# Back to the Future with Description Logics

## Have Linked Data, Microformats Stumbled into an Adaptive Design?; Benefits from Keeping the TBox and ABox Separate

I was glad to see Kendall Clark pick up on parts of my earlier piece on Thinking ‘Inside the Box’ with Description Logics. He took one point of view in his posting — that I mostly agree with — but I’d also like to reinforce some other thoughts. And, those thoughts are: description logics (DL) provides earlier lessons and insights that our current zeal for linked data should not overlook, and the lessons we can gain from DL are really fundamental and architectural.

For those of you who have not read Kendall’s piece — which I heartily recommend — let me give you my Cliffs Note’s summary: there are those within the semantic Web community that want to capture the conceptual relationships within knowledge and domains, the Maximum Fidelity tribe, and then those that want to link and describe as many things as possible, the Maximum Scalability tribe, with those (like Kendall’s firm, Clark & Parsia) residing in the middle and following the precepts of DL. The theme is that extremes exist and need to be bridged. [1]

Posing these contrasts is an effective way to describe different ideas and approaches, but, like all straw men, perhaps it hides nuances and complexity. And, as I note below, it may also pose the wrong straw man dichotomy.

### We Are All Tribes of One

Jim Hendler, for one, took exception to Kendall’s characterization to make the obvious point that different use cases demand different approaches. What was interesting, however, in these interchanges was that a nerve was seemingly struck about differences in viewpoints and approaches. Indeed, the very reference to “tribes” seemed to bring out the (ahem) tribal response.

So, just so we are clear, in what I say below I take on the position of a tribe of one; that is, my own opinion. Of course, this is what all of us do. By positing tribes and viewpoints we simplify what is nuanced and subtly convey that opinions are cultural (“tribal”) and not subject to learning and change. Perhaps within the temporal viewpoint of whatever may be today’s trends and “memes” such thinking may hold, but I fundamentally disagree with such a static view of collective understanding and communities over more meaningful periods of years or decades. But, I digress. . . .

At the risk of being simplistic, I think we can say that there was a rich academic and intellectual history behind description logics going back to the early 1990s [2]. Then, with the seminal semantic Web paper built from thinking in the late 90s by Berners-Lee and published by him and Hendler and Lassila in 2001 in Scientific American [3], a real marker was put down for machine-readable and -actionable data (via “agents”) accessible on the Web. Many have been disappointed at the slow pace of the semWeb’s unfolding and some have blamed and rejected AI and “big” ontologies for this slowness. As usable standards finally emerged, a newer set of acolytes pushed “just getting data out there” and RDF linked data began to assume prominence from about 2006 onward, spearheaded by DBpedia and the linked open data community.

In so many ways we are coming full circle — coming back to the future — in seeing how our new linked data techniques can again benefit from this earlier DL thinking. Rather then poles and spectrums, I think we are experiencing the need to revisit our intellectual past now that workable publishing mechanisms and scalability and organization assume real prominence. Though clearly not intentional, the linked data community (and, in a related way, microformats), may just have stumbled upon a very cool architectural design that can leverage DL precepts.

### Some Terminology Revisited

Some of this DL and semWeb terminology can be off-putting. But it is helpful to know the lingo if one wants to look into the technical literature. Though most of this stuff can be described without resorting to such terms and can be readily grasped on an intuitive basis, here are some important grounding terms:

• TBox — according to [2], a TBox “contains intensional knowledge in the form of a terminology (hence the term ‘TBox,’ but ‘taxonomy’ could be used as well) and is built through declarations that describe general properties of concepts. Because of the nature of the subsumption relationships among the concepts that constitute the terminology, TBoxes are usually thought of as having a lattice-like structure; this mathematical structure is entailed by the subsumption relationship — it has nothing to do with any implementation.” A TBox uses a controlled vocabulary to define the concepts and roles of a domain of interest and the relations or properties amongst them
• ABox — according to [2], an ABox “contains extensional knowledge — also called assertional knowledge (hence the term ‘ABox’) — that is specific to the individuals of the domain of discourse.” An ABox provides the concept and role membership assertions for instance data, as well as assertions or “facts” about the attributes of those instances using the same controlled vocabulary as defined in the TBox
• First-order logic (FOL) — is a formal deductive system with unambiguous logic and mathematical structures for declaring, testing and inferring propositions (statements) and predicates (relations). A first-order theory consists of a set of axioms (usually finite or recursive) and the statements deducible from them based on FOL’s base logical axioms (such as the operators found in classical set theory, which itself is built on FOL)
• Description logics (DL) — are any of a family of knowledge representation languages that can be translated and characterized according to first-order logic. A DL language has a syntax that consists of unary predicate symbols to denote concepts, binary relations to denote roles, and recursion. DL semantics define concepts as sets of individuals and roles as sets of pairs of individuals. The expressivity of a DL language is a function of the logical operators the language supports (shown with representations such as $\mathcal{SROIQ}^\mathcal{(D)}$, the expressiveness of OWL 2). DL languages can be translated into other DL languages that support the same expressivity, regardless of syntax, but more expressive languages can not be equivalently represented by less expressive ones. The current OWL dialects of OWL Lite and OWL DL are DL languages
• Axiom — in traditional logic or FOL, an axiom (also called a ‘postulate’) is a proposition that is not proved or demonstrated but considered to be either self-evident or consistent with the base logic of the system. As such, its truth is taken for granted and the axiom serves as a starting point for deducing and inferring other (theory-dependent) truths
• Intensional — is a form of set membership that is based on the propositions and concepts which defines the set; there may be many possible members that remain unenumerated so long as they meet the conditions for membership. The intensional principal judges objects to be a member based on the properties or conditions they must have
• Extensional — is a form of set membership that arises from (“extends”) its listed set members. The extensional principle judges objects to be a member if they have the same external characteristics (whether as explicitly defined properties or not)
• Ontology — as used in knowledge representation or information science, this term is most often defined using Tom Gruber’s “explicit specification of a conceptualization” [4]. In practice on the semantic Web, it is any defined schema or data record structure including the most lightweight controlled vocabularies and structures (such as microformats). In DL, both ABox and TBox specifications and statements are lumped under the term
• Vocabulary — also ‘controlled vocabulary,’ is an organized, variously structured set of terms used for information retrieval or characterization. In its simplest form, a controlled vocabulary is merely a list for checking possible matches for set membership or not; at its more complex, it is the set of terms contained within a detailed and specified ontology or schema with formalized (axiomatized) relationships
• Knowledge base — in the DL community, a knowledge base is simply defined as TBox + Abox. In other words, a knowledge base is a logical schema of roles and concepts and the relationships between them (the TBox) as populated by the actual data (instances) asserting memberships and attributes (“facts”) (the ABox).

