by Tom Baker, Karen Coyle, Sean Petiya
Published in: Library Hi Tech, v. 32, n. 4, 2014 pp 562-582 DOI:10.1108/LHT-08-2014-0081
Open Access Preprint
The above article was just published in Library hi Tech. However, because the article is a bit dense, as journal articles tend to be, here is a short description of the topic covered, plus a chance to reply to the article.
We now have a number of multi-level views of bibliographic data. There is the traditional "unit card" view, reflected in MARC, that treats all bibliographic data as a single unit. There is the FRBR four-level model that describes a single "real" item, and three levels of abstraction: manifestation, expression, and work. This is also the view taken by RDA, although employing a different set of properties to define instances of the FRBR classes. Then there is the BIBFRAME model, which has two bibliographic levels, work and instance, with the physical item as an annotation on the instance.
In support of these views we have three RDF-based vocabularies:
FRBRer (using OWL)
RDA (using RDFS)
BIBFRAME (using RDFS)
The vocabularies use a varying degree of specification. FRBRer is the most detailed and strict, using OWL to define cardinality, domains and ranges, and disjointness between classes and between properties. There are, however, no sub-classes or sub-properties. BIBFRAME properties all are defined in terms of domains (classes), and there are some sub-class and sub-property relationships. RDA has a single set of classes that are derived from the FRBR entities, and each property has the domain of a single class. RDA also has a parallel vocabulary that defines no class relationships; thus, no properties in that vocabulary result in a class entailment. [1]
As I talked about in the previous blog post on classes, the meaning of classes in RDF is often misunderstood, and that is just the beginning of the confusion that surrounds these new technologies. Recently, Bernard Vatant, who is a creator of the Linked Open Vocabularies site that does a statistical analysis of the existing linked open data vocabularies and how they relate to each other, said this on the LOV Google+ group:
"...it seems that many vocabularies in LOV are either built or used (or both) as constraint and validation vocabularies in closed worlds. Which means often in radical contradiction with their declared semantics."What Vatant is saying here is that many vocabularies that he observes use RDF in the "wrong way." One of the common "wrong ways" is to interpret the axioms that you can define in RDFS or OWL the same way you would interpret them in, say, XSD, or in a relational database design. In fact, the action of the OWL rules (originally called "constraints," which seems to have contributed to the confusion, now called "axioms") can be entirely counter-intuitive to anyone whose view of data is not formed by something called "description logic (DL)."
A simple demonstration of this, which we use in the article, is the OWL axiom for "maximum cardinality." In a non-DL programming world, you often state that a certain element in your data is limited to the number of times it can be used, such as saying that in a MARC record you can have only one 100 (main author) field. The maximum cardinality of that field is therefore "1". In your non-DL environment, a data creation application will not let you create more than one 100 field; if an application receiving data encounters a record with more than one 100 field, it will signal an error.
The semantic web, in its DL mode, draws an entirely different conclusion. The semantic web has two key principles: open world, and non-unique name. Open world means that whatever the state of the data on the web today, it may be incomplete; there can be unknowns. Therefore, you may say that you MUST have a title for every book, but if a look at your data reveals a book without a title, then your book still has a title, it is just an unknown title. That's pretty startling, but what about that 100 field? You've said that there can only be one, so what happens if there are 2 or 3 or more of them for a book? That's no problem, says OWL: the rule is that there is only one, but the non-unique name rule says that for any "thing" there can be more than one name for it. So when an OWL program [2] encounters multiple author 100 fields, it concludes that these are all different names for the same one thing, as defined by the combination of the non-unique name assumption and the maximum cardinality rule: "There can only be one, so these three must really be different names for that one." It's a bit like Alice in Wonderland, but there's science behind it.
What you have in your database today is a closed world, where you define what is right and wrong; where you can enforce the rule that required elements absolutely HAVE TO be there; where the forbidden is not allowed to happen. The semantic web standards are designed for the open world of the web where no one has that kind of control. Think of it this way: what if you put a document onto the open web for anyone to read, but wanted to prevent anyone from linking to it? You can't. The links that others create are beyond your control. The semantic web was developed around the idea of a web (aka a giant graph) of data. You can put your data up there or not, but once it's there it is subject to the open functionality of the web. And the standards of RDFS and OWL, which are the current standards that one uses to define semantic web data, are designed specifically for that rather chaotic information ecosystem, where, as the third main principle of the semantic web states, "anyone can say anything about anything."
I have a lot of thoughts about this conflict between the open world of the semantic web and the needs for closed world controls over data; in particular whether it really makes sense to use the same technology for both, since there is such a strong incompatibility in underlying logic of these two premises. As Vatant implies, many people creating RDF data are doing so with their minds firmly set in closed world rules, such that the actual result of applying the axioms of OWL and RDF on this data on the open web will not yield the expected closed world results.
This is what Baker, Petiya and I address in our paper, as we create examples from FRBRer, RDA in RDF, and BIBFRAME. Some of the results there will probably surprise you. If you doubt our conclusions, visit the site http://lod-lam.slis.kent.edu/wemi-rdf/ that gives more information about the tests, the data and the test results.
[1] "Entailment" means that the property does not carry with it any "classness" that would thus indicate that the resource is an instance of that class.
[2] Programs that interpret the OWL axioms are called "reasoners". There are a number of different reasoner programs available that you can call from your software, such as Pellet, Hermit, and others built into software packages like TopBraid.
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