UMBEL (Upper-level Mapping and Binding Exchange Layer) is a lightweight reference structure for placing Web content, named entities and data in context with other data. It is comprised of about 21,000 subject concepts and their relationships — with one another and with external vocabularies and named entities.
There’s Some Cool Tools in this Box of Crackerjacks
And, UMBEL is What, Again?
UMBEL (Upper-level Mapping and Binding Exchange Layer) is a lightweight reference structure for placing Web content and data in context with other data. It is comprised of about 21,000 subject concepts and their relationships — with one another and with external vocabularies and named entities.
Each UMBEL subject concept represents a defined reference point for asserting what a given chunk of content is about. These fixed hubs enable similar content to be aggregated and then placed into context with other content. These subject context hubs also provide the aggregation points for tying in their class members, the named entities which are the people, places, events, and other specific things of the world.
The backbone to UMBEL is the relationships amongst these subject concepts. It is this backbone that provides the contextual graph for inter-relating content. UMBEL’s subject concepts and their relationships are derived from the OpenCyc version of the Cyc knowledge base.
UMBEL’s backbone is also a reference structure for more specific domains or ontologies, thereby enabling further context for inter-relating additional content. Much of the sandbox shows these external relationships.
These first set of Web services provide online demo sandboxes, and descriptions of what they are about and their API documentation. The first 11 services are:
- Find Subject Concepts — look up via name, URI or alias one of the 21,000 UMBEL subject concepts
- Subject Concept Report — basic information about the subject concept; description, canonical name, and the like
- Subject Concept Detailed Report — very cool! see below
- List Sub-Concepts & Sub-Classes — for developers and ontology geeks
- List Super-Concepts & Super-Classes — for developers and ontology geeks
- List Equivalent External Classes — for developers and ontology geeks
- Verify Sub-Class Relationship — for developers and ontology geeks
- Verify Super-Class Relationship — for developers and ontology geeks
- Verify Equivalent Class Relationship — for developers and ontology geeks
- Subject Concepts Explorer — good golly, Miss Molly, we have visuals!
- Yago Ontology — a little help from our friends.
A CLASSy Detailed Report
The single service that provides the best insight to what UMBEL is all about is the Subject Concept Detailed Report. (That is probably because this service is itself an amalgam of some of the others.)
Starting from a single concept amongst the 21,000, in this case ‘Mammal’, we can get descriptions or definitions (the proper basis for making semantic relationships, not the ‘Mammal’ label), aliases and semsets, equivalent classes (in OWL terms), named entities (for leaf concepts), more general or specific external classes, and domain and range relationships with other ontologies. Here is the sample report for ‘Mammal’:
The discerning eye likely observes that while there are a rich set of relationships to the internal UMBEL subject concepts, coverage is still light for external classes and named entities. This sandbox is, after all, a first release and we are early in the mapping process.
But, it should also start to become clear that the ability of this structure to map and tie in all forms of external concepts and class structures is phenomenal. Once such class relationships are mapped (to date, most other Linked Data only occurs at the instance level), all external relationships and properties can be inherited as well. And, vice versa.
So, for aficionados of the network effect, stand back! You ain’t seen nothing yet. If we have seen amazing emergent properties arising from the people and documents on the Web, with data we move to another quantum level, like moving from organisms to cells. The leverage of such concept and class structures to provide coherence to atomic data is literally primed to explode.
To put it mildly, trying to get one’s mind around the idea of 21,000 concepts and all of their relationships and all of their possible tie in points and mappings to still further ontologies and all of their interactions with named entities and all of their various levels of aggregation or abstraction and all of their possible translations into other languages or all of their contextual descriptions or all of their aliases or synonyms or all of their clusterings or all of their spatial relationships or all of the still more detailed relationships and instances in specific domains or, well, whew! You get the idea.
It is all pretty complex and hard to grasp.
One great way to wrap one’s mind around such scope is through interactive visualization. The first UMBEL service to provide this type of view is the Subject Concept Explorer, a screenshot of which is shown here:
But really, to gain the true feel, go to the service and explore for yourself. It feels like snorkeling through those schools of billions of tiny silver fish. Very cool!