### TBox v ABox: Different Purposes and Roles

Within description logics and for our purposes herein, the two concepts we will most focus upon are the ABox and the TBox. As the definitions above suggest, the TBox is more structural and reflects the logical and conceptual relationships within a domain; that is, the role and concept and class relationships. The ABox provides the data (instance) records and characterizations within that schema; that is the instances and facts assertions. By analogy, in a conventional relational database system, the database or logical schema would correspond to the TBox; the actual data records or tables would correspond to the ABox.

These distinctions suggest very different purposes and roles, then, for the TBox and the ABox:

 TBox ABox Definitions of the concepts and properties (relationships) of the controlled vocabularyDeclarations of concept axioms or rolesInferencing of relationships, be they transitive, symmetric, functional or inverse to another propertyEquivalence testing as to whether two classes or properties are equivalent to one anotherSubsumption, which is checking whether one concept is more general than anotherSatisfiability, which is the problem of checking whether a concept has been defined (is not an empty concept)Classification, which places a new concept in the proper place in a taxonomic hierarchy of conceptsLogical implication, which is whether a generic relationship is a logical consequence of the declarations in the TBox Membership assertions, either as concepts or as rolesAttributes assertions Consistency checking of instancesEntailments, which are whether other propositions are implied by the stated conditionSatisfiability checks, which are that the conditions of instance membership are metInfer property assertions implicit through the transitive propertyInstance checking, which verifies whether a given individual is an instance of (belongs to) a specified conceptKnowledge base consistency, which is to verify whether all concepts admit at least one individualRealization, which is to find the most specific concept for an individual objectRetrieval, which is to find the individuals that are instances of a given concept

While certainly many of the ABox tests and checks require TBox structure, there is a pretty clear separation of purpose and role. Moreover: 1) the scale of the information in each “box” is vastly different (perhaps a few to hundreds to at most thousands of concepts in the TBox in contrast to potentially millions or more instances in ABoxes); and 2) ABox dataset repositories may also be (indeed, often are!) numerous, spatially distributed and semantically heterogeneous.

### The Wisdom of Separating Concerns

DL and semantic Web stuff in general are data and logic models, not architectural guidance. So, rarely does one see discussion of the architectural imperatives that some of these logical underpinnings provide. We see knowledge bases and ontologies both used as umbrella terms encompassing both the ABox and the TBox.

However, our own deployment experiences and the literature suggest there are manifest advantages to keeping the TBox and ABox separate:

 Advantages of Keeping the TBox and ABox Separate Better performance by keeping inference and reasoning purposes separateBetter scalability through separation of functionUse of tailored reasoners and rules engines based on purpose [5]More modular design, including keeping attribute information separate from structural and conceptual relationshipsFaster, global instance checking using summary ABox tests [6]Assignment of named entities (instances) to distinct and disjoint super types [7] that can bring significant tableaux benefits to ABox reasoningEasier partitioning of ABoxes [8]Easy swapping in and mix-and-matching of varied, multiple and private or public named entity dictionaries (ABoxes)Integration with extant relational (RDBMs) data structures and data stores for instance (ABox) data [9]Integration with other lightweight structures (microformats, other) for instance (ABox) dataFaster retrievals via TBox routing to appropriate ABoxesSimpler ABox vocabularies that are easier to understand and extend (including continued reliance on RDF and RDFS)More capable TBox ontologies, including integration of rules systemsRelatively easy extension of the TBox schema ontology into specific domainsEasy ABox data entry and updating via wiki or sematic wikiAbility to triangulate between separate concept (TBox) and instance (ABox) disambiguation approaches to improve overall precision and recall.

It would be useful to refrain from lumping the very different purposes of ABoxes and TBoxes under the umbrella rubric of ‘ontology’. It would also be useful for designers and vocabulary authors to be more explicit in their own minds as to purpose and content when formulating new ontologies. Smushing all of these concepts into one bubbling mess may not lead to clarity nor good performance.

