deal with digital objects in four guises:
As physical objects, consisting of „inscriptions‟ (usually binary states of „on-ness‟ or
„off-ness‟) on carrier media such as computer disks or tapes. (Despite the impression of that they exist in „cyberspace‟, even online resources must exist on physical carriers somewhere)
As logical objects consisting of computer readable code, whose existence at any particular time depends on the physical inscriptions but is not tied to any particular carrier
As conceptual objects that have meaning to humans, unlike the logical or physical objects that encode them at any particular time. (This is recognisable as the performance presented to a user)
As bundles of essential elements that embody the message, purpose, or features for which the material was chosen for preservation.
This multi-layered nature of digital objects has profound implications for digital preservation.
Preservation means different things for each layer. 2
Preservation programmes for non-digital heritage have traditionally worried about preserving the physical object as the embodiment of the object‟s meaning. However, individual physical manifestations of a digital object are almost inevitably lost, one after another, because the 2 This concept is adapted from Thibodeau K (2002).
media used for physical storage are typically unstable and liable to short-term deterioration.
Preservation requires a succession of data transfers from one physical carrier to another.
Despite this shift in focus from the physical object to a conceptual object inherent in digital preservation, it must never be forgotten that digital objects cannot survive without some kind of appropriate physical form.
The logical encoding normally has a much longer life than any particular physical inscription, but it is by no means sacrosanct. As the layers of technology used for access – hardware such as computer processors, disk drives and peripheral equipment, and many layers of software such as operating systems, specific applications, and presentation tools - become obsolete, it may be necessary to change the logical encoding so that it can present the same conceptual object using different technology. The conceptual object is the ultimate focus of preservation concern; as noted above, it is at this level that digital objects convey meaning to human users.
However, for most digital objects there is a further layer that must be considered. Many objects consist of several elements, some of which are more important than others in carrying the object‟s essential message. Preservation programmes have to decide which sub-set of elements should be preserved for re-presentation to users.
7.6 Strategies for preserving digital materials
Digital preservation involves choosing and implementing an evolving range of strategies to achieve the kind of accessibility discussed above, addressing the preservation needs of the different layers of digital objects. The strategies include:
Working with producers (creators and distributors) to apply standards that will prolong the effective life of the available means of access and reduce the range of unknown problems that must be managed
Recognising that it is not practical to try to preserve everything, selecting what material should be preserved
Placing the material in a safe place
Controlling material, using structured metadata and other documentation to facilitate access and to support all preservation process
Protecting the integrity and identity of data
Choosing appropriate means of providing access in the face of technological change
Managing preservation programmes to achieve their goals in cost-effective, timely, holistic, proactive and accountable ways.
REFERENCES
– where to look for more information Cross referencesSome issues and concepts in this chapter are discussed further elsewhere in these Guidelines:
Essential elements see chapters 12 and 17
Strategies for preservation see appropriate chapters in section 3 Offsite references (all links viewed March 2003)
Heslop H., Davis S. (2002) (unpublished). An Approach to the Preservation of Digital Records. National Archives of Australia, Canberra
Thibodeau K. (2002). Overview of Technologic al Approaches to Digital Preservation and Challenges in Coming Years. In: The State of Digital Preservation: An International
Perspective – Conference Proceedings, Documentation Abstracts, Inc., Institutes for Information Science, Washington, D.C., April 24025, 2002. Council on Library and Information Resources, Washington, D.C.
http://www.clir.org/pubs/reports/pub107/thibodeau.html