These amazing visualizations are being brought to us by Moritz Stefaner, imho one of the best visualization and Flash gurus around. We will be showcasing more about Moritz’s unbelievable work in some forthcoming posts, where some even cooler goodies will be on display. His work is also on display at a couple of other sites that you can spend hours drooling over. Thanks, Moritz!
Missing Endpoints and Next Steps
You should note that developer access to the actual endpoints and external exposure of the subject concepts as Linked Data are not yet available. The endpoints, Linked Data and further technical documentation will be forthcoming shortly.
The currently displayed services and demos provided on this UMBEL Web services site are a sandbox for where the project is going. Next releases will soon provide as open source under attribution license:
- The formal UMBEL ontology written in OWL Full and SKOS
- Technical documentation for the ontology and its use and extension
- Freely accessible Web services according to the documentation already provided
- Technical documentation and reports for the derivation of the subject concepts from OpenCyc and the creation and extension of semsets and named entities related to that structure.
When we hit full stride, we expect to be releasing still further new Web services on a frequent basis.
BTW, for more technical details on this current release, see Fred Giasson’s accompanying post. Fred is the magician who has brought much of this forward.
Part 4 of 4 on Foundations to UMBEL
Just as DBpedia has provided the nucleating point for linking instance data (see Part 2), UMBEL is designed to provide a similar reference structure for concepts. These concepts provide some fixed positions in space to which other sources can link and relate. And, like references for instance data, the existence of reference concepts can greatly diminish the number of links necessary in the Linked Data environment.
Clearly, the combination of the representativeness of UMBEL’s subject concepts (the “scope” of the ontology) and their relationships (the “structure” of the backbone) is fundamental. These factors in turn express the functional capabilities of the system.
First Things First
The first fundamental point deserving emphasis is that a reference structure of almost any nature has value. We can argue later about what is the best reference structure, but the first task is to just get one in place and begin bootstrapping. Indeed, over time, it is likely that a few reference structures will emerge and compete and get supplemented by still further structures. This evolution is expected and natural and desirable in that it provides choice and options.
A reference structure of concepts has the further benefit of providing a logical reference structure for instances as well. While Wikipedia is perhaps the most comprehensive collection of humanity-wide instances, no single source can or will be complete in scope. Thus, we foresee specialty sources ranging from the companies in Wikicompany to plants and animals in the Encyclopedia of Life or thousands of other rich instance sources also acting as reference hubs.
How do each of these rich instance sources relate to one another? What is the subject concept or topical basis by which they overlap or complement? What is the framework and graph structure of knowledge to give this information context? These are the benefits brought by a structure of reference concepts, independent from the specifics of the reference structure itself.
Another key consideration is that broad-scale acceptance is important. An express purpose of UMBEL is to aid the interconnection of related content using broadly accepted foundations.
Since the Web’s inception fifteen years ago, there have been various alternatives tried or in ascendance for organizing and bringing structure to Web content. Some of these may be too static and inflexible, others perhaps too arbitrary or parochial. All approaches to date have had little collective success.
There are also new and exciting developments in social networks and user-driven content and structure arising from areas such as tagging or Wikipedia (and wikis in general). But it is not clear that bottom-up contributions suitable to individual articles or topics can lead to coherent structural frameworks; arguably, they have not yet so far. And then there are sporadic government or corporate or trade association initiatives as well.
