As part of the development of the concept of the semantic web in recent years have been created a large number of OWL ontologies and RDF datasets based on them. However, in the design and using the properties of OWL ontologies, is extremely important to correctly reflect real-world information, because something that is totally logical in the world of abstract data may poorly correlate with the expected behavior of web applications for the user. For example, SPARQL queries that using the transitive properties of the OWL language can create loops and return incorrect information. In this article we show that in some cases it is preferable to abandon the use of transitivity of properties in ontologies and describe an algorithm for traversing related entities, which allows solving the problem of loops. As an example, illustrating this problem is considered the web application "Linked Open Specialties" (LOS), which sends SPARQL queries to the ontology "Specialties". The ontology “Specialties” represents the structure of official lists of specialties, bachelors and masters graduate programs and research specialties that were valid in recent years in Russian Federation and allows us to establish their correspondence using the transitive property “equalsTo”. Notably, that, although the developed algorithm is formulated and used in terms of a specific subject area to solve the problem of a separate application, it is quite universal and can be used to solve the transitivity problem in RDF-datasets of other subject areas.
The past two decades have been characterized by the rapid development of
Semantic Web technologies. [
However, the capabilities of the RDF language are limited by binary predicates and
efficient computer processing of data is possible only when semantics are added to
RDF data sets. In practice, for this purpose are developed RDF datasets based on
OWL ontologies. In accordance with the definition of the W3C consortium, ontology
is understood as a formal model of knowledge representation in a certain subject area
that describes types of objects (classes), interaction between them (properties), and
ways of joint use classes and properties (axioms) [
The OWL language implements the basic requirements for ontology languages: clearly defined syntax, formal semantics, sufficient power of expression, ease of expressing knowledge and effective support for logical inference. Support for logical deduction allows to obtain new knowledge using existing knowledge and to detect different kinds of contradictions in ontologies, for example, unforeseen relations between classes or individuals. This process is possible by the mechanism of axioms (restrictions) - a kind of rules that operate in this ontology. Axioms of the OWL language allow us to present “complex” knowledge, for example, to set restrictions of the cardinality (there cannot be more than 20 students in a group), or characteristics of properties, such as transitivity (if lecture1 is included in topic1, and topic1 is included in section1, then this means that lecture 1 is included in section 1).
In this paper, we consider in detail an axiom of the OWL language such as the transitivity of properties and the problem of using transitive properties in RDF datasets. Queries that use transitive properties can create a kind of loops and return incorrect information. In this paper, this problem is described and solved using a specific RDF-dataset as an example. The set is developed on the basis of the OWL-ontology “Speciality”, which is for to present data of the lists of specialties, bachelors and masters graduate programs and research specialties ever operating in the Russian Federation. We propose a method for solving this problem by developing and implementing an algorithm for traversing related entities. 2.
According to the OWL notation, property P is called transitive in the case from the totality of facts that individuals A and B are connected by property P and individuals B and C are connected by property P, it follows that individuals A and C are also connected by property P.
Transitive properties are composite properties because they are created in a few steps. For example, on the basis of the statements “Mari isDescendantOf Tom” and “Tom isDescendantOf Mike”, it may be concluded that “Mari isDescendantOf Mike”.
The fact that transitive properties are composite may be the source of some
problems associated with the so-called “loops” which can occur unpredictably in the case
of sufficiently long transitive chains. This means that if the connection chain of this
property is nonlinear and represents some semblance of graph, then at the moment
there is no mechanism for specifying the order of its traversal or any restrictions on it,
therefore SPARQL queries to resources associated with this property can return
results that do not meet expectations [
More specifically, this problem can be illustrated by the example of the ontology “Specialties”. 3.
The ontology “Specialties” [
The ontology “Specialties” and RDF dataset were developed by a team of teachers and students of Yuri Gagarin State Technical University of Saratov (SSTU) [9]. They can be accessed in several ways: through the Web application Linked Open Specialties (LOS) [10], or through the SPARQL endpoint [11]. The described ontology is also published in the cloud of open linked dictionaries (Linked Open Vocabulary) [12] and can be used by any developers to create any web applications in the area of higher education of the Russian Federation that require data on former or current lists of specialties and educational programs.
Using the data of this ontology, it is possible by specialty in the diploma and the year of its receipt to determine the relevant specialty or educational program currently in force. That is, in particular, it associates the name and code of the specialty or educational program valid at one time with the name and code of the specialty or educational program valid at another time.
