OWL Web Ontology Language Use Cases and Requirements
OWL Web Ontology Language Use Cases and Requirements
W3C Recommendation 10 February 2004
New Version
Available: OWL 2
(Document Status Update, 12 November 2009)
The OWL Working Group has produced
a W3C Recommendation for a new version of OWL which adds
features to this 2004 version, while remaining compatible.
Please see OWL 2
Document Overview for an introduction to OWL 2 and a guide
to the OWL 2 document set.
This document specifies usage scenarios, goals and requirements
for a web ontology language. An ontology formally
defines a common set of terms that are used to describe and
represent a domain. Ontologies can be used by automated tools to
power advanced services such as more accurate web search,
intelligent software agents and knowledge management.
This document has been reviewed by W3C Members and other interested
parties, and it has been endorsed by the Director as a W3C
Recommendation. W3C's role in making the Recommendation is to
draw attention to the specification and to promote its widespread
deployment. This enhances the functionality and interoperability of
the Web.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
The Semantic Web is a vision for the future of the Web in
which information is given explicit meaning, making it easier for
machines to automatically process and integrate information
available on the Web. The Semantic Web will build on XML's
ability to define customized tagging schemes
[XML] and RDF's flexible
approach to representing data
[RDF Concepts]. The next element
required for the
Semantic Web is a web ontology language which can formally
describe the semantics of classes and properties used in web
documents. In order for machines to perform useful reasoning
tasks on these documents, the language must go beyond the basic
semantics of RDF Schema [RDF Vocabulary].
This document is one part of the specification of OWL, the Web Ontology
Language. The
Document
Roadmap section of the OWL Overview document describes each of the
other documents. This document enumerates the
requirements of a web ontology language as perceived by the working group.
However, it is expected that future
languages will extend OWL, adding, among other things,
greater logical capabilities and the ability to establish trust
on the Semantic Web.
We motivate the need for a web ontology language
by describing six use
cases. Some of these use cases are based on efforts currently
underway in industry and academia, others demonstrate more
long-term possibilities. The use cases are followed by design
goals that describe high-level objectives and guidelines for
the development of the language. These design goals will be
considered when evaluating proposed features. The section on Requirements
presents a set of features that should be in the language and
gives motivations for those features. The Objectives
section describes a list of features that might be useful for
many use cases but may not necessarily be addressed by the
working group.
The
Web Ontology Working Group charter tasks the group to produce
this more expressive semantics and to specify mechanisms by which
the language can provide "more complex relationships between
entities including: means to limit the properties of classes with
respect to number and type, means to infer that items with various
properties are members of a particular class, a well-defined model
of property inheritance, and similar semantic extensions to the
base languages." The detailed specification of the web ontology
language will take into consideration:
the design goals and requirements that are contained in this
document
review comments on this document from public feedback,
invited experts and working group members
specifications of or proposals for languages that meet many
of these requirements
An ontology defines the terms used to describe and represent
an area of knowledge. Ontologies are used by people, databases,
and applications that need to share domain information (a domain
is just a specific subject area or area of knowledge, like
medicine, tool manufacturing, real estate, automobile repair,
financial management, etc.). Ontologies include computer-usable
definitions of basic concepts in the domain and the relationships
among them (note that here and throughout this document, definition
is not used in the technical sense understood by logicians).
They encode knowledge in a domain and also knowledge
that spans domains. In this way, they make that knowledge
reusable.
The word ontology has been used to describe artifacts with
different degrees of structure. These range from simple
taxonomies (such as the Yahoo hierarchy), to metadata schemes
(such as the Dublin Core), to logical theories. The Semantic Web
needs ontologies with a significant degree of structure. These
need to specify descriptions for the following kinds of
concepts:
Classes (general things) in the many domains of interest
The relationships that can exist among things
The properties (or attributes) those things may have
Ontologies are usually expressed in a logic-based language, so
that detailed, accurate, consistent, sound, and meaningful
distinctions can be made among the classes, properties, and
relations. Some ontology tools can perform automated reasoning
using the ontologies, and thus provide advanced services to
intelligent applications such as: conceptual/semantic search and
retrieval, software agents, decision support, speech and natural
language understanding, knowledge management, intelligent
databases, and electronic commerce.
Ontologies figure prominently in the emerging Semantic Web as
a way of representing the semantics of documents and enabling the
semantics to be used by web applications and intelligent agents.