### A Simple Schematic of Best Practice

Taking these basic ideas we can visualize a general schematic for best practice splits within the ontology or knowledge base:

The TBox is clearly focused on the domain at hand, but also includes links and equivalents to external ontologies. The TBox level should be entirely free of instance data, though all attributes, properties and concepts that might be found at the ABox level are also defined with their relationships at the TBox level. Like any semWeb ontology, this TBox level should also re-use common Web ontologies such as FOAF, SIOC, UMBEL, etc.

It is also the case that because of the reasoning needs at the TBox layer, the semantic Web language used should likely be a dialect of OWL (see below).

(BTW, for my own practice, I will try to limit my use of the ‘ontology’ term to the concepts and classes at this TBox level.)

The ABox level, in contrast, may consist of multiple datasets and name spaces. These structures are most appropriately seen as lightweight controlled vocabularies with limited structure; if written by scratch perhaps limited to RDF or RDFS (the schema variant). This layer, however, can also remain in non-semWeb native form — such as RDBMS data tables, microformats or other formats — that are wrapperized for interoperability through one or more ‘RDFizers‘ or GRDDL.

These structures should likely not make many external assertions, if any, and if done, perhaps in separate mapping or linkage file that can be processed and analyzed independently. It is important, however, to make sure that all attributes at this ABox layer have a counterpart with relationships and structure defined at the TBox layer.

This architectural design enables complete independence of the instance datasets from the inferencing logic or federation that might be applied to them.

### The Relevance to Linked Data Instances

Since it first took off in 2006, linked data and the various datasets now shown in the ‘LOD cloud‘ have been dominated by instance data. There are perhaps 10 million to 20 million instance objects available as linked data, many of which are derived from Wikipedia (via DBpedia) with attributes or structure coming from the Wikipedia infoboxes.

“There need not be a trade-off between expressiveness and scalability. Proper design, language choice and architecture can readily achieve both — while maintaining independence of scope or purpose.”

A similarly fast explosion has taken place with structured records via microformats and other simple data structures. For example, some earlier estimates suggest there are perhaps more than 2 billion pages that include microformats [10].

I have at times recently made comments about the dominance of instance data within the linked data community and the need for organizing structure. While this observation, I believe, remains true and provides a rationale for UMBEL as an organizing subject structure (or any other organizing structure, for that matter), perhaps I have been missing a more fundamental point: linked data (at least as practiced to date) is really about exposing ABoxes with simple structure. Perhaps, by serendipity, linked data (and other light structures like microformats) are showing the way to a distributed, mixed ABox-TBox structure for the Web.

With this altered viewpoint, a number of new observations emerge:

• Linked data instance structures perhaps need to be consciously designed as such, with lightweight structure and limited external (OWL-based or TBox-oriented) class structure
• The recent interest shown in so-called VoCamps (for lightweight vocabulary development) and voiD (vocabulary of interlinked datasets) might be usefully viewed specifically through an ABox “lens” with perhaps best practices for structures and vocabulary syntaxes to emerge
• TBox class and relationship structures should remain apart from the instance data and can be used to operate independently of the datasets
• Architectural and design changes might also emerge through clear separation of ABox and TBox that can benefit semantic Web scalability.

Linked data and microformats and other lightweight structures are now giving us the exposed instance data to begin reasoning and showing differences due to inferencing and other logic advantages for the semantic Web. Now that the ABox is being proven, let’s move on and stress-test the TBox!

### OWL 2 and Query Rewriting

Since the first version of OWL there has been confusion and some limitations with the dialects of the language. Only OWL Full allowed classes to be treated both as instances and classes (so-called metamodeling), and was therefore used as the basis for mapping UMBEL, for example, to RDF and RDFS vocabularies and to Cyc. This design was necessary, but left UMBEL undecidable using standard DL reasoners; only the two dialects of OWL DL and OWL Lite met description logics requirements.

Indeed, it was even hard to determine what dialect an OWL file represented, among many other problems and issues. The technical committee behind OWL 2, in fact, has written an excellent critique of issues with this first version of OWL [11].

For nearly two years the next version of OWL, OWL 2, has been undergoing development, with the last draft now published and available for last comment before January 23 [12]. Lessons and refinements to the use of DL have also occurred. Some have criticized this effort and have criticized the need for OWL 2′s growing expressiveness and vocabulary [see 1, for example]. I believe these criticisms to be unfair and to miss many of the thoughtful improvements in this new version.

Version control and expressiveness are two of these benefits. A broader benefit, though, has been the keen attention the developers have given to compliance with description logics and the ability to formulate fragments (called “profiles”) that only present subsets of DL useful for computational considerations [13]. For example, one profile, OWL 2 QL, appears well suited to the ABox; another, EL, appears well suited to the TBox. Users and tools builders may define other subsets of OWL 2 to deal with different use cases.

What is emerging are possible design patterns that would have comprehensive TBox guidance and inference structures that first receive a query, then do query rewriting for less capable OWL dialects and mapping to distributed ABox datasets, some of which might be kept in native relational DB or other structural forms [9, 14, 15]. Other approaches and designs, such as overviewed for DLDB2, KAON2, OWLIM, BigOWLIM and Minerva, are testing other architectural and DL combinations [see 15]. And, at the level of the specific triplestore, other optimizations are being made such as owl:sameAs or query rewrite with Virtuoso [16]. This new version of OWL and its profiles have adapted to past lessons and can be matched well to the emerging hardware and architectural designs.

