This is the subject line of a thread that's been running on the XML4LIB email list over the last couple of days. The question has been around almost as long as XML itself, but MARC is still very much with us.
Several writers in the thread argue that the question is wrong: XML cannot replace MARC because they are different things. For me, though, that confuses three different components in the MARC world: the MARC standard (ISO 2709), the different flavours such as MARC 21, which are like application profiles defining content designators and lists of values, and content standards, predominantly Anglo-American Cataloguing Rules (AACR). ISO 2709 is a kind of extensible markup language designed for data exchange and so could be replaced by XML, but it can only be done effectively when the other two components are re-aligned to modern requirements and to the flexible power of XML.
Not surprisingly, the Library of Congress' MARCXML framework is discussed in the thread. In a strict sense, it replaces MARC, i.e. ISO 2709, with XML. But it deliberately emulates precisely the restrictive structural characteristics of MARC, enabling round trip no loss conversion, to allow MARC data to be manipulated and transformed to other formats and contexts, using modern XML tools. Undoubtedly, this has been a tonic for the geriatric MARC, or (switching metaphors) it is a useful bridge or stopgap between the the old world of MARC and a new world, as yet not fully realised, based on XML. It allows a little more value to be squeezed from the huge investment of systems and data in MARC.
Some writers in the thread, however, criticise MARCXML for not being the panacea that it makes no claim to be. Its structure means that it is not well suited to XML indexing systems so performance is sub-optimal and, more importantly, it is not capable of articulating metadata in ways that are now required. Several writers call not only for better articulation of the metadata but also for a different set of metadata elements, more suited to modern requirements for search, navigation, data presentation and interchange between heterogeneous environments. Peter Binkley (University of Alberta) puts it well:
... we need metadata to aid not just searching but also clustering, linking to relevant external resources, etc. - all the things we can do in the new environment to enhance search results and other forms of access. The XML tools for using web services etc. are great and will get better much faster than anything MARC-based.Here, though, we move into the territory of application profiles and content rules. As several other writers in the thread point out, an area of activity that could be leading the way to a full replacement of MARC is that based on the Functional Requirements for Bibliographic Records (FRBR). In the publishing world, it provided the conceptual model for the bibliographic elements of the Indecs framework, which led to the development of the ONIX format. Now, its principles are being built into the next edition of the Anglo-American Cataloguing Rules, AACR3. Although AACR3 will be capable of expression in MARC 21, it will push MARC's capabilities closer to the limits. MARC records have been 'FRBRised' in a number of different initiatives with some success, but the work has clearly discovered shortcomings in the MARC format.
MARC will not be replaced by a single, dominant and self-contained metadata format. We can no longer even scope the contents of a 'self-contained' record. Increasingly, we require and have the ability to connect pieces of content dynamically and unrestrictedly, as we move towards the semantic web. The 'replacement' will be a metadata infrastructure. This is well argued by Roy Tennant in his article A bibliographic metadata infrastructure for the 21st century, summed up in his catchphrase 'I never metadata I didn't like'.
Dick Miller and his colleagues at the Lane Medical Library, Stanford University Medical Center, have done a great deal of impressive work to show the way forward for bibliographic data, in their development of XOBIS. A quote from his post to the thread makes a fitting end:
Some may think that MARC is robust since so many ILS systems use it, but ILS systems themselves are endangered, not able to respond with agility to changing technologies. For libraries to flourish, bibliographic data needs to be flexibly deployable in broader environments lest we will gradually lose relevance.