Here is a summary of alternate approaches:
- Existing library systems — Dewey Decimal Classification, Library of Congress, UDC and many other library classification schemes have been touted for the Web and all have failed. Some reasons cited for this failure are physical books are very different from free digital bits; Web schema need to evolve quickly; and lack of stewards and curation
- Market share — at various times certain successful vendors have held temporary minor ascendance with content organizational frameworks, generally directory structures. Examples include About, Yahoo!, Open Directory Project (DMOZ), Northern Light, etc. Yet even at their peaks, market shares were low, external adoption was rare, scope was questioned and arbitrary, with interest in directories now nearly absent
- WordNet — though of strong interest and use to computational linguists, and quite popular for many content analyses, WordNet has seen little consumer or commercial interest. However, the synset structure and its coverage is extremely valuable for concept disambiguation, and therefore has a role in UMBEL (as it does in many other online systems)
- Standards efforts — some sporadic success and some notable failures have occurred in the standards arenas. Generally, the successful initiatives tend to be in close communities where there are clear financial benefits for adherence, such as in the exchange of financial or commerce data; broader and more ambitious efforts have tended to be less successful
- Professional organizations and associations — areas such a finance, pharmaceuticals, biologists, physicists and many bounded communities have enjoyed sporadic and sometimes notable success in developing and using domain-specific schema; none have yet transferred beyond their beginning boundaries to the broader Web
- Government initiatives — there are episodic successes for government-sponsored content organizational initiatives, mostly in metadata, controlled vocabularies and ontologies, often where contractors or suppliers may be compelled to comply. NIH’s National Library of Medicine (and other NIH branches) have also seen significant domain successes, due to its foresight and its receptive biology, genetics and medical communities
- Upper ontologies — UMBEL investigated this area considerably in the early months of the project. Most of the upper ontologies have relatively sparse subject concept content, being geared to smaller, abstract-oriented “upper” structures. Some such as SUMO and DOLCE and now PROTON, have concerted initiatives to extend to middle- and domain-level ontologies . To date, penetration of these systems into general Web or commercial realms has been quite limited
- Wikipedia — a clear and phenomenal success, Wikipedia and related initiatives like Wikinvest and Wikicompany and scores more have proven to be a rich fount for named entities and article-length content, but not for the category and content organization structures in which that content is embedded. This is an area of keen academic and collective interest  and it may still result is useful organizational schema as these popular wikis continue to evolve and mature. However, they have not yet done so, and while a rich source for entities and data, UMBEL decided to pass on their use for “backbone” structure at this time
- No collective structure — tagging or folksonomies or doing nothing have perhaps the greatest market share at present.
Since inception, the stated intent of the UMBEL project was to base its subject structure on extant systems. To minimize development time, the structure needed to be drawn from one of the categories above. Possible development of a de novo structure was rejected because of development time and the low probability of gaining acceptance in the face of so many competing alternatives.
Rationale for OpenCyc
The granddaddy of knowledge bases suitable to all human content and knowledge is Cyc. Because of its more than 20-year history, Cyc brings with it considerable strengths and some weaknesses.
Amongst all alternatives, Cyc rapidly emerged as the leading candidate. While its strengths warranted close attention, its weaknesses also suggested a considerable effort to overcome them. This combination compelled the need for a significant investigation and due diligence.
First, here are OpenCyc’s strengths:
- Venerable and solid — through an estimated 200 person years of engineering and effort, the Cyc structure has been tested and refined through many projects and applications. While a few years back such groundings were unparalleled in the field, we are also now seeing some Internet-wide projects tap into the law of large numbers to get significant inputs of human labor. Cyc has also tapped this venue for ongoing expansion of its KB using the online FACTory game 
- Community — there is a large community of Cyc users and supporters from academic, government, commercial and non-profit realms. Moreover, the formation of The Cyc Foundation has also served as a vehicle for tapping into volunteer effort as well
- Upgrade Path — OpenCyc has an upgrade path to the more capable ResearchCyc, full Cyc and the services of Cycorp
- Comprehensive — no existing system has the scope, breadth and coverage of human concepts to match that of Cyc (however, sources for named entities such as Wikipedia have recently passed Cyc in scope; see next section)
- Common sense — since its founding as a project and then backed by the standalone Cycorp, Cyc has set for itself both a more pragmatic but harder challenge than other knowledge systems. Cyc has set out to capture the common sense at the heart of human reasoning. This objective means codifying generally unstated logic and rules-of-thumb — not unlike teaching a baby to walk and talk and read — all of which are lengthy tasks of trial and error. However, as Cyc has gained this foundation, it has also led to a more solid basis for its reasoning and conceptual relationships
- Power and inference — ultimately the purpose of a knowledge base is to support reasoning and inference by computer when presented with a (often small) starting set of assertions or facts. Cyc has literally thousands of microtheories now governing its inference domains, giving it a scope and power unmatched by other systems. The importance of such reasoning is not the silly science fiction of autonomous intelligent robots, but as achievable aids to make connections, determine relationships and filter and order results
- Robust supporting capabilities — such knowledge base-wide capabilities can also be deeply leveraged in such areas as entity extraction, machine translation, natural language processing, risk analysis or one of the other dozens of specialty modules available in Cyc, and
- Free and open — last, but not least, is the fact that a mostly complete Cyc was released as a free and open source version in 2002. OpenCyc has now been downloaded more than 100,000 times and is in production use for many applications. Non-profits and academics can also obtain access to the full capabilities of the Cyc system through ResearchCyc. This open character is an absolute essential because leading Web applications and leading innovators of the Web eschew proprietary systems.