These changes appear to now provide the option for various dialects of OWL to be matched with reasoners and architectural designs in order to optimize for different purposes. Rather than a spectrum, we appear to be learning and maturing. Hopefully, getting back to the architectural implications of the TBox – ABox split can show us there need not be a trade-off between expressiveness and scalability. Proper design, language and dialect choice, and architecture can readily achieve both — while maintaining independence of scope or purpose.

Thanks, OWL 2! You have fulfilled your commitment to description logics. It is now our turn to figure out the best practices for working with these tools.

[1] For those with a spare 90 minutes or so, you may also want to view this panel session and debate that took place on “An OWL 2 Far?” at ISWC ’08 in Karlsruhe, Germany, on October 28, 2008. The panel was chaired by Peter F. Patel-Schneider (Bell Labs, Alcathor) with the panel members of Stefan Decker (DERI Galway), Michel Dumontier (Carleton University), Tim Finin (University of Maryland) and Ian Horrocks (University of Oxford), with much audience participation. See http://videolectures.net/iswc08_panel_schneider_owl/.
[2] F. Baader, D. Calvanese, D. McGuinness, D. Nardi, and P. F. Patel-Schneider, editors. The Description Logic Handbook: Theory, Implementation and Applications. Cambridge University Press, 2003. See Chapter 1. Sample chapters may be viewed from Enrico Franconis Description Logics course notes and tutorial at http://www.inf.unibz.it/~franconi/dl/course/, which is an excellent starting reference point on the subject.
[3] Tim Berners-Lee, James Hendler and Ora Lassila, 2001. “The Semantic Web,” in Scientific American, 284(5):34-43, May 2001; see http://www.sciam.com/article.cfm?id=the-semantic-web.
[4] Thomas R. Gruber, 1993. “A Translation Approach to Portable Ontology Specifications,” in Knowledge Acquisition 5(2): 199-220; see http://tomgruber.org/writing/ontolingua-kaj-1993.pdf.
[5] Georgios Meditskos and Nick Bassiliades, 2008. “Combining a DL Reasoner and a Rule Engine for Improving Entailment-based OWL Reasoning,” presented at the 7th International Semantic Web Conference (ISWC2008); see http://lpis.csd.auth.gr/publications/med-iswc08.pdf.
[6] Achille Fokoue, Aaron Kershenbaum, Li Ma, Edith Schonberg, and Kavitha Srinivas, 2006. “The Summary Abox: Cutting Ontologies Down to Size,” presented at the 5th International Semantic Web Conference, Athens, GA, USA, November 5-9, 2006; see http://iswc2006.semanticweb.org/items/Kershenbaum2006qo.pdf.
[7] These are akin to the lexicographer supersenses that have been applied in WordNet for nouns and verbs (though only nouns are used here). See Massimiliano Ciaramita and Mark Johnson, 2003. Supersense Tagging of Unknown Nouns in WordNet, in Proceedings of the Conf. on Empirical Methods in Natural Language Processing, pp. 168173, 2003. See http://www.aclweb.org/anthology-new/W/W03/W03-1022.pdf.
[8] Yuanbo Guo and Jeff Heflin, 2006. “A Scalable Approach for Partitioning OWL Knowledge Bases,” in Proceedings of the 2nd International Workshop on Scalable Semantic Web Knowledge Base Systems (SSWS2006), Athens, Georgia, USA, November, 2006; see http://swat.cse.lehigh.edu/pubs/guo06c.pdf.
[9] Diego Calvanese, Giuseppe De Giacomo, Domenico Lembo, Maurizio Lenzerini and Riccardo Rosati, 2007. “Tractable Reasoning and Efficient Query Answering in Description Logics: The DL-Lite Family,” in Journal of Automated Reasoning, 39 (3), October 2007; see http://www.dis.uniroma1.it/~degiacom/papers/2007/calv-etal-JAR-2007.pdf.
[10] The original citation could not be found, but it is referenced on the Microformats mailing list on Oct. 1, 2008; see http://microformats.org/discuss/mail/microformats-discuss/2008-October/012550.html.
[11] Bernardo Cuenca Grau, Ian Horrocks, Boris Motik, Bijan Parsia, Peter Patel-Schneider and Ulrike Sattler, 2008. “OWL2: The Next Step for OWL,” in Journal of Web Semantics, 6(4): 309-322, November 2008; see http://www.comlab.ox.ac.uk/people/ian.horrocks/Publications/download/2008/CHMP+08.pdf.
[12] Boris Motik, Peter F. Patel-Schneider, Bijan Parsia, eds., 2008. OWL 2 Web Ontology Language: Structural Specification and Functional-Style Syntax, W3C Working Draft, December 2, 2008. See http://www.w3.org/TR/owl2-syntax/.
[13] Boris Motik, Bernardo Cuenca Grau, Ian Horrocks, Zhe Wu, Achille Fokoue and Carsten Lutz, eds., 2008. OWL 2 Web Ontology Language: Profiles, W3C Working Draft, December 2, 2008. See http://www.w3.org/TR/owl2-profiles/.
[14] Luciano Serafini and Andrei Tamilin, 2007. “Instance Migration in Heterogeneous Ontology Environments,” in Proceedings of 6th International Semantic Web Conference / 2nd Asian Semantic Web Conference (ISWC/ASWC 2007), pages 452-465, 2007. See http://iswc2007.semanticweb.org/papers/449.pdf.
[15] Zhengxiang Pan, Xingjian Zhang and Jeff Heflin, 2008. “DLDB2: A Scalable Multi-Perspective Semantic Web Repository,” in WI 08: Proceedings of the International Conference on Web Intelligence, IEEE Computer Society Press, pp. 489-495; see http://swat.cse.lehigh.edu/pubs/pan08a.pdf.
[16] Orri Erling and Ivan Mikhailov, 2007. “RDF Support in the Virtuoso DBMS,” in Proceedings of the 1st Conference on Social Semantic Web, Leipzig, Germany, Sep 26-28, 2007; see http://aksw.org/cssw07/paper/5_erling.pdf.
###### Posted:January 14, 2009