Ontologies can prove very useful for a community as a way of
structuring and defining the meaning of the metadata terms that
are currently being collected and standardized. Using ontologies,
tomorrow's applications can be "intelligent," in the sense that
they can more accurately work at the human conceptual level.
Ontologies are critical for applications that want to search
across or merge information from diverse communities. Although
XML DTDs and XML Schemas are sufficient for exchanging data
between parties who have agreed to definitions beforehand, their
lack of semantics prevent machines from reliably performing this
task given new XML vocabularies. The same term may be used with
(sometimes subtle) different meaning in different contexts, and
different terms may be used for items that have the same meaning.
RDF and RDF Schema begin to approach this problem by allowing
simple semantics to be associated with identifiers. With RDF Schema, one
can define classes that may have multiple subclasses and super
classes, and can define properties, which may have sub properties,
domains, and ranges. In this sense, RDF Schema is a
simple ontology language. However, in order to achieve
interoperation between numerous, autonomously developed and
managed schemas, richer semantics are needed. For example, RDF
Schema cannot specify that the Person and Car classes are
disjoint, or that a string quartet has exactly four musicians as
members.
One of the goals of this document is to specify what is needed
in a web ontology language. These requirements will be motivated
by potential use cases and general design objectives that take
into account the difficulties in applying the standard notion of
ontologies to the unique environment of the Web.
Ontologies can be used to improve existing Web-based
applications and may enable new uses of the Web. In this section
we describe six representative use cases of web ontologies. Note
that this is not an exhaustive list, but instead a cross-section
of interesting use cases. These use cases served as a guideline in
choosing requirements for the OWL language (see
Section 4). The requirements were
chosen based on the aspects of the use cases that the working group
considered most important, while considering the scope of the OWL
charter and other design constraints. As such, one should not assume
that OWL will directly support every aspect of the use cases.
A web portal is a web site that provides information content on a
common topic, for example a specific city or domain of interest. A web
portal allows individuals that are interested in
the topic to receive news, find and talk to one another, build a
community, and find links to other web resources of common
interest.
In order for a portal to be successful, it must be a starting
place for locating interesting content. Typically this content is
submitted by members of the community, who often index it under
some subtopic. Another means of collecting content relies on the
content providers tagging the content with information that can
be used in syndicating it. Typically, this takes the form of
simple metatags that identify the topic of the content, etc.
However, a simple index of subject areas may not provide the
community with sufficient ability to search for the content that
its members require. In order to allow more intelligent syndication,
web portals can define an ontology for the community.
This ontology can provide a terminology for describing
content and axioms that define terms using other terms from the
ontology. For example, an ontology might include terminology such as
"journal paper," "publication," "person," and "author." This
ontology could include definitions that state things such as
"all journal papers are publications" or "the authors of all
publications are people." When combined with facts, these definitions
allow other facts that are necessarily true to be inferred. These
inferences can, in turn, allow users to obtain search results from
the portal that are impossible to obtain from conventional retrieval
systems.
Such a technique relies on content providers using the web ontology
language to capture high-quality ontology relationships, and an objective
of OWL is to enable sufficiently rich and useful metadata content to
motivate the necessary effort. It is a separate challenge to minimize this
effort and an ontology language will likely have a greater impact if it can
facilitate metadata capture as an nonintrusive part of any information
creation process.
One example of an ontology based portal is OntoWeb. This portal serves
the academic and industry community that is interested in
ontology research. Another example of a portal that uses Semantic
Web technologies and could benefit from an ontology language is
The Open Directory Project; a large,
comprehensive human-edited directory of the Web. It is constructed
and maintained by a vast, global community of volunteer editors.
RDF dumps of the Open Directory database are available for download.
Ontologies can be used to provide semantic annotations for collections of
images, audio, or other non-textual objects.
It is even more difficult for machines to extract
meaningful semantics from multimedia than it is to extract
semantics from natural language text. Thus, these types of resources
are typically indexed by captions or metatags. However, since
different people can describe these non-textual objects in
different ways, it is important that the search facilities go
beyond simple keyword matching. Ideally, the ontologies would
capture additional knowledge about the domain that can be used to
improve retrieval of images.
Multimedia ontologies can be of two types: media-specific and
content-specific. Media specific ontologies could have taxonomies of
different media types and describe properties of different media.
For example, video may include properties to identify length of
the clip and scene breaks. Content-specific ontologies could
describe the subject of the resource, such as the setting or
participants. Since such ontologies are not specific to the media,
they could be reused by other documents that deal with the same
domain. Such reuse would enhance search that was simply looking
for information on a particular subject, regardless of the
format of the resource. Searches where media type was important
could combine the media-specific and content-specific ontologies.