The class hierarchy of this ontology is presented in Figure 1. Ontology object properties (properties that connect instances of two classes) are presented in Table 1.
Property from the ontology "Dublin Core". Represents a resource that is physically or logically included in the described resource. Property from the ontology "Dublin Core". Represents a resource in which the described resource is physically or logically included in any resource.
A property that indicates that a particular object is part of another object. It is transitive and inverse to the “consistsOf” property and is a subclass of a class dcterms: isPartOf.
Subproperty properties «PartOf», shows that certain UGSN included in a certain list. Domain is UGSN, the range – is a list, and it is the inverse of the property «listConsistsOf». Property that indicates that a certain specialty included in certain UGSN. Domain is a specialty, the range – is a UGSN, and it is the inverse of the property «UGSNConsistsOf».
A property that indicates that a particular object has specific components. It is transitive and inverse to the property “isPartOf” , is a subclass of the class dcterms: hasPart class. The sub-property of the “consistsOf” property, showing that a particular list has components (UGSN). Domain is the list, the range – is a UGSN, and it is the inverse of the property ugsnConsistsOf (UGSN сonsists of) hasLevelEducation (has level education) owl:sameAs equalsTo (equals to) UGSNHasUDC «isPartOfList».
The sub-property of the “consistsOf” property, showing that a certain UGSN has components (specialties). Domain is UGSN, the range - is a specialty, and it is the inverse of the property «isPartOfUGSN», Shows the level of training for a particular specialty. Domain is a specialty, and the range - is a level of education. Also is a functional property.
A property from the ontology "Web Ontology Language", showing that two links actually refer to the same object, that is, the objects are identical.
Property showing that one specialty from one list corresponds to another specialty from another list. Domain and range - "Specialty". Is transitive and symmetric property, is a subclass of owl: sameAs.
A property indicating that a specific UGSN corresponds to a specific UDC from the ontology "UDC-Scheme". 4.
From the object properties of the ontology “Specialties”, the most important in the context of this work is the property equalsTo. It shows that one specialty from one list corresponds to another specialty from another list. The domain and range of this property is a specialty. It is transitive and symmetric property and is a subclass of a class owl: sameAs.
The transitivity of this property is necessary in order to establish correspondence through related lists, which is logical when establishing consistency between specialties.
However, the results of SPARQL queries directed at obtaining a list of equivalent specialties did not meet the needs of the task, since they contained the full path of property transitivity. Perhaps this is correct from the point of view of the abstract application of this characteristic of the property, but it does not correspond to the general logic of the LOS application, since it contains a kind of “cycle”. In this case, “loop” means the non-linear course of transitivity in which the output chain returns to the specialty from the same list as the initial specialty for which compliance is establishing. This leads to the fact that the user of the application often gets a result that looks incorrect.
An example of such a result can be seen in Table 2. When requesting correspondences for the specialty "Economics" from the list “OKSO” with the code "080100", output contain such specialties as "Foreign regional studies", "Regional studies of Russia", "Applied mathematics and computer science", which is not the expected result for the user.
Foreign Regional Studies Regional Studies of Russia Regional Studies of Russia Foreign Regional Studies Applied Mathematics and Computer Science Applied Mathematics and Computer Science Business Informatics 032000 032200 Liberal arts Liberal arts Political sciences and regional studies Political sciences and regional studies Physics and Mathematics sciences Mathematics Mechanics Economics and Management Economics and Management Computer Science and Computer Engineering Computer Science and Computer Engineering and 1061
It should be noted that in the above example already including the filtering by specialties, which belong to the same list as the initial specialty. However, as mentioned above, the filters in the SPARQL-query works directly with its results and have no influence on the inference process and the construction of the transitivity chain, therefore, they do not fulfill the required task. 5.
In order to solve the considered problem, we suggest removing the transitivity restriction on property equalsTo and using the following algorithm that obtains the list of related specialty for a given specialty and returns values, which is the correct from the user’s point of view.
This algorithm allows getting the set of all equivalent specialties from other lists by the code of a given specialty.