This is the post that sums up the transition that is 2009 to come:

So, this brings to mind a couple of thoughts:

• Linked Open Data community — stop talking only with yourselves, and get active helping to spread the word about benefits and approaches to the broader market; see if you can do so without mentioning any technical terms
• Media and content enterprises of all stripes — extracting structure from your content is where it is at; if you don’t do it, someone else will.

We are now being buffeted by the biggest tsunami in creative destruction that has occurred at least within the past 50 years. The Web provides the key to the emerging opportunities. So, too, does structured data and the ability to aggregate it.

Oh, and by the way. All of you start-ups hoping to make it through an ad-based revenue model?: Bend over and kiss it goodbye.

I actually kind of like this environment.

Posted by AI3's author, Mike Bergman Posted on January 14, 2009 at 10:39 pm in Adaptive Information, Linked Data | Comments (0)

# Multi-part Federated Search Interview

## Topics Range from the Deep Web to Semantic Web in this Search Luminaries Series

I’m pleased to wrap up a multi-part interview with the Federated Search Blog as part of their ongoing ‘Search Luminaries’ series. Sol Lederman, editor of the blog, does a thorough and comprehensive job! Over the past month on every Friday, I have answered some 25 or so of his detailed questions.

Federated Search Blog was particularly interested in the deep Web, its discovery and size. Many of the early questions deal with those themes. However, by Part 4 things get a bit more current, with the topics shifting to the semantic Web, linked data and Zitgist.

Here are the links to the series:

To give you a flavor of the interview, here is an example of one of the questions (and probably my favorite):

20. Tim Berners-Lee, credited with inventing the World Wide Web, has been talking about the importance and value of the Semantic Web for years yet common folks don't see much evidence of the Semantic Web gaining traction. Is there substance to the Semantic Web? What's happening with it now and what does its future look like?

Wow, in 10,000 words or less?

No, actually, this is a very good question. As things go, I am a relative newbie to the semantic Web, only having studied and followed it closely since about 2005. I'm sure my perspective in coming later to the party may not be shared by those at the beginning, which dates to the mid-1990s as Berners-Lee's vision naturally progressed from a Web of documents, as most of us currently know the Web, to a Web of data.

I think there is indeed incredibly important substance to the semantic Web. But, as I have written elsewhere, the semantic Web is more of a vision than a discernable point in time or a milestone.

The basic idea of the semantic Web is to shift the focus from documents to data. Give data a unique Web address. Characterize that data with rich metadata. Describe how things are related to one another so that relationships and connections can be traced. Provide defined structures for what these things and relationships "mean"; this is what provides the semantics, with the structures and their defined vocabularies known as "ontologies" (which in one analog can be seen as akin to a relational database schema).

As these structures and definitions get put in place, the Web itself then becomes the infrastructure for relating information from everywhere and anywhere on any given topic or subject. While this vision may sound grandiose, just think back to what the Web itself has done for us and documents over the past decade or so. This same architecture and infrastructure can and should be extended to the actual information in those documents, the data. And, oh, by the way, conventional databases can now join this party as well. The vision is very powerful and very cool.

Progress has indeed been slow. Many advocates fairly point to how long it takes to get standards in place and for a while people spoke of the "chicken-and-egg" problem of getting over the threshold of having enough structured data to consume to make it worthwhile to create the tools and applications and showcases that consume that data.

From my perspective, the early visions of the semantic Web were too abstract, a bit off perhaps. First, there was the whole idea of artificial intelligence and machines using the data as opposed to better ways for humans to draw use from the data at hand. The fundamental and exciting engine underneath the semantic Web — the RDF (Resource Description Framework) data model — was not initially treated on its own. It got admixed with XML that made understanding difficult and distinctions vague. There is and remains too much academia and not enough pragmatics driving the bus.

But that is changing and fast.

There is now an immediate and practical "flavor" of the semantic Web called linked data. It has three simple bases:

(1) RDF as the simple but adaptable data model that can represent any information — structured or unstructured — as the basic "triple" statement of subject-predicate-object. That sounds fancy, but just substitute verb for predicate and noun for subject and object. In other words: Dick sees Jane; or the ball is round. It sounds like a kindergartner reader, but that is how data can be easily represented and built up into more complex structures and stories

(2) Give all objects a unique Web identifier. Unique identifiers are common to any database; in linked data, we just make sure those identifiers conform to the same URIs we see constantly in the address bar of our Web browsers, and:

(3) Post and expose this stuff as accessible on the Web (namely, HTTP).

My company adds some essential "spice" to these flavors with respect to reference structures and concepts to give the information context, but these simple bases remain the foundation.