As an example of a multimedia collection, consider an archive of
images of antique furniture. An ontology of antique furniture would be
of great use in searching such an archive. A taxonomy can be used
to classify the different types of furniture. It would also be
useful if the ontology could express definitional knowledge.
For example, if an indexer selects the value "Late Georgian" for the
style/period of (say) an antique chest of drawers, it should be
possible to infer that the data element "date.created" should have a
value between 1760 and 1811 A.D. and that the "culture" is
British. Availability of this type of background knowledge
significantly increases the support that can be given for
indexing as well as for search. Another feature that could be
useful is support for the representation of default
knowledge. An example of such knowledge would be that a
"Late Georgian chest of drawers," in the absence of other information,
would be assumed to be made of mahogany.
This knowledge is crucial for real semantic queries, e.g. a
user query for "antique mahogany storage furniture" could match
with images of Late Georgian chests of drawers, even if nothing
is said about wood type in the image annotation.
Large corporations typically have numerous web pages
concerning things like press releases, product offerings and case
studies, corporate procedures, internal product briefings and
comparisons, white papers, and process descriptions. Ontologies
can be used to index these documents and provide better means of
retrieval. Although many large organizations have a taxonomy for
organizing their information, this is often insufficient. A
single ontology is often limiting because the constituent categories are
likely constrained to those representing one view and one granularity of a
domain; the ability to simultaneously work with multiple ontologies
would increase the richness of description. Furthermore, the ability to
search on values for different parameters is often more useful than
a keyword search with taxonomies.
An ontology-enabled web site may be used by:
A salesperson looking for sales collateral relevant to a
sales pursuit.
A technical person looking for pockets of specific
technical expertise and detailed past experience.
A project leader looking for past experience and templates
to support a complex, multi-phase project, both during the
proposal phase and during execution.
A typical problem for each of these types of users is that
they may not share terminology with the authors of the desired
content. The salesperson may not know the technical name for
a desired feature or technical people in different fields might
use different terms for the same concept. For such problems,
it would be useful for each class of user to have different
ontologies of terms, but have each ontology interrelated so
translations can be performed automatically.
Another problem is framing queries at the right level of
abstraction. A project leader looking for someone with expertise
in operating systems should be able to locate an employee who
is an expert with both Unix and Windows.
One aspect of a large service organization is that it may have
a very broad set of capabilities. But when pursuing large
contracts these capabilities sometimes need to be assembled in
new ways. There will often be no previous single matching
project. A challenge is to reason about how past templates and
documents can be reassembled in new configurations, while
satisfying a diverse set of preconditions.
This use case is for a large body of engineering documentation,
such as that used by the aerospace industry. This documentation can be of
several different types, including design documentation, manufacturing
documentation, and testing documentation. These document sets each have
a hierarchical structure, but the structures differ between the sets.
There is also a set of implied axes which cross-link the documentation
sets: for example, in aerospace design documents, an item such as a wing
spar might appear in each.
Ontologies can be used to build an information model which allows
the exploration of the information space in terms of the items which
are represented, the associations between the items, the properties
of the items, and the links to documentation which describes and
defines them (i.e., the external justification for the existence of
the item in the model). That is to say that the ontology and taxonomy
are not independent of the physical items they represent, but may be
developed/explored in tandem.
A concrete example of this use case is design
documentation for the aerospace domain, where typical users include:
Maintenance engineer looking for all information relating
to a particular part (e.g., "wing-spar").
Design engineer looking at
constraints on re-use of a particular sub-assembly.
To support this kind of usage, it is important
that constraints can be defined. These constraints may be used to
enhance search or check consistency. An example of a constraint might
be:
biplane(X) => CardinalityOf(wing(X)) = 2
wingspar(X) AND wing(Y) AND isComponentOf(X,Y) => length(X) < length(Y)
Another common use of this kind of ontology is to support the
visualization and editing of charts which show snapshots of the information
space centered on a particular concept (e.g., a class or instance). These
are typically activity/rule diagrams or entity-relationship diagrams.
The Semantic Web can provide agents with the capability to
understand and integrate diverse information resources. A
specific example is that of a social activities planner, which
can take the preferences of a user (such as what kinds of films
they like, what kind of food they like to eat, etc.) and use
this information to plan the user's activities for an evening.