Literally, after months of investigation and involvement, the richness of practical uses to which the OpenCyc knowledge base can be applied are still revealing themselves.
Drawbacks to OpenCyc
But there are weaknesses and problems with Cyc.
To be sure, there are some individuals and perhaps some historical criticisms of Cyc that involved fears of Big Brother or grandiose claims about artificial intelligence or machine reasoning. These are criticisms of hype, immaturity or ignorance; they are different than the drawbacks observed by our UMBEL project and not further discussed here.
In UMBEL’s investigation of Cyc, we observed these drawbacks:
- Obscure upper ontology — the Cyc upper ontology, shown in the figure below, is perhaps not as clean as more recent upper ontologies (Proton,  for example, is a very clean system). The various sub-classifications of ‘Thing’ and degrees of “tangibility” seem particularly problematic. However, since these are not direct binding concepts for UMBEL and provide appropriate “glue” for the upper portions of the graph, these criticisms can be easily overlooked
- Cruft — twenty years of projects and forays into obscure domains (many for the military or intelligence communities) have left a significant degree of cruft marbled through the knowledge base. Indeed, as our vetting showed, perhaps about 30% of the concepts in Cyc are holdovers from prior projects or relate to internal Cyc-only topics
- Reasoning concepts — another 15% or so of Cyc concepts are abstract or for reasoning purposes, such as reasoning over colors, beliefs, the sizes of objects, their orientations in space, and so forth. These are certainly legitimate concepts and appropriate to Cyc’s purposes, but are not needed or desired for UMBEL’s purposes
- Greater expressivity — Cyc is grounded in the LISP language and has many higher-order logic constructs. Paradoxically, this greater expressiveness may make translation to UMBEL more difficult
- Older conventions — also related to these groundings in an earlier era are the reliance on functions and functional predicates for many relations, and the absence of the current triple data model underlying RDF. While it is true that OWL versions of OpenCyc have been made and are the basis for UMBEL’s work to date, there are also errors in these translations perhaps in some instances due to the lesser expressiveness of RDF and OWL
- Documentation — while complete reference materials can ultimately be found, it is difficult to do so and introductory and entry-level tutorials could stand to be augmented
- Named entities — for many years, but now especially with the emergence of Wikipedia, Cyc has been criticized for its relative paucity of named entity coverage and imbalances of what it does contain. While from UMBEL’s perspective this appears to be strictly correct, such criticism misses the mark of Cyc’s special purpose and contributions as a solid conceptual and common sense framework. Those common-sense portions of the system are more immutable, and can be readily mapped to named entity sources. Indeed, perhaps Cyc will now see new vigor as the Web becomes a superior source for contemporary named entity coverage while Cyc fulfills its more natural (and needed) structural role.
Surprisingly, for a system of its age and evolution, Cyc seems to have adhered well to naming conventions and other standards.
UMBEL’s project diligence thus found the biggest issue going forward to be the cruft in the system. There is a solid structure underneath Cyc, but one that is too often obscured and not made as shiny and clean as it deserves.
The Decision and Design
Five months of nearly full-time due diligence was devoted to this question of the suitability of Cyc as the intellectual grounding for UMBEL.
On balance, OpenCyc’s benefits significantly outweighed its weaknesses. This balance also stands considerably superior to all potential alternatives.
An important factor through this deliberation was the commitment of Cycorp and The Cyc Foundation to the aims of UMBEL, and the willingness of those organizations to lend time and effort to promote UMBEL’s aims. Twenty years of development and the investment of decades of human effort and scrutiny provides a foundation of immense solidity.