These are really not complex steps. They are really no different than the early phases of posting documents on the Web. Only now, we are exposing data.

More importantly, we can forget the chicken-and-egg problem. Each new data link we make brings value, in the similar way that adding a node to a network brings value according to Metcalfe's Law. Only with linked data, we already have the nodes — the data — we are just establishing the link connections (the verbs, predicates or relations) to flesh out the network graph. Same principle, only our focus is now to connect what is there rather than to add more nodes. (Of course, adding more linked nodes helps as well!)

The absolutely amazing thing about our current circumstance as Web users is that we truly now have simple and readily deployable mechanisms available to finally overcome the decades of enterprise stovepipes. The whole answer is so simple it can be mistaken as snake oil when first presented and not inspected a bit.

As an industry accustomed to hype and cynical about so much of this, I only ask that your readers check out these assertions for themselves and suspend their normal and expected disbelief. For me, in a career of more than 30 years focusing on information and access, I feel like we finally now have the tools, data model and architecture at hand to actually achieve data interoperability.

Thanks again to Sol and Federated Search Blog for this opportunity.

# Thinking ‘Inside the Box’ with Description Logics

## Linked Data Need Not Rediscover the Past; A Surprise in Every Box

A standard cliché of management consultants is the exhortation to think “outside the box.” Of course, what is meant by this is to question assumptions, to think differently, to look at problems from new perspectives.

With our recent release of the (linked open data) ‘LOD constellation‘ of linked data classes based around UMBEL, I have been fielding a lot of inquiries on what the relationship is of UMBEL to DBpedia. (See, for example, this current interview by the Semantic Web Company with me and Sören Auer of the DBpedia project.) This also fits into the ongoing distinction we have made in the UMBEL project between our subject concepts (classes) and named entities (instances).

What has actually most been helping my thinking is to get fully inside the box (or, rather, boxes, hehe). Let me explain.

The problem with urging outside-the-box thinking is that many of us do a less-than-stellar job of thinking inside the box. We often fail to realize the options and opportunities that are blatantly visible inside the box that could dramatically improve our chances of success.

Naomi Karten [1]

### The Description Logics Underpinnings of the Semantic Web

Description logics are one of the key underpinnings to the semantic Web. They grew out of earlier frame-based logic systems from Marvin Minsky and also semantic networks; the term and discipline was first given definition in the 1980s by Ron Brachman, among many others [2].

Description logics (DL, most often expressed in the plural) are a logic semantics for knowledge representation (KR) systems based on first-order predicate logic (FOL). They are a kind of logical metalanguage that can help describe and determine (with various logic tests) the consistency, decidability and inferencing power of a given KR language. The semantic Web ontology languages, OWL Lite and OWL DL (which stands for description logics), are based on DL and were themselves outgrowths of earlier DL languages.

Description logics and their semantics traditionally split concepts and their relationships from the different treatment of individuals and their attributes and roles, expressed as fact assertions. The concept split is known as the TBox (for terminological knowledge, the basis for T in TBox) and represents the schema or taxonomy of the domain at hand. The TBox is the structural and intensional component of conceptual relationships.

Thus, the model is an abstraction of a concrete world where the concepts are interpreted as subsets of the domain as required by the TBox and where the membership of the individuals to concepts and their relationships with one another in terms of roles respect the assertions in the ABox.

Franz Baader and Werner Nutt [3]

The second split of individuals is known as the ABox (for assertions, the basis for A in ABox) and describes the attributes of individuals, the roles between individuals, and other assertions about individuals regarding their class membership with the TBox concepts. Both the TBox and ABox are consistent with set-theoretic principles.

TBox and ABox logic operations differ and their purposes differ. TBox operations are based more on inferencing and tracing or verifying class memberships in the hierarchy (that is, the structural placement or relation of objects in the structure). ABox operations are more rule-based and govern fact checking, instance checking, consistency checking, and the like [3]. ABox reasoning is generally more complex and at a larger scale than that for the TBox.

Early semantic Web systems tended to be very diligent about maintaining these “box” distinctions of purpose, logic and treatment. One might argue, as I do herein, that the usefulness and basis for these splits has been lost somewhat in our first implementations and publishing of linked data systems.

### ABox and TBox Analogs in the Linked Data Web

Most of the semantic Web work at the beginning of this decade was pretty explicit about references to description logics and related inferencing engines and computational efficiency. Some of the early commercial semantic Web vendors are still very much focused on this space.

However, with the first release and emphasis on linked data about two years ago, the emphasis seemed to shift to the more pragmatic questions of actually posting and getting data out there. Best practices for cool URIs and publishing and linkage modes assumed prominence. The linking open data (LOD) movement began in earnest and gained mindshare. Of course, many in the DL and OWL development communities continued to discuss logic and inferencing, but now seemingly more as a separate camp to which the linked data tribe paid little heed.

The central hub of this linked data effort has been DBpedia and its pivotal place within the ‘LOD cloud.’ What is remarkable about the LOD cloud, however, is that it is almost entirely an ABox representation of the world and its instances. Starting from the core set of individual instances within Wikipedia, this cloud has now grown to many other sources and the central place for finding linked instance data. If one looks carefully at the LOD cloud and its linkages we can see the prevalence of instance-level relationships and attributes.

