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Documents authored by Ahmetaj, Shqiponja

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DOI: 10.4230/TGDK.3.3.1
A Logic Programming Approach to Repairing SHACL Constraint Violations

Authors: Shqiponja Ahmetaj, Robert David, Axel Polleres, and Mantas Šimkus

Published in: TGDK, Volume 3, Issue 3 (2025). Transactions on Graph Data and Knowledge, Volume 3, Issue 3

Abstract
The Shapes Constraint Language (SHACL) is a recent standard, a W3C recommendation, for validating RDF graphs against shape constraints to be checked on target nodes of a data graph. The standard also describes the notion of validation reports, which detail the results of the validation process. In case of violation of constraints, the validation report should explain the reasons for non-validation, offering guidance on how to identify or fix violations in the data graph. Since the specification left it open to SHACL processors to define such explanations, a recent work proposed the use of explanations in the style of database repairs, where a repair is a set of additions to or deletions from the data graph so that the resulting graph validates against the constraints. In this paper, we study such repairs for non-recursive SHACL, the largest fragment of SHACL that is fully defined in the specification. We propose an algorithm to compute repairs by encoding the explanation problem - using Answer Set Programming (ASP) - into a logic program, where the answer sets contain (minimal) repairs. We then study a scenario where it is not possible to simultaneously repair all the targets, which may be the case due to overall unsatisfiability or conflicting constraints. We introduce a relaxed notion of validation, which allows to validate a (maximal) subset of the targets and adapt the ASP translation to take into account this relaxation. Finally, we add support for repairing constraints which use property paths and equality of paths. Our implementation in clingo is - to the best of our knowledge - the first implementation of a repair program for SHACL.
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Shqiponja Ahmetaj, Robert David, Axel Polleres, and Mantas Šimkus. A Logic Programming Approach to Repairing SHACL Constraint Violations. In Transactions on Graph Data and Knowledge (TGDK), Volume 3, Issue 3, pp. 1:1-1:36, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2025)

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@Article{ahmetaj_et_al:TGDK.3.3.1,
  author =	{Ahmetaj, Shqiponja and David, Robert and Polleres, Axel and \v{S}imkus, Mantas},
  title =	{{A Logic Programming Approach to Repairing SHACL Constraint Violations}},
  journal =	{Transactions on Graph Data and Knowledge},
  pages =	{1:1--1:36},
  ISSN =	{2942-7517},
  year =	{2025},
  volume =	{3},
  number =	{3},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/TGDK.3.3.1},
  URN =		{urn:nbn:de:0030-drops-252124},
  doi =		{10.4230/TGDK.3.3.1},
  annote =	{Keywords: SHACL, Shapes Constraint Language, Database Repairs, Knowledge Graphs, Semantic Web, Answer Set Programming}
}
Document
DOI: 10.4230/DagRep.14.3.9
Shapes in Graph Data: Theory and Implementation (Dagstuhl Seminar 24102)

Authors: Shqiponja Ahmetaj, Slawomir Staworko, Jan Van den Bussche, and Maxime Jakubowski

Published in: Dagstuhl Reports, Volume 14, Issue 3 (2024)

Abstract
This report documents the program and the outcomes of Dagstuhl Seminar "Shapes in Graph Data: Theory and Implementation" (24102). The seminar brought together active expert and junior researchers, both from academia and industry, to discuss the many open problems and research directions that arise from shapes in graph data, and, more generally, flexible and expressive schema and constraint languages for graph databases. The participants informed each other on how we perceive the research area, reported on the most recent results, discussed open problems and future directions, and in particular, four working groups were formed with promising intentions to work on new research and vision papers.
Cite as

Shqiponja Ahmetaj, Slawomir Staworko, Jan Van den Bussche, and Maxime Jakubowski. Shapes in Graph Data: Theory and Implementation (Dagstuhl Seminar 24102). In Dagstuhl Reports, Volume 14, Issue 3, pp. 9-30, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@Article{ahmetaj_et_al:DagRep.14.3.9,
  author =	{Ahmetaj, Shqiponja and Staworko, Slawomir and Van den Bussche, Jan and Jakubowski, Maxime},
  title =	{{Shapes in Graph Data: Theory and Implementation (Dagstuhl Seminar 24102)}},
  pages =	{9--30},
  journal =	{Dagstuhl Reports},
  ISSN =	{2192-5283},
  year =	{2024},
  volume =	{14},
  number =	{3},
  editor =	{Ahmetaj, Shqiponja and Staworko, Slawomir and Van den Bussche, Jan and Jakubowski, Maxime},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/DagRep.14.3.9},
  URN =		{urn:nbn:de:0030-drops-211818},
  doi =		{10.4230/DagRep.14.3.9},
  annote =	{Keywords: constraint languages, data for the semantic web, graph data, schema languages}
}
Document
DOI: 10.4230/LIPIcs.ICDT.2018.4
Rewriting Guarded Existential Rules into Small Datalog Programs

Authors: Shqiponja Ahmetaj, Magdalena Ortiz, and Mantas Simkus

Published in: LIPIcs, Volume 98, 21st International Conference on Database Theory (ICDT 2018)

Abstract
The goal of this paper is to understand the relative expressiveness of the query language in which queries are specified by a set of guarded (disjunctive) tuple-generating dependencies (TGDs) and an output (or 'answer') predicate. Our main result is to