Though perhaps Wikipedia (or something like it also based on broad Web input) might emerge with the scope and completeness of Cyc, that prospect is at minimum some years away and by no means certain. No other current framework than Cyc can meet UMBEL’s immediate purposes. Moreover, as stated at the outset, UMBEL’s purpose is pragmatic. We will leave it to others to argue the philosophical nuances of ontology design and “truth” while we get on with the task of creating context of real value.
The next decision was to base all UMBEL subject concepts on existing concepts in OpenCyc.
This means that UMBEL inherits all of the structural relations already in OpenCyc. It also means that UMBEL can act as a sort of contextual middleware between unstructured Web content and the inferential and tools infrastructure within OpenCyc (and beyond into ResearchCyc and Cyc for commercial purposes) and back again to the Web. We term this “roundtripping” and the capability is available for any of the 21,000 subject concepts vetted from OpenCyc within UMBEL.
Having made these commitments, our next effort was to break out the brushes, roll up the sleeves, and plunge into a Spring session of deep cleaning. This effort to vet and clean OpenCyc will be documented in the Technical Report to accompany the first release of the UMBEL ontology. We think you’ll like its shiny new look.
Part 3 of 4 on Foundations to UMBEL
UMBEL (Upper-level Mapping and Binding Exchange Layer) is a lightweight ontology for relating Web content and data to a standard set of subject concepts. It is being designed to apply to all types of data on the Web from RSS and Atom feeds to tagging to microformats and topic maps to RDF and OWL (among others). The project Web site is at http://www.umbel.org.
The first portion and priority for UMBEL is to prepare the lightweight subject concept ontology, the focus of this four-part foundations series. After the UMBEL ontology is released in first draft, the project will then turn to the binding protocols for non-RDF formats.
The previous part in this series discussed at length RDF classes and instances or individuals. We are now tightening these terms down to reflect the specific intents and usage within UMBEL. UMBEL’s main classes categorize subject concepts; notable instances are specifically termed named entities.
UMBEL defines subject concepts as a distinct subset of the more broadly understood concept  such as used in the SKOS RDFS controlled vocabulary , conceptual graphs, formal concept analysis or the very general concepts common to many upper ontologies . We define subject concepts as a special kind of concept: namely, ones that are concrete, subject-related and non-abstract .
UMBEL contrasts subject concepts with abstract concepts and with named entities. Abstract concepts represent abstract or ephemeral notions such as truth, beauty, evil or justice, or are thought constructs useful to organizing or categorizing things but are not readily seen in the experiential world. Named entities are the real things or instances in the world that are themselves natural and notable class members of subject concepts.
Subject concepts are a special kind of concept: namely, ones that are concrete, subject-related and non-abstract. Note in other systems or ontologies similar constructs may alternatively be called topics, subjects, concepts or perhaps interests. UMBEL has adopted the term subject concept to distinguish from these uses, which have different nuances of meaning and use, as well as to highlight the subject or topic nature of UMBEL's concrete concepts.
Each subject concept is a class. While subject concepts have a preferred label (using SKOS terminology), they are representative or a proxy for that concept, and not to be confused with the thing itself. Every UMBEL subject concept can be expressed and referred to by a different preferred label in alternate languages. Indeed, in a given language, different preferred labels may be swapped out without affecting the identity or use of the subject concept itself. The name for a subject concept is therefore merely a handle.
Subject concepts are the core constituents to the UMBEL framework. All subject concepts are based on existing concepts in OpenCyc, the open source version of the Cyc knowledge base (see Part 4). About 21,000 of them have been distilled and are part of the UMBEL backbone.
Semsets are semantically close terms or phrases synonomous or nearly so with the meanings of a subject concept or a named entity. Semsets are akin to WordNet synsets or Cyc aliases, but can also include more contemporary jargon or slang as may be drawn from Web tagging or folksonomies. The term semset has been chosen to distinguish this consolidated meaning.
Semsets may apply to either subject concepts or named entities. In the latter case, their use is closer to the sense of an alias (such as nicknames, or "great satan" or "uncle sam" for the "United States").
Abstract concepts represent abstract or ephemeral notions such as truth, beauty, evil or justice, or are thought constructs useful to organizing or categorizing things but are not readily seen in the experiential world. They are included in the UMBEL specification because they help maintain the integrity of the UMBEL subject concept graph.