In fact, the LOD cloud diagram to upper right from the Wikipedia article on linked data has become the key visual metaphor for the movement. But, as noted, this view is almost exclusively one at the ABox instance level.

The UMBEL project began at roughly the same time and as a response to the release of DBpedia. My question in looking at the first data linked to DBpedia was, What is this content about? Sure, I might be able to find multiple records discussing Abraham Lincoln as a US president regarding attributes like birth date and a list of children, but where could I retrieve records about other presidents or, more broadly, other types of leaders such as prime ministers, kings or dictators?

The intuition was that the linked data and the various FOAF and other distributed instance records it was combining lacked a coherent reference structure of subject topics or concepts with which to describe content. The further intuition was that — while tagging systems and folksonomies would allow any and all users to describe this content with their own metadata — a framework for relating these various assignments to one another was still lacking.

In the nearly two years of development leading to the first beta release of UMBEL we have tried many analogies and metaphors to describe the basis of the 20,000 subject concept classes within UMBEL in relation to its role and other linked data initiatives. While many of those metaphors help visualize use and role, the more formal basis offered by description logics actually helps to most precisely cast UMBEL’s role. For example, in today’s interview with the Semantic Web Company, I note:

“. . . we have described UMBEL as a roadmap, or middleware, or a backbone, or a concept ontology, or an infocline, or a meta layer for metadata, and others. Today, what I tend to use, particularly in reference to DBpedia, is the TBox-ABox distinction in computer science and description logics. UMBEL is more of a class or structural and concept relationships schema — a TBox — while DBpedia is more of an an instance and entity layer with attributes — an ABox. I think they are pretty complementary. . . “

The resulting class level structure produced by UMBEL and its mappings to other classes within existing linked data enabled us to create and then publish the ‘LOD constellation‘, a complementary TBox structure to the linked data’s existing ABox one. This diagram to the lower right from the Wikipedia article on linked data now shows this complement.

### Completeness and Sufficiency

Description logics have arisen to aid our creating and understanding of knowledge representation systems. From this basis, we can see that the first efforts of the linked data initiative have lacked context, the TBox. At a specific level, the question is not DBpedia v. UMBEL or cloud v. constellation. Both types of structure are required in order to complete the logical framework. By thinking inside the box — by paying attention to our logical underpinnings — we can see that both TBoxes and ABoxes are essential and complementary to creating a useful knowledge representation system.

By more explicitly adopting a description logics framework we can also better address many prior questions of context, coherence and sufficiency. These have been constant themes in my recent writings that I will be revisiting again through the helpful prism of formal description logics.

My interview today with Sören Auer also brought up some important points regarding context. As we have said in other venues, it is important that any TBox be available for context purposes. Whether that should be UMBEL or some other framework depends on the use case. As I noted in the interview, “UMBEL’s specific purpose is to provide a coherent framework for serious knowledge engineers looking to federate data.” Other uses may warrant other frameworks, and certainly not always UMBEL.

But, in any event, I have two cautions to the linked data community: 1) do not take the suggestion to have a reference framework of concepts as being equivalent to adopting a single ontology for the Web; think of any reference structure as an essential missing TBox, and not some call to adopt “one ontology to rule them all,” but 2) in adopting alternative frameworks, take care that whatever is designed or adopted itself be able to meet basic DL logic tests of consistency and coherence.

### A Serendipitous Surprise

No one has yet elaborated the significant advantages from design, performance, architectural and flexibility perspectives from a distinct and explicit separation of TBox from ABox — but they’re there!

The many advantages from separate TBox and ABox frameworks are one serendipitous surprise coming from the early development of linked data. To my knowledge, no one has yet elaborated the significant advantages from design, performance, architectural and flexibility perspectives from a distinct and explicit separation of TBox from ABox. We believe these advantages to be substantial.

Realize, as distributed, UMBEL already has both TBox and ABox components. The TBox component is the lightweight UMBEL ontology, with its 20,000 subject concept classes and their hierarchical and other relationships. This component has a vocabulary (or terminology) for aiding the linking to external ontologies. The vocabulary is quite suitable for extension into new domains as well.

The ABox component is the named entities part of instances drawn from Wikipedia and the BBC’s John Peel sessions. Besides being of common, broad interest, these 1.5 million instances (per the current version) are included in the distribution to instantiate the ontology for demonstration and sandbox purposes.

So, UMBEL’s world is quite simple: subject concepts (SCs) and named entities (NEs). Subject concepts are the TBox and classes that define the structure and concept relationships. Named entities are the individual “things” in the world (some lower case such as animals or foods) and are the ABox of instances that populate this structure.

In our early efforts, we concentrated on the SC portion of UMBEL. Most recently, we have been concentrating on the NE component and its NE dictionaries. It was these investigations that drew us into an ABox perspective when looking at design options. The logic and rationale had been sitting there for some years, but it took cracking open the older textbooks to become reacquainted with it.

Once we again began looking inside the box, we began to see and enumerate some significant advantages to an explicit TBox-ABox design, as well as advantages for keeping these components distinct:

• Easier understood ontologies with a very limited number of predicates
• Lightweight schema design that is easy to extend
• Ability to “triangulate” between separate SC (concept) and NE (instance) disambiguation approaches to improve overall precision and recall
• Attribute information is kept separate from structural and conceptual relationships
• Easy to swap in varied, multiple and private or public named entity dictionaries
• Relatively easy extension of the schema ontology into specific domains
• A design suitable to computation efficiency (rules for ABox; inference and standard reasoning for TBox), and
• Assignment of NEs to distinct and disjoint “super types” [4] that can bring significant tableaux benefits to ABox reasoning.