The task of planning these activities will depend upon the
richness of the service environment being offered and the needs
of the user. During the service determination / matching process,
ratings and review services may also be consulted to find closer
matches to user preferences (for example, consulting reviews and
rating of films and restaurants to find the "best").
This type of agent requires domain ontologies that represent the terms
for restaurants, hotels, etc. and service ontologies to represent the terms
used in the actual services. These ontologies will enable the capture of
information necessary for applications to discriminate and balance among
user preferences. Such information may be provided by a number of
sources, such as portals, service-specific sites, reservation sites and
the general Web.
Agentcities is an
example of an initiative that is exploring the use of agents in a
distributed service environment across the Internet. This will
involve building a network of agent platforms that represent real
or virtual cities, such as San Francisco or the Bay
Area, and populating them with the services of those cities.
Initially, these services will be oriented towards business to
consumer services, such as hotels, restaurants, entertainment,
etc., but eventually, they will be expanded to include business
to business services, such as payroll, and business to enterprise
services.
This will require a number of different domain and service
ontologies: Key issues include:
Use and integration of multiple separate ontologies across
different domains and services
Distributed location of ontologies across the Internet
Potentially different ontologies for each domain or service
(ontology translation/cross-referencing)
Simple ontology representation to make the task of defining
and using ontologies easier
Ubiquitous computing is an emerging paradigm of personal
computing, characterized by the shift from dedicated computing
machinery to pervasive computing capabilities embedded in our
everyday environments. Characteristic to ubiquitous computing are
small, handheld, wireless computing devices. The pervasiveness
and the wireless nature of devices require network architectures
to support automatic, ad hoc configuration. An additional reason
for development of automatic configuration is that this
technology is aimed at ordinary consumers.
A key technology of true ad hoc networks is service discovery,
functionality by which "services" (i.e., functions offered by
various devices such as cell phones, printers, sensors, etc.) can
be described, advertised, and discovered by others. All of the
current service discovery and capability description mechanisms
(e.g., Sun's JINI, Microsoft's UPnP) are based on ad hoc
representation schemes and rely heavily on standardization (i.e.,
on a priori identification of all those things one would want to
communicate or discuss).
The key issue (and goal) of ubiquitous computing is
"serendipitous interoperability," interoperability under
"unchoreographed" conditions, i.e., devices which weren't
necessarily designed to work together (such as ones built for
different purposes, by different manufacturers, at a different
time, etc.) should be able to discover each others' functionality
and be able to take advantage of it. Being able to "understand"
other devices, and reason about their services/functionality is
necessary, since full-blown ubiquitous computing scenarios will
involve dozens if not hundreds of devices, and a priori
standardizing the usage scenarios is an unmanageable task.
The interoperation scenarios are dynamic in nature (i.e., devices appear
and disappear at any moment as their owners carry them from one room or
building to another) and do not involve humans in the loop as far as
(re-)configuration is concerned. The tasks involved in the utilization of
services involve discovery, contracting, and composition. The contracting
of services may involve representing information about security, privacy
and trust, as well as about compensation-related details (the provider of a
service may have to be compensated for services rendered). In particular,
it is a goal that corporate or organizational security policies be
expressed in application-neutral form, thus enabling constraint
representation across the diversity of enforcement mechanisms (e.g.,
firewalls, filters/scanners, traffic monitors, application-level routers
and load-balancers).
Thus, an ontology language will be used to describe the characteristics of
devices, the means of access to such devices, the policy established by the
owner for use of a device, and other technical constraints and requirements
that affect incorporating a device into a ubiquitous computing
network. The needs established for
DAML-S (particularly the issues
surrounding the expressiveness of the language) and the
RDF-based schemes for representing information about device characteristics
(namely, W3C's Composite
Capability/Preference Profile (CC/PP) and WAP Forum's
User Agent Profile or UAProf) directly relate to this use case
and the resource infrastructure which will support applications which
will negotiate and dynamically configure ad hoc networks.
Design goals describe general motivations for the language
that do not necessarily result from any single use case. Along
with the Use Cases, these motivate
the Requirements and
Objectives in Sections 4 and 5.
In this section, we describe eight design goals for the web ontology
language, in particular those dealing with:
using established ontologies
changing established ontologies
integrating established ontologies
detecting inconsistencies across ontologies and instances of use
providing a balance between expressivity and scalability when
creating ontologies
avoiding unnecessary complexity which may discourage widespread
adoption
maintaining compatibility with other standards
supporting internationalization
For each goal, we describe the tasks it supports and
explain the rationale for the goal. We also describe the degree
to which RDF and RDF Schema supports the goal.