Like subject concepts, abstract concepts are based strictly on those already in OpenCyc. Abstract concepts may be viewed in the UMBEL graph, and may be used for ontology mapping, but are not generally displayed when doing standard content mapping or concept look-ups via Web services. For various domain extraction or relatedness determinations, abstract concepts may be excluded from UMBEL's internal processing.
Named entities are the real things or instances in the world that are themselves natural and notable class members of subject concepts. The initial named entities are drawn from Wikipedia as processed via YAGO, and other online fact-based repositories. Named entities are the instances of the subject concepts in the standard definition of the term .
Named entities and the sources for them are also a major avenue for growth and expansion of UMBEL moving forward. Named entities are more contemporary and changing, while the reference subject concept backbone is more fixed and stable.
Each named entity is mapped to a governing subject concept for ontology purposes. There are no relations between named entities except as mediated through a subject concept(s). As noted, named entities may also have semset aliases.
Subject Concepts v. Abstract Concepts
The following table helps draw the distinction between subject concepts and abstract concepts. Technical documentation at the time of the UMBEL ontology release will list the 520 or so abstract concepts presently within UMBEL. Looking at those can help draw the distinction.
|Subject Concepts|| Abstract Concepts|
Subject Concepts v. Named Entities
The following table helps draw the distinction between subject concepts and named entities. Technical documentation at the time of the UMBEL ontology release will describe certain "gray" categories and the determination as to whether they should be treated as one or the other.
For example, most geographical places clearly belong to the named entity category. But, on somewhat arbitrary grounds, all nations, countries, states and provinces were assigned as subject concepts so that they would act as classes with other entities mapped to them. It should also be noted that entites or concepts in the gray zone may be treated both as a named entity and a subject concept.
|Subject Concepts||Named Entities|
Though there are shades of gray between subject concepts and named entities, we have found this distinction to be a powerful means for gaining clarity in UMBEL’s design. It provides a clean path for keeping an ontology lightweight while in essence providing infinite extensibility for all manner of named entities and the datasources that contain them. Moreover, the ability to classify named entities into types orthogonal to subject concepts also provides useful guidance for presentation templates that may be automatically invoked in data meshups. But, that is a topic for another day.
Part 2 of 4 on Foundations to UMBEL
Linked Data follows recommended practices for identifying, exposing and connecting data. A robust Linking Open Data (LOD) community has developed around the practice in the past year with the size of compliant data now exceeding several billion RDF triples.
Like any new development, there has been the need for best practices to be articulated and documented. Some of the best guides are How to Publish Linked Data on the Web from Chris Bizer, Richard Cyganiak and Tom Heath; Cool URIs for the Semantic Web from Leo Sauermann and Richard Cyganiak; the Linked Data for the Web chapter from Joshua Tauberer; and Deploying Linked Data from OpenLink Software. Also, to see and experience Linked Data just follow Kingsley Idehen’s blog and prolific mailing list postings, almost always with valuable demos and links.
The techniques these documents most often explain deal with such items as exposing and dereferencing URIs, content negotiation and naming and distinguishing so-called (unfortunately) information resources and non-information resources . The above references cover these topics with good clarity. The general tenor of these guides is on the techniques of exposing and publishing Linked Data.
OK About Exposing, What About Linking?
UMBEL is really a mechanism for aiding the linkage of data, not exposing it per se, so we will leave the discussion of exposing and publishing Linked Data to these other venues. Those other venues deal well with the Data portion of Linked Data. We want to focus here on the Linked portion.
At first blush, it is surprising how little is actually said or written about this linkage portion. For example, in the best practices How to Publish Linked Data there is fairly minor discussion of external links in Section 2.2 and then the sole discussion on links limited to Section 6.
To quote in part:
RDF links enable Linked Data browsers and crawlers to navigate between data sources and to discover additional data. The application domain will determine which RDF properties are used as predicates. . . . It is common practice to use the owl:sameAs property for stating that another data source also provides information about a specific non-information resource. An owl:sameAs link indicates that two URI references actually refer to the same thing. . . . RDF links can be set manually, which is usually the case for FOAF profiles, or they can be generated by automated linking algorithms. This approach is usually taken to interlink large datasets.