Input data: code of the specialty s; Output data: set of specialties A corresponding to the specialty s filed by the input. The steps of the algorithm: 1. Get the list code p, to which specialty s belongs; 2. Get a list of specialties B, associated with s by property equalsTo; 337 337 1061 1061 337 337 1061 337 1061 2011-2012 2012-our days 2012-our days 2011-2012
Remove specialty s from list B, if B is contained s; For each specialty si from list B do the following steps: 4.1. Get the code of list pi that contain specialty si; 4.2. If pi corresponds with p, break the current iteration of the loop and go to specialty si+1; 4.3. If the set of specialties A, contains si, break the current iteration of the loop and go to specialty si+1; 4.4. Add specialty si to the set of specialties A; 4.5. Do steps 2-4 for specialty si.
For ease of developing and testing this algorithm, on the base of the ontology “Specialties” a small ontology “Test” was developed and filled with data. It is an abstraction of that part of the original ontology, which will interact directly mechanism for establishing correspondences.
The class hierarchy of the developed ontology is presented in Figure 2 and includes only two classes: EducationalProgramm - the specialty of education, and List - a list that contains specialties.
The object properties hierarchy of the ontology «Test» is presented in Figure 3. The main properties in it are the following properties: equalsTo - establishes the correspondence between specialties from various lists, isPartOfList - shows that the specialty is part of the list, listConsistOf - shows that the list consists of components specialties.
The resulting ontology was filled with data about individuals - specialties, and for the convenience of testing the name of each contains names of those specialties with which it is directly related by the property equalsTo. An example of a description of such specialty is presented in Figure 4.
The next step was the development of a console application that implements this algorithm in Java. The main reason for choosing this programming language was the fact that LOS application is written in this language and thus further integration of the developed mechanism would be the most simple.
The following classes were created during application development: App, directly responsible for the logic of establishing correspondence between specialties and the formation of the required result; SparqlQuery containing the texts of all used SPARQL-queries; QueryService, necessary for substitution of variable values and correct formation of SPARQL queries before sending to the server; Util, responsible for connecting to a SPARQL-endpoint, sending
SPARQL-queries and receiving their results.
The App class requires special attention, since the algorithm for obtaining the list of correspondences for the specialty is implemented precisely in it. The main function in it is recursiveTraversal, a recursive function to bypass the list of specialties related by property equalsTo, its code is shown in Figure 5.
The result of work of the application for the specialty "a11a01a21a22" is presented in Figure 6. All necessary correspondences were obtained, including those whose connection by the property equalsTo with this individual was not setup directly, but was obtained as a result of a recursive traversal of related specialties.
The integration of this algorithm in the LOS application will provide correct results when performing a request to search for specialties that correspond to the specialty "Economics" from the list “OKSO” with the code "080100" (Fig. 8.)
Thus, we investigated the problem of property transitivity in RDF-datasets using a specific example. To do this, we analyzed the work of the LOS application that performs SPARQL queries to the RDF-dataset, which was based on the ontology "Specialty". As a result, a problem was revealed in constructing the output chain of the transitive property equalsTo: queries using this property returned an incorrect result containing "loops". A recursive traversal algorithm for related specialties, described in this article is correct and can be successfully implemented in the application “Specialties of higher education of the Russian Federation” to solve the problem of transitivity of the property equalsTo.
In the future, the re-engineering of the application “Specialties of higher education of the Russian Federation” will be held using this algorithm, for which the developed Java application that implements this algorithm will be useful.
Notably, that, although the developed algorithm is formulated and used in terms of a specific subject area to solve the problem of a separate application, it is quite universal and can be used to solve the transitivity problem in RDF-datasets of other subject areas. 9. Sytnik A.A., Shulga T.E. Ontological engineering knowledge in the field of higher education of the Russian Federation // Engineering enterprises and knowledge management (IP & UZ-2018): collection of scientific papers of the XXI-th Russian scientific conference. April 26-28, 2018 / under scientific. ed. Yu. F. Telnova: in 2.t. - Moscow: FGBOU VO "REU them. G. V. Plekhanov", 2018.T1. Pp. 234-239. ISBN 978-5-7307-1359-8. (v.1) 10. SPARQL-endpoint to the ontology "Specialties". Available online: http://sparql.sstu.ru:3030 (accessed on 30.09.2020) 11. Web application "Specialties of higher education of the Russian Federation”. Available online: http://los.sstu.ru (accessed on 30.09.2020) Ontology "Specialties" in the open dictionary of related data LOV. Available online: http://lov.okfn.org/dataset/lov/vocabs/losp (accessed on 30.09.2020)