3.1 Shared ontologies
Ontologies should be publicly available and different data
sources should be able to commit to the same ontology for shared
meaning. Also, ontologies should be able to extend other
ontologies in order to provide additional definitions.
Supported Tasks:
Any use case in which distributed data sources use shared
terminology.
Justification:
Interoperability requires agreements on the definitions of
identifiers. Ontologies can provide standard sets of identifiers and formal
descriptions of those identifiers. Data sources that commit to the same
ontology explicitly agree to use the same identifiers with the same
meanings.
Often, shared ontologies are not sufficient. An organization
may find that an existing ontology provides 90% of what it needs,
but the remaining 10% is critical. In such cases, the
organization should not have to create a new ontology from
scratch, but instead be able to create an ontology which extends
an existing ontology and adds any desired identifiers and
definitions.
RDF(S) Support:
In RDF, each schema has its own namespace identified by a URI.
Each resource in the schema has an ID, and a globally unique
identifier can be created by combining the ID with the
URI of the namespace. Any resource that uses this URI
references the term as defined in that schema. However, RDF is
unclear on the definition of a term that has partial definitions
in multiple schemas. The specification appears to assume that the
definition is the union of all descriptions that use the same
identifier, regardless of source. However, this may lead to
problems in a distributed environment, where some schemas may
contain incorrect or false definitions. There is no way in RDF
for a resource to indicate which set of definitions it agrees
to.
3.2 Ontology evolution
An ontology may change during its lifetime. A data source should specify
the version of an ontology to which it commits.
An important issue
is whether or not documents that commit to one version
of an ontology are compatible with those that commit to another.
Both compatible and incompatible revisions should be allowed, but
it should be possible to distinguish between the two. Note that
since formal descriptions only provide approximations for the
meanings of most identifiers,
it is possible for a revision to change the intended meaning of an
identifier without changing its formal description. Thus determining
semantic backwards-compatibility requires more than a simple
comparison of term descriptions. As such, the ontology author
needs to be able to indicate such changes explicitly.
Supported Tasks:
Any use case in which the ontology could potentially change,
and in particular those in which the owner of the ontology is
different from the data providers.
Justification:
Since the Web is constantly growing and changing, we must
expect ontologies to change as well. Ontologies may need to
change because there were errors in prior versions, because a new
way of modeling the domain is preferred, or because
new terminology has been created (e.g.,
as the result of the invention of new technology). A web ontology
language must be able to accommodate ontology revision. Note that
ontology evolution is different from ontology extension, which
does not change the original ontology.
RDF(S) Support:
The RDF Schema Specification recommends that each version of a
schema should be a separate resource with its own URI. The
rdfs:subClassOf and rdfs:subPropertyOf properties can be used to
relate new versions of classes and properties to older versions.
However, this has the drawback that incorrect definitions cannot
be retracted. For example, assume that in schema v1, v1:Dolphin
is a rdfs:subClassOf v1:Fish. When this mistake is noticed, the
new version of the schema, v2, says that v2:Dolphin is a
rdfs:subClassOf v2:Mammal. However, if we make v2:Dolphin a
rdfs:subClassOf v1:Dolphin, then we also say that v2:Dolphin is
an rdfs:subClassOf v1:Fish which perpetuates the error.
3.3 Ontology interoperability
Different ontologies may model the same concepts in different
ways. The language should provide primitives for relating
different representations, thus allowing data to be converted to
different ontologies and enabling a "web of ontologies."
Supported Tasks:
Any use case in which data from different providers with
different terminologies must be integrated.
Justification:
Although shared ontologies and ontology extension allow a
certain degree of interoperability between different
organizations and domains, there are often cases where there are
multiple ways to model the same information. This may be due to
differences in the perspectives of different organizations,
different professions, different nationalities, etc. In order for
machines to be able to integrate information that commits to
heterogeneous ontologies, there need to be primitives that allow
ontologies to map concepts to their equivalents in other ontologies.
RDF(S) Support:
RDF provides minimal support for interoperability by means of
the rdfs:subClassOf and rdfs:subPropertyOf properties.
3.4 Inconsistency detection
Different ontologies or data sources may be contradictory. It
should be possible to detect these inconsistencies.
Supported Tasks:
Any use cases in which decentralization of data and lack of
controlling authority can lead to conflicts in the data. Any
ontology extension task that may result in incoherent
descriptions (possibly by extending an ontology in a way that
generated an over constrained concept).