Upon reflection, though, perhaps less coverage of the linkage portion of Linked Data is not that surprising. The Linked Data practice is barely one year old and, while growing at a most impressive rate, is still in the very earliest phases. Frankly, until recently, there has not been really that much data to Link.
We can see the status of Linked Data via the now-famous Linked Open Data diagram maintained by Richard Cyganiak (see  for the most recent interactive version; this one is current as of the date of posting):
Many have used this figure before (including me) to make general statements about the state of Linked Data. In this post, however, I want to comment on some different aspects.
While new data sources (or bubbles) are being added constantly, I count 43 “mappings” on this diagram (the arrows, and ignoring bi-directional) and 34 different sources (the bubbles). Nineteen of those mappings involve DBpedia, the exposed data of Wikipedia, and 11 involve FOAF.
owl:sameAs relations between possible datasets are in essence a pairwise mapping, similar to how a group of people might toast one another by clinking glasses. This type of pairwise relationship is kind of like an additive analog to a factorial, which is actually a quadratic function more specifically known as a triangular number. As new datasets get added, we see a progression of the form 1, 3, 6, 10, 15, 21, 28, 36, etc., representing the number of these possible pairwise mappings (“glass clinks”) between datasets.
The actual equation for this progression is n*((n+1)/2), where n is the number of datasets (N) – 1. Nominally, then, the number of 34 dataset bubbles could lead to as many as 561 pairwise mappings. But, again, only 43 are shown.
Of course, we are still only talking about potential pairwise mappings between datasets, and not the number of actual instance mappings themselves. DBpedia alone contains 1.5 million or so instances.
We can factor our progression into the Big O computer science notation consistent with the quadratic form of O(n2). Now, with instances numbering into the millions compounded by only a very few datasets and their pairwise mappings, we are still talking about potentially astronomical numbers to express as linked triples.
Yet the actual number of mapped triples is much lower than these potential maximums. The amount of Linked Data remains tractable. Why?
The first and obvious answer is that not all pairwise mappings make sense and not all instances are equivalent (sameAs). This factor will always be true. But it does not alone account for the efficiency.
The second less obvious answer is that certain of the datasets act as reference nodes or hubs. By having them, everything need not be mapped to everything else. We can express linkages as N to one or N to a few, and not the asymptotically growing N-to-N. Newly added datasets may often and for a notable portion of instances only need to be mapped to the reference nodes in order to link their data into the network.
DBpedia, with its scope and richness of notable instances, therefore, plays an essential role in Linked Data as expressed to date. Other comprehensive and authoritative sources can act similarly. In this manner, the development of the Linked Data graph may mirror the hub aspects of the existing World Wide Web document graph.
Rich reference nodes acting as hubs appears to be a key to the scalability of the Linked Data Web.
A Short Aside on These Links
[A publisher exposes Linked Data by making the URI of an RDF resource ("data") accessible via HTTP. When encountered, an agent (such as a browser or crawler) can then dereference this resource to a Web-based URL address for retrieval.
Any attribute or relationship that describes such Linked Data may be accessed at time of retrieval. A relationship defines an external link when either the subject or object of the triple is an external URI. If that resource's URI has also been properly exposed, we can now trace it to still new relations and resources, akin to data surfing (so long as the trail of resources remains exposed). In a parallel analogy to document hyperlinks, some have termed this 'hyperdata' .
But that leads to a funny thing. Without this fetch or retrieval, there is generally no explicit publishing or knowledge of the external links for these resources. In other words, these external links are not “publicly” obvious (so to speak), until the Linked Data resource is discovered or stumbled upon. So, while our current recipes give us best practices for how to expose and publish resources (the Data), we have no similar guidance — and, frankly, no practice — for the Linked portion of Linked Data.
To carry the analogy a bit further, while Linked Data is acting to break the barriers of data silos, the relations and linkages of that data remain in those silos until accessed. This may indeed be the proper thing, but somehow it has a feeling of the early years of the document Web before services like Yahoo! began publishing listings of useful links.