We are still learning about these advantages and will document them further in pending work on coherence and named entity dictionary (NED) creation.

### Thinking Inside the TBox and ABox

The two main points of this article have been to: 1) recognize the important intellectual legacy of description logics and how they can inform the linked data enterprise moving forward; and 2) be explicit about the functional and architectural splits of the TBox from the ABox. Making this split brings many advantages.

There will continue to be many design challenges as linked data proliferates and actually begins to play its role of aiding meaningful knowledge work. The grounding in description logics and the use of DL for testing alternative designs and approaches is a powerful addition to our toolkit.

Sometimes there are indeed many benefits to thinking inside the box.

[2] F. Baader, D. Calvanese, D. McGuinness, D. Nardi, and P. F. Patel-Schneider, editors. The Description Logic Handbook: Theory, Implementation and Applications. Cambridge University Press, 2003. See Chapter 1. Sample chapters may be viewed from Enrico Franconi’s Description Logics course notes and tutorial at http://www.inf.unibz.it/~franconi/dl/course/, which is an excellent starting reference point on the subject.
[3] Ibid.; see Chapter 2.
[4] These are akin to the lexicographer supersenses that have been applied in WordNet for nouns and verbs (though only nouns are used here). See Massimiliano Ciaramita and Mark Johnson, 2003. Supersense Tagging of Unknown Nouns in WordNet, in Proceedings of the Conf. on Empirical Methods in Natural Language Processing, pp. 168173, 2003. See http://www.aclweb.org/anthology-new/W/W03/W03-1022.pdf.

# WOA! So RESTful it is UMBELievable!

## UMBEL’s New Web Services Embrace a Full Web-Oriented Architecture

I recently wrote about WOA (Web-oriented architecture), a term coined by Nick Gall, and how it represented a natural marriage between RESTful Web services and RESTful linked data. There was, of course, a method behind that posting to foreshadow some pending announcements from UMBEL and Zitgist.

Well, those announcements are now at hand, and it is time to disclose some of the method behind our madness.

As Fred Giasson notes in his announcement posting, UMBEL has just released some new Web services with fully RESTful endpoints. We have been working on the design and architecture behind this for some time and, all I can say is, it’s UMBELievable!

As Fred notes, there is further background information on the UMBEL project — which is a lightweight reference structure based on about 20,000 subject concepts and their relationships for placing Web content and data in context with other data — and the API philosophy underlying these new Web services. For that background, please check out those references; that is not my main point here.

### A RESTful Marriage

We discussed much in coming up with the new design for these UMBEL Web services. Most prominent was taking seriously a RESTful design and grounding all of our decisions in the HTTP 1.1 protocol. Given the shared approaches between RESTful services and linked data, this correspondence felt natural.

What was perhaps most surprising, though, was how complete and well suited HTTP was as a design and architectural basis for these services. Sure, we understood the distinctions of GET and POST and persistent URIs and the need to maintain stateless sessions with idempotent design, but what we did not fully appreciate was how content and serialization negotiation and error and status messages also were natural results of paying close attention to HTTP. For example, here is what the UMBEL Web services design now embraces:

• An idempotent design that maintains state and independence of operation
• Language, character set, encoding, serialization and mime type enforced by header information and conformant with content negotiation
• Error messages and status codes inherited from HTTP
• Common and consistent terminology to aid understanding of the universal interface
• A resulting componentization and design philosophy that is inherently scalable and interoperable
• A seamless consistency between data and services.

There are likely other services out there that embrace this full extent of RESTful design (though we are not aware of them). What we are finding most exciting, though, is the ease with which we can extend our design into new services and to mesh up data with other existing ones. This idea of scalability and distributed interoperability is truly, truly powerful.

It is almost like, sure, we knew the words and the principles behind REST and a Web-oriented architecture, but had really not fully taken them to heart. As our mindset now embraces these ideas, we feel like we have now looked clearly into the crystal ball of data and applications. We very much like what we see. WOA is most cool.

### First Layer to the Zitgist ‘Grand Vision’

For lack of a better phrase, Zitgist has a component internal plan that it calls its ‘Grand Vision’ for moving forward. Though something of a living document, this reference describes how Zitgist is going about its business and development. It does not describe our markets or products (of course, other internal documents do that), but our internal development approaches and architectural principles.

Just as we have seen a natural marriage between RESTful Web services and RESTful linked data, there are other natural fits and synergies. Some involve component design and architecting for pipeline models. Some involve the natural fit of domain-specific languages (DSLs) to common terminology and design, too. Still others involve use of such constructs in both GUIs and command-line interfaces (CLIs), again all built from common language and terminology that non-programmers and subject matter experts alike can readily embrace. Finally, some is a preference for Python to wrap legacy apps and to provide a productive scripting environment for DSLs.

If one can step back a bit and realize there are some common threads to the principles behind RESTful Web services and linked data, that very same mindset can be applied to many other architectural and design issues. For us, at Zitgist, these realizations have been like turning on a very bright light. We can see clearly now, and it is pretty UMBELievable. These are indeed exciting times.

BTW, I would like to thank Eric Hoffer for the very clever play on words with the UMBELievable tag line. Thanks, Eric, you rock!