Abstract The success of the use of ontology-based systems depends on efficient and user-friendly methods of formulating queries against the ontology. We propose a method to query a class of ontologies, called facet ontologies (fac-ontologies), using a faceted human-oriented approach. A fac-ontology has two important features: (a) a hierarchical view of it can be defined as a nested facet over this ontology and the view can be used as a faceted interface to create queries and to explore the ontology; (b) the ontology can be converted into an ontological database, the ABox of which is stored in a database, and the faceted queries are evaluated against this database. We show that the proposed faceted interface makes it possible to formulate queries that are semantically equivalent to $${\mathcal {SROIQ}}^{Fac}$$ SROIQ Fac , a limited version of the $${\mathcal {SROIQ}}$$ SROIQ description logic. The TBox of a fac-ontology is divided into a set of rules defining intensional predicates and a set of constraint rules to be satisfied by the database. We identify a class of so-called reflexive weak cycles in a set of constraint rules and propose a method to deal with them in the chase procedure. The considerations are illustrated with solutions implemented in the DAFO system (data access based on faceted queries over ontologies).

The Ontology-Based Data Access (OBDA) systems allows users to access external databases through a conceptual domain view, given in terms of an ontology. This semantic technology addresses such problems as conceptual modeling, query rewriting, an source-to-target mapping. We show how these issues have been defined and implemented in the DAFO system, highlighting those features that distinguish it from the standard OBDA system. We propose an original approach to cope with the trade-off between expressiveness of the ontology-based conceptual modelling and ontology-mediated query rewriting in the DAFO systems. For this purpose, we divide ontological rules into three parts: (a) rewriting rules, (b) rules defining intensional predicates (views), and (c) constraint rules (satisfied in the database). This allows to balance the semantic power of the conceptual modeling and the effciency of query answering in the DAFO system, where queries are based on the faceted query paradigm.

Faceted search is an attractive and user friendly information retrieval paradigm. It combines two traditional approaches, namely keyword search (allowing the direct search) and navigational search (allowing for browsing the information space by iteratively narrowing the scope). In creating a faceted query, the user is provided with a tree-shaped faceted interface, which is a hierarchical view of an ontology describing the information domain. In this paper, we discuss a transformation and translation process leading from a graphical form of a faceted query, through first order forms (without and with rewriting) to a SQL query. The crucial in this is the mapping of the ontology into a relational database, and translating to SQL. In DAFO system we have used a commercial RDBMS with optimization capabilities. We report our experimental results which show very promising execution times.

Highly distributed NoSQL key-value databases have been designed to meet the needs of various applications in the field of social networks, Web search and e-commerce. High availability, fault tolerance and scalability (i.e. Quality of Service, QoS) of such systems is in conflict with the strong consistency (Quality of Data, QoD). Thus, a new functionality of services, balancing QoS and QoD, is needed. This functionality is referred as Service Level Agreements, SLA) (Terry et al., 2013). We present a system Lorq as a proposal of a solution to this problem. Lorq is a consensus quorum based algorithm for NoSQL data replication. In this paper, we discuss how different consistency levels are guaranteed by Lorq.

We discuss the problem of creating a knowledge base representing a relational database exchange system and reasoning about schema mappings in such scenario. The knowledge base is developed by describing both databases and mappings in the data exchange system by means of OWL ontology. The key issue is to decide about preservation of semantics of data while the data are being transformed. Then some reasoning about consistency of schema mappings with integrity constraints of databases and with application domain knowledge is necessary.

In this paper we discuss an approach to generate executable mappings between XML schemas in a P2P data integration system. The process consists of four steps: (a) schema annotation in a global domain ontology; (b) deriving schema matches using ontology knowledge and some inferring rules; (c) deriving schema mappings based on the schema matches; (d) generating XQuery program as an executable form of schema mappings. We propose a set of rules for inferring matching and mapping formulas between XML schemas as well as an algorithm translating a mapping formula into an XQuery program. We discuss also some issues concerning schema annotation. Both schema matches and mappings are based on graph-pattern formulas allowing vertical and horizontal navigations. The presented approach underlies the implementation of SixP2P (Semantic Integration of XML data in P2P environment) system.

A new method for viewing and manipulating identity-based object structures is proposed. The method is founded on the viewing the object structure as an object graph which in turn is represented as a set of paths. Over a power set of paths we construct a powerdomain which forms a lattice with well-defined meet and join operations. In our construction of the powerdomain of paths we combine the Hoare ordering (appropriate for ordering of tuple objects) with the Smyth ordering (appropriate for ordering of set objects) into a new ordering appropriate for set of paths capturing information conveyed by both tuple and set objects. The approach allows to deal with incomplete tuple objects as well as with partially described sets. The goal of the powerdomain construction is to provide a semantic domain for a path calculus language [20], where path calculus variables range over paths and path calculus queries give sets of paths as their answers.