several recurring suggestions for international cooperation in developing effec- tive strategies for long-term preservation’ (Beagrie, 2003, p.6), and the example of a preservation technology watch was offered.
4) Standardize formats
5) Persistent object preservation (Walton, 2003, pp.5–7).
Walton discusses digital records within the context of the InterPARES Project’s concern to establish the characteristics of authentic digital records. His five cate- gories fall into two types: those concerned primarily with the technology, and those primarily with the data formats. The two categories of strategies that focus primarily on the technology are ‘preserve old technology’ and ‘emulate old tech- nology’. Both of these leave ‘the digital components of the record exactly as received’, concentrating instead on the technology (software, operating system, hardware) that is required to reproduce the digital object. Walton suggests that the merit of this approach is that authenticity is much more likely to be main- tained, but it is impractical because ‘it requires multiple solutions (that is, one for every distinct category of equipment being preserved) rather than a single one’, and writing emulator software is labour-intensive and costly. There is also, for the first method, the major issue of technology obsolescence.
Walton’s three categories that focus primarily on the data formats are ‘format migration’, ‘standardize formats’, and ‘persistent object preservation’. Format migration attempts to ensure that the data remains ‘live’ by continually updating it so it ‘can be reproduced using current technology’. Format standardization converts the data to a limited number of non-proprietary formats (for example, TIFF for images, XML for text) and there is thus no need to rely on one tech- nology to read them, as many kinds of software can do this. Both of these categories minimize the need to rely on software and hardware that will become obsolete, but both require high levels of resources to manage ongoing migra- tion projects or convert to standard formats. Walton’s fifth category, ‘persistent object preservation’, attempts to separate data completely from the technology.
‘All aspects of the record are abstracted, or represented by other data, to the highest possible degree in order to render them independent of any specific technology.’ Digital materials are described in terms that are independent of software or hardware: an example is electronic records converted to XML and then encapsulated with metadata. Walton notes, however, that ‘its operational viability remains to be demonstrated’. (All quotes in the preceding two para- graphs come from Walton, 2003, pp.5–7.)
Walton’s approach is based on distinguishing between data preservation and maintaining the technology. Thibodeau extends this approach. He describes a
‘preservation spectrum’, which has at one end ‘preserve technology’, and at the other ‘preserve objects’. He suggests principles to guide us in positioning ‘candi- date methods’ across the spectrum:
• On the ‘preserve technology’ end of the spectrum, methods that attempt to keep data in specific logical or physical formats and to use technology orig- inally associated with those formats to access the data and reproduce the objects
• In the middle of the spectrum, methods that migrate data formats as tech- nology changes, enabling use of state-of-the-art technology for discovery, access, and reproduction
• On the ‘preserve objects’ end of the spectrum, methods that focus on preserving essential characteristics of objects that are defined explicitly and independently of specific hardware or software (Thibodeau, 2002).
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Typologies of principles, strategies and practices 111
But these are not sufficient: also to be taken into account is the suitability of a method for the material being preserved. Some methods are general, and others apply only to specific technologies. Thibodeau accordingly proposes that the method’s suitability (which he designates as applicability) needs also to be taken account of. He has suggested a graph of preservation methods, reproduced here as Figure 6.3.
Rothenberg takes a different tack. His classification is based on the extent to which strategies and practices are complete. His ‘overview of proposed approaches to preservation’ (Rothenberg, 2003) has three categories: non- solutions, partial solutions, and potentially complete solutions. Non-solutions include ‘do nothing’ and digital archaeology. The longer list is of partial solutions:
Save page-images of artifacts, Extract and save ‘core contents’ of arti- facts, Translate artifacts into standard or ‘canonical’ forms (without migration), Rely on ‘viewer’ programs to render obsolete formats in the future, Save metadata to help interpret saved bit streams (‘assisted archaeology’), Save source-code of rendering software (for future reverse-engineering).
It is Rothenberg’s potentially complete solutions that we should pay most attention to. It has only three entries, suggesting that there is still significant work to be done to develop a range of viable strategies and practices:
• Formalization (replace artifacts by formal descriptions of themselves)
• Migration (repeatedly convert artifacts into new artifacts)
• Emulation (run original rendering software on virtually recreated hardware) (Rothenberg, 2003).
