The urgency for preservation of digital material becomes widely recognised not merely in scholarly circles. Anybody who creates and uses digital information also should be concerned with the maintenance of these documents. Retaining digital documents is not done with finding a suitable location for storage. Conventional media such as print or paintings are durable enough to be maintained for decades, even centuries in this fashion. Yet, this does not apply for digital media.
The inherent fragility of digital information is due to several reasons. Electronic storage media is prone to decay within time-scales that cover not even one human generation by far. Additionally, media formats become obsolete rapidly due to advances in technology. In order to elude these impediments, the data has to be migrated to other types of media, before the information becomes permanently irretrievable. These two issues have already been discussed in the Section 2.3.2, Longevity of archival media. However, physical decay of media and technological obsolescence cover only a minor part of the difficulties in guaranteeing long-term preservation.
Preserving the pure data, the bit stream, does not guarantee that the information it holds is preserved along. Software is necessary for making the digital documents accessible and meaningful. Only by running these application programs the data can be decoded and accurately displayed. Yet, software underlies an evolution itself, perhaps at an even larger scale than hardware does due to advances of technology being made. Programs are supplemented by others that offer greater convenience, even data formats in a single application are adapted in order to extend their functionality. Often a new version of the same software is not similar enough to its predecessor to ensure no information is lost on conversion [Rus99]. Exacerbating this is the dependency of application software on the underlying operating system, which in turn requires a specific type of hardware platform. Consequently, although the software being just another stream of bits can be preserved together with the documents, a future system will most probably not be able to run the program on its computer hardware. In a nutshell, the following chain of dependencies exists:
data => application programs, plug-ins => operating system => hardware
Long-term preservation is a process which ensures the usability and integrity of digital material retaining its meaning and, where possible, re-create the original form and function of the object to establish its authenticity, validity and evidential value. Those concerned with the preservation of digital materials must look far into the future to guarantee that access to material is ensured over centuries [Rus99]. Also, digital preservation raises organisational, legislative and economic concerns. Yet, these issues can hardly be addresses in the absence of a sound, accepted technical solution to the digital longevity problem [Rot99]. Physical decay of media, technology obsolescence, development of software and its dependency on hardware - for all those obstacles in long-term preservation of digital material differing solutions have been proposed, which, in a nutshell, are