Given that the mappings between data sources represent new and often expensive manual or automated effort, it seems like invested value is not being sufficiently shared. Fortunately, there is nothing preventing us from explicitly dereferencing these linked mapping triples along with standard resource triples. We just need to begin doing so.
But I digress. ]
Another Little Known Secret of Linked Data
The careful reader may have noticed a couple of earlier implications. Current Linked Data is useful for linking data for given instances from different data sources (say, for combining political, demographic and mapping information for a geographic place like Quebec City) via owl:sameAs. But that predicate is an instance-level relationship that only works for the very same individuals . Our current ability to make external linkages is largely constrained to the instance level.
Moreover, such instance-level links lack context and a conceptual framework for inferencing or determining relatedness between concepts or in relation to other instances. Today’s state-of-the-art is not really about linking “things” (as quoted before) when we establish Linked Data. It is more about linking atomic instances, the members of “things”. We have no current framework for relating things at the concept level.
Put another way, Linked Data presently lacks practical frameworks or mechanisms for linking at the class level.
On the face of it this sounds contradictory to what we know about RDF and how it is designed. The language and its formalisms and indeed many of the popular RDF schema have a rich set of classes. Classes are easy to design and spin-off on the fly.
So, the commentary about lack of a framework is NOT about the lack of logic or vocabulary or even schema or ontologies (though there are certainly some gaps there). Rather, it is based on the lack of reference nodes or structures upon which to base those connections. When there is no fixed or defined point in information space, everything floats; there is no framework for connections. There is no grounding.
So, technically, while class-level data connections are not prevented and can be made with Linked Data, to our knowledge few or none presently exist . This is a little known secret with far-reaching implications.
Just as DBpedia provided the nucleating hub for linking instance data, UMBEL is designed to provide a similar reference node for concepts. These concepts provide some fixed positions in the information space to which other sources can link and relate. And, like references for instance data, the existence of reference concepts can greatly diminish the number of links necessary for an efficient Linked Data environment.
Though the nature of the reference set is important (and we describe UMBEL’s choice of OpenCyc in Part 4 of this series), a more fundamental point is that a reference structure of almost any nature has this value. We can argue later about what is the best reference structure.
But the first task is to just get one in place and begin bootstrapping. Indeed, over time, it is likely that a few reference structures will emerge and compete and get supplemented by still further structures. This evolution is expected and natural and desirable in that it provides choice and options.
A reference structure of concepts has the further benefit of providing a logical reference structure for instance data as well. While a DBpedia (based on Wikipedia) is perhaps the most comprehensive collection of humanity-wide instances, no single source can or will be complete in scope. Thus, we foresee specialty sources ranging from the companies in Wikicompany to plants and animals in the Encyclopedia of Life or thousands of other rich instance sources also acting as reference hubs.
How do each of these rich instance sources relate to one another? What is the subject concept or topical basis by which they overlap or complement? What is the framework and graph structure of knowledge to give this information context?
These roadmaps and signposts are UMBEL’s formal purpose.
Taking Linked Data to the Next Level
Mapping between classes is a much different — and more complicated — matter than mapping instances. As editors we are still grappling with design choices here and are playing with ideas such as confidence metrics to capture the relative accuracy of set matching methods. Later ontology documentation will discuss these designs further.
The rationale for UMBEL and our observations on the state of current Linked Data is not meant to be critical. The community is early in its understanding of how to do Linked Data and scale it. Personally as editors and then on behalf of our company, we have clearly committed to Linked Data as a practice and objective.
In summary, our review of the current state of Linked Data suggests that we:
- Need reference sets to aid scalability
- Need context (UMBEL classes) for adding new reference instances, and
- Need context for relating classes to one another.
Reference sets are a real key, both for instances and concepts. Using them by no means implies centrality or a loss of the distributed advantages of the Web. DBpedia (Wikipedia) has not had this effect for instances and UMBEL will not do so for concepts.
Nor does the use of reference sets imply the need to reach some global consensus or to close out any alternatives. Reference hubs and choice and freedom are not in conflict. Placing data in context will show clear advantages over data absent context. The argument will be settled as simply as that.
Now that Linked Data has put forward the recipes and mechanisms for opening up and sharing data on the Web, it is now time to take the initiative to the next level by providing the contextual signposts and roadmaps for those linkages.