Justification:
The Web is decentralized, allowing anyone to say anything. As
a result, different viewpoints may be contradictory, or even
false information may be provided. In order to prevent agents
from combining incompatible data or from taking consistent data
and evolving it into an inconsistent state, it is important that
inconsistencies can be detected automatically.
RDF(S) Support:
RDF and RDFS do not allow inconsistencies to be expressed.
3.5 Balance of expressivity and scalability
The language should be able to express a wide variety of
knowledge, but should also provide for efficient means to reason
with it. Since these two requirements are typically at odds, the
goal of the web ontology language is to find a balance that
supports the ability to express the most important kinds of
knowledge.
Supported Tasks:
Any use case that uses large ontologies or large data sets and
requires the representation of diverse knowledge.
Justification:
There are over one billion pages on the Web, and the potential
application of the Semantic Web to embedded devices and agents
poses even larger amounts of information that must be handled.
The web ontology language should support reasoning systems that scale
well. However, the language should also be as expressive
as possible, so that users can state the kinds of knowledge that
is important to their applications.
Expressivity determines what can be said in the language, and
thus determines its inferential power and what reasoning
capabilities should be expected in systems that fully implement
it. An expressive language contains a rich set of primitives that
allow a wide variety of knowledge to be formalized. A language
with too little expressivity will provide too few reasoning
opportunities to be of much use and may not provide any
contribution over existing languages.
RDF(S) Support:
RDF is very scalable (with the exception of the XML syntax
being extremely verbose) but is not very expressive.
3.6 Ease of use
The language should provide a low learning barrier and have
clear concepts and meaning. The concepts should be independent
from syntax.
Supported Tasks:
Markup and querying of semantic web documents by users, either
directly or indirectly.
Justification:
Although ideally most users will be isolated from the language
by front end tools, the basic philosophy of the language must be
natural and easy to learn. Furthermore, early adopters, tool
developers, and power users may work directly with the syntax,
meaning human readable (and writable) syntax is desirable.
RDF(S) Support:
RDF is fairly easy to use, but RDF Schema is more complex. The
syntax appears to be a major barrier for many.
3.7 Compatibility with other standards
The language should be compatible with other
commonly used Web and industry standards. In particular, this includes XML
and related standards (such as XML Schema and RDF), and possibly other
modeling standards such as UML.
Supported Tasks:
Exchange of ontologies and data in a standard format.
Justification:
Compatibility with other standards eases tool
development and deployment of the language.
RDF(S) Support:
RDF has an XML serialization syntax [RDF Syntax].
3.8 Internationalization
The language should support the development of multilingual
ontologies, and potentially provide different views of ontologies
that are appropriate for different cultures.
Supported Tasks:
Tasks where the same ontology is used by multiple countries or
cultures.
Justification:
The Web is an international tool. The Semantic Web should aid
in the exchange of ideas and information between different
cultures, and should thus make it easy for members of different
nations to use the same ontologies.
RDF(S) Support:
To the extent that XML supports internationalization, so does
RDF.
4 Requirements
The use cases and design goals motivate a number of
requirements for a web ontology language. The Working Group
currently feels that the requirements described below are
essential to the language. Each requirement includes a short
description and is motivated by one or more
design goals from the previous section.
R1. Ontologies as distinct resources
Ontologies must be resources that have their own unique
identifiers, such as a URI reference.
Two concepts in different ontologies must have distinct absolute
identifiers (although they may have identical relative
identifiers). It must be possible to uniquely identify a concept in
an ontology using a URI reference.
Ontologies must be able to explicitly extend other ontologies
in order to reuse concepts while adding new classes and properties.
Ontology extension must be a transitive relation; if ontology A
extends ontology B, and ontology B extends ontology C, then
ontology A implicitly extends ontology C as well.
It must be possible to provide meta-data for each ontology,
such as author, publish-date, etc. These properties may
or may not be borrowed from the Dublin Core element set.
The language must provide features for comparing and relating
different versions of the same ontology. This should include
features for relating revisions to prior versions, explicit
statements of backwards-compatibility, and the ability to
deprecate identifiers (i.e., to state they are available for
backwards-compatibility only, and should not be used in new
applications/documents.)
The language must be able to express complex definitions of
classes. This includes, but is not limited to, sub classing and
Boolean combinations of class expressions
(i.e., intersection, union, and complement).
The language must be able to express the definitions of
properties. This includes, but is not limited to, sub properties,
domain and range constraints, transitivity, and inverse
properties.
The language must include features for stating that pairs of
identifiers represent the same individual (note that consistent
with the terminology used in other OWL documents, an individual
here is any instance of an OWL class, and does not necessarily
mean a person). Due to the distributed
nature of the Web, it is likely that different identifiers will
be assigned to the same individual. The use of a standard URL
does not solve this problem, because some individuals may have
multiple URLs, such as a person who has home and work web pages
or e-mail addresses.
The language must provide a way to allow statements to be "tagged" with
additional information such as source, timestamp, confidence level,
etc. The language need not provide a standard set of properties
that can be used in this way, but should instead provide a
general mechanism for users to attach such information.
RDF reification may be one possible way to
accommodate the requirement, although reification is a somewhat
controversial feature.
The language must support the ability to treat classes as
instances. This is because the same concept can often be seen as
a class or an individual, depending on the perspective of the
user. For example, in a biological ontology, the class Orangutan
may have individual animals as its instances. However, the class
Orangutan may itself be an instance of the class Species. Note, that Orangutan
is not a subclass of Species, because then that would say that each
instance of Orangutan (an animal) is an instance of Species.
The language must support the specification of cardinality
restrictions on properties. These restrictions set minimum and
maximum numbers of individuals that any single individual can be related
to via the specified property.
The language should have an XML serialization syntax. XML has become
widely accepted by industry and numerous tools
for processing XML have been developed. If the web ontology
language has an XML syntax, then these tools can be extended and
reused.
The language should support the specification of multiple alternative
user-displayable labels for the resources specified by an ontology. This can be
used, for example, to view the ontology in different natural languages.
In some character encodings, e.g. Unicode based encodings,
there are some cases where two different character sequences look
the same and are expected, by most users, to compare equal. An
example is one using a pre-composed form (just one c-cedilla
character) and another using a decomposed form (a 'c' character
followed by a cedilla accent character). Given that the W3C I18N
WG has decided that early uniform normalization (to Unicode
Normal Form C) as the usual approach to solving this problem, any
other solution needs to be justified.
In addition to the set of features that should be in the
language (as defined in the previous section), there are other
features that would be useful for many use cases. These features
will be addressed by the working group if possible, but the group
may decide that there are good reasons for excluding them from
the language or for leaving them to be implemented by a later
working group. Some of these objectives are not fully defined,
and as such need further clarification if they are to be
addressed by the language. Note that the order of the objectives
below does not imply relative priority or degree of consensus.
O1. Layering of language features
The language may support different levels of complexity for
defining ontologies. Applications can conform to a particular
layer without supporting the entire language. A guideline for
identifying layers may be based on functionality found in
different types of database and knowledge base systems.
The language should support the specification of default
values for properties. Such values could be used to make
inferences about typical members of classes. However, true
default values (such as those used in defeasible inheritance reasoning)
are nonmonotonic, which can be problematic on the
Web where new information is constantly being discovered or
added. Furthermore, there is no commonly accepted method for
dealing with defaults. An alternative is for the language
specification to recommend to users how they can create their own
default mechanisms.
Due to the size and rate of change on the Web, the
closed-world assumption (which states that anything that
cannot be inferred is assumed to be false) is inappropriate.
However, there are many situations where closed-world information
would be useful. Therefore, the language must be able to state that a given
ontology can be regarded as complete. This would then sanction additional
inferences to be drawn from that ontology. The precise semantics of such a
statement (and the corresponding set of inferences) remains to be defined,
but examples might include assuming complete property information about
individuals, assuming completeness of class-membership, and assuming
exhaustiveness of subclasses.
The language should support the ability to specify ranges of
values for properties. This mechanism may borrow from XML Schema
data types [XML-SCHEMA2].
The language may support the composition of properties in statements
about classes and properties. An example of the use of property
composition would be the assertion that a property called uncleOf
is the same as the composition of the fatherOf and brotherOf
properties.
The language should support the ability to commit to portions
of an ontology, as well as committing to an entire ontology.
However, it is unclear what granularity should be used here.
Possible choices are to choose a subset of concepts with their entire
definitions, or to choose individual pieces of definitions. Note
that borrowing partial definitions of concepts must address the potential
interoperability problems that can arise since different applications will be
using the same identifier to mean different things.
The language should support the ability to create ontology
views, in which subsets of an ontology can be specified or concepts can
be assigned alternate names. This is particularly useful in
developing multicultural versions of an ontology. Note that this
requirement may be satisfied by having multiple ontologies and
using an ontology mapping mechanism.
The W3C XML Digital Signature specification is an important building block
for communication among untrusted properties, which is important for many
ontology applications. The web ontology language should be designed in
a way that makes it straightforward to use XML Signatures.
The language should support string concatenation and simple
pattern matching. These features can be used to establish
interoperability between ontologies that treat complex
information as a formatted string and those that have separate
properties for each component. For example, one ontology may
represent an e-mail address as a single string, while another may
divide it into a string for user name and a string for server name.
To integrate the two ontologies, one would need to specify that
the concatenation of the user name, the '@' character, and the server
name is equivalent to the single value used in the first
ontology.
The language should support the ability to aggregate
information in a way similar to SQL's GROUP BY clause. It should
allow counts, sums, and other operations to be computed for each
group. This would allow interoperability between ontologies that
represented information at different levels of granularity, and
could relate things such as budget category totals to budget
line item amounts, or the number of personnel to individual data on
employees.
The language should support the use of executable code to
evaluate complex criteria. Procedural attachments greatly extend
the expressivity of the language, but are not well-suited to
formal semantics. A procedural attachment mechanism for web
ontologies should specify how to locate and execute the
procedure. One potential candidate language would be Java, which
is already well-suited to intra-platform sharing on the Web.
In general, the language will not make a unique names
assumption. That is, distinct identifiers are not assumed to
refer to different individuals (see Requirement
R11).
However, there are many applications where the unique names
assumption would be useful. Users should have the option of
specifying that all of the names in a particular namespace or
document refer to distinct individuals.
The language should support the definition and use of complex /
structured data types. These may be used to specify dates,
coordinate pairs, addresses, etc.
Industrial Strength Ontology Management,
Aseem Das, Wei Wu, and Deborah L. McGuinness. In Isabel Cruz, Stefan Decker, Jerome Euzenat, and Deborah L. McGuinness, editors. The Emerging Semantic Web. IOS Press, 2002. http://www.ksl.stanford.edu/people/dlm/papers/ontologyBuilderVerticalNet-abstract.html
RDF/XML Syntax Specification (Revised), Dave Beckett, Editor, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/ . Latest version available at http://www.w3.org/TR/rdf-syntax-grammar/ .
XML Schema Part 2: Datatypes, Paul V. Biron and Ashok Malhotra, Editors, W3C Recommendation, 2 May 2001. http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/ . Latest version available at http://www.w3.org/TR/xmlschema-2/ .
Appendix A: Change Log
Changes since Proposed Recommendation are listed in reverse chronological order.
Reformatted references section.
Acknowledgments
Raphael Volz and Jonathan Dale made significant authorial contributions
to this document, and along with the current editor, were co-editors
of the initial working drafts.
The initial effort to identify design goals and requirements was co-led
by Deborah McGuinness and the editor. Some of the requirements were
directly contributed by Dr. McGuinness [DWM]
based upon over a decade of work in building ontology-based systems.
Other requirements were identified as part of the editor's Ph.D. thesis
work in building a prototype Semantic Web system [Hef].
A draft version of the Corporate Web Site Management section was written
by Michael Smith.
The content of this document is the result of extensive discussions within the
Web Ontology Working Group
as a whole. The participants in this working group included:
Yasser alSafadi, Jean-François Baget, James Barnette, Sean
Bechhofer, Jonathan Borden, Stephen Buswell, Jeremy Carroll, Dan
Connolly, Peter Crowther, Jonathan Dale, Jos De Roo, David De
Roure, Mike Dean, Larry Eshelman, Jérôme Euzenat, Tim
Finin, Nicholas Gibbins, Sandro Hawke, Patrick Hayes, Jeff Heflin,
Ziv Hellman, James Hendler, Bernard Horan, Masahiro Hori, Ian
Horrocks, Jane Hunter, Ruediger Klein, Natasha Kravtsova, Ora
Lassila, Deborah McGuinness, Enrico Motta, Leo Obrst, Mehrdad
Omidvari, Martin Pike, Marwan Sabbouh, Guus Schreiber, Noboru
Shimizu, Michael K. Smith, John Stanton, Lynn Andrea Stein, Herman
ter Horst, David Trastour, Frank van Harmelen, Bernard Vatant,
Raphael Volz, Evan Wallace, Christopher Welty, Charles White,
Frederik Brysse, Francesco Iannuzzelli, Massimo Marchiori, Michael
Sintek and John Yanosy.