RACER stands for Renamed ABox and Concept Expression Reasoner.
RacerPro is the commercial name of the software.
The origins of RacerPro are within the area of description logics. Since description logics provide the foundation of international approaches to standardize ontology languages in the context of the so-called semantic web, RacerPro can also be used as a system for managing semantic web ontologies based on OWL (e.g., it can be used as a reasoning engine for ontology editors such as Protégé). However, RacerPro can also be seen as a semantic web information repository with optimized retrieval engine because it can handle large sets of data descriptions (e.g., defined using RDF). Last but not least, the system can also be used for modal logics such as Km.
The semantic web is aimed at providing "machine-understandable" web resources or by augmenting existing resources with "machine-understandable" meta data. An important aspect of future systems exploiting these resources is the ability to process OWL (Web Ontology Language) documents (OWL KBs), which is the official semantic web ontology language. Ontologies may be taken off the shelf or may be extended for domain-specific purposes (domain-specific ontologies extend core ontologies). For doing this, a reasoning system is required as part of the ontology editing system. RacerPro can process OWL Lite as well as OWL DL documents (knowledge bases). Some restrictions apply, however. OWL DL documents are processed with approximations for nominals in class expressions and user-defined XML datatypes are not yet supported.
A first implementation of the semantic web rule language (SWRL) is provided with RacerPro1.9 (see below for a description of the semantics of rules in this initial version).
The following services are provided for OWL ontologies and RDF data descriptions:
Future extensions for OWL (e.g., OWL-E) are already supported by RacerPro if the system is seen as a description logic system. RacerPro already supports qualified cardinality restrictions as an extension to OWL DL.
RacerPro is a knowledge representation system that implements a highly optimized tableau calculus for a very expressive description logic. It offers reasoning services for multiple T-boxes and for multiple A-boxes as well. The system implements the description logic ALCQHIR+ also known as SHIQ. This is the basic logic ALC augmented with qualifying number restrictions, role hierarchies, inverse roles, and transitive roles.
In addition to these basic features, RacerPro also provides facilities for algebraic reasoning including concrete domains for dealing with:
For these domains, no feature chains can be supported due to decidability issues. RacerPro supports the specification of general terminological axioms. A T-box may contain general concept inclusions (GCIs), which state the subsumption relation between two concept terms. Multiple definitions or even cyclic definitions of concepts can be handled by RacerPro.
RacerPro implements the HTTP-based quasi-standard DIG for interconnecting DL systems with interfaces and applications using an XML-based protocol . RacerPro also implements most of the functions specified in the older Knowledge Representation System Specification (KRSS).
Given a T-box, various kinds of queries can be answered. Based on the logical semantics of the representation language, different kinds of queries are defined as inference problems (hence, answering a query is called providing inference service). As a summary, we list only the most important ones here:
Note that whenever a concept is needed as an argument for a query, not only predefined names are possible, instead concept expressions allow for adaptive formulation of queries that have not been anticipated at system construction time.
If also an A-box is given, among others, the following types of queries are possible:
RacerPro provides another semantically well-defined query language (nRQL, new Racer Query Language), which also supports negation as failure, numeric constraints w.r.t. attribute values of different individuals, substring properties between string attributes, etc. In order to support special OWL features such as annotation and datatype properties, special OWL querying facilities have been incorporated into nRQL. The query language OWL-QL  is the W3C recommendation for querying OWL documents. nRQL has been used as a basic engine for implementing a very large subset of the OWL-QL query language (see above).
For some representation purposes, e.g., reasoning about spatial relations such as contains, touching, etc., relational algebras and constraint propagation have proven to be useful in practice. RacerPro combines description logics reasoning with, for instance, reasoning about spatial (or temporal) relations within the A-box query language nRQL. Bindings for query variables that are determined by A-box reasoning can be further tested with respect to an associated constraint network of spatial (or temporal) relationships.
Although RacerPro is one of the first systems supporting this kind of reasoning in combination with description logics (or OWL), we expect that international standardization efforts will also cover these important representation constructs in the near future. Note also that the semantically well-founded treatment can hardly be efficiently achieved using rule systems.
Description logic systems are no databases. Although one can use RacerPro for storing data using A-boxes, probably, databases provide better support with respect to persistency and transactions. However, databases can hardly be used if indefinite information is to be handled in an application (e.g., ӊohn was seen playing with a ball, but I cannot remember, it was soccer or basket ball, so he must be a soccer player or a basket-ball playerԩ. Being able to deal with indefinite information of this kind is important when information about data comes in from various sources, and in situations where sources are, for instance, used to exclude certain cases that are possible given the information at hand. Description logics are also important if data descriptions are to be queries with respect to varying T-boxes (or ontologies). Note that this is a common scenario in modern information technology infrastructure. Due to the rapid pace in technology evolution, also the vocabulary used to access data descriptions changes quite frequently. New concepts are introduced, and can be set into relation to older terminology using description logics (or semantic web ontologies).
There are numerous papers describing how description logic in general (and RacerPro in particular) can be used to solve application problems (see the International Workshops on Description Logics and the workshops on Applications of Description Logics ADL-01, ADL- 02, or ADL-03). Without completeness one can summarize that applications come from the following areas:
Among others the RacerPro system offers the following new features:
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