The categorization in the UNESCO Guidelines (2003) is different again, being based in part on the length of time that the strategies are likely to provide a
112 Overview of Digital Preservation Strategies
Objective Digital preservation methods
ApplicabilitySpecificGeneral
Preserve objects Preserve
technology Maintain
original technology
Universal virtual computer
Object interchange
format Persistent
archives
Programmable chips
Re-engineer software
Version migration
Format standardization
Rosetta Stone translation
Typed object conversion Virtual
machine Emulation
Viewer
Figure 6.3 Digital Preservation Methods (From Thibodeau, 2002, p.19)
viable result, and in part on what stage of the preservation process the principal effort is needed:
‘Investment’ strategies (primarily involving investment of effort at the start):
• Use of standards
• Data extraction and structuring
• Encapsulation
• Restricting the range of formats to be managed
• ‘UVC’ (Universal Virtual Computer) approach
Short-term strategies (likely to work best over the short-term only):
• Technology preservation
• Backwards compatibility and version migration
• Migration (which may also work over longer periods)
Medium- to long-term strategies (likely to work over longer periods):
• (Migration)
• Viewers
• Emulation
• (UVC approach) Alternative strategies:
• Non-digital approaches
• Data recovery
• Combinations (UNESCO, 2003, pp.126–127).
These Guidelinesdistinguish between general principles, and specific strategies and practices. For example, they suggest that the principles (which they label as ‘strategies’) for preserving digital materials include:
• Working with producers (creators and distributors) to apply standards
• Recognising that selection of material to be preserved is essential
• Placing the material in a safe place
• Using structured metadata and other documentation to control material, facil- itate access and support preservation processes
• Protecting the integrity and identity of data
• Effectively managing preservation programs (UNESCO, 2003, p.37).
Nine further specific principles on which current strategies and practices are based are articulated in the UNESCO Guidelines. These can be grouped into four categories: conversion to analogue form, maintaining the original format, stan- dardizing formats, and migration. Conversion to analog forminvolves converting data to a human readable form on a stable carrier such as paper, film or metal.
Maintaining the original format is the most popular category, with four strategies:
‘Making the data ‘self-describing’ and ‘self-sustaining’ by packaging it with metadata and with links to software that will continue to provide access for some time’; ‘Maintaining the data in its original form . . . and providing tools 1111
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Typologies of principles, strategies and practices 113
that will re-present it . . . using the original software and hardware . . . or using new software that emulates the behaviour of the original software and/or hard- ware’; ‘Maintaining the data and providing new presentation software (viewers) that will render an acceptable presentation of it for each new operating envi- ronment’; ‘Providing specifications for emulating the original means of access on a theoretical intermediate computer platform, as a bridge to later emulation in a future operating environment’. Standardising formatsincludes either ‘creating data in, or converting data to, a highly standardised form of encoding and/or document structure (or file format) that will continue to be widely recognised by computer systems for a long time’, or ‘converting the data to a format where the means of access will be easier to find’. Migration involves converting the data to new formats that can be read by each new technology; it also includes providing the ability for migration on demand ‘by maintaining the data and recording enough information about it to allow a future user or manager to convert it to a then-readable form’ (UNESCO, 2003, p.125). Finally, preservation programmes, these guidelines suggest, must be organizationally viable and financially sustainable, if they are to be effective and credible (UNESCO, 2003, pp.43–44).
This classification is noteworthy because of its emphasis on the combination of strategies and practices. In the absence of a single, universally applicable solution these guidelines support the application of multiple strategies in preser- vation programmes. These are likely to be based on the use of
standards for data encoding, structuring and description that can be expected to remain recognisable for long periods; emulationof obsolete software or hardware in a new environment; and migrationof data from one operating technology to another (UNESCO, 2003, pp.124–126).
The doubt expressed in the Guidelines as to the likelihood of a single solution ever being found is now generally shared. Whatever we might consider as main- stream approaches – the consensus is that these are migration, technology preservation, emulation, and perhaps persistent object preservation – they are not mutually exclusive, and will be deployed according to the kind of digital materials to which they are applied and their access requirements (Smith, 2003, p.43). Rothenberg is also aware of this, noting that
For now, consider using multiple approaches in parallel: Digital archae- ology (for records that are unlikely to be accessed), Page-image techniques (for simple records), Formal methods (when applicable and affordable), Standards (when available), Migration (when it needs to be done anyway, i.e. for active records; or as a stopgap), Emulation (if orig- inal behaviour is needed; oras a cheap backup, to preserve everything) (Rothenberg, 2003).
None of the typologies described here are complete. They do not, for instance, note the principle of redundancy (multiple copies at multiple locations), best exemplified by the LOCKSS Project. However, they allow us to identify some of the principles that are important and to make some judgments about the applicability and viability in the longer term of specific strategies and prac- tices. For example, they help us distinguish more clearly between ‘preserve
114 Overview of Digital Preservation Strategies
technology’ and ‘preserve objects’ approaches, they encourage us to consider the nature of the digital object and the different requirements that each may have, with consequent different ‘best’ strategies and technologies for their preservation, and they suggest that the tools we have available to us at present should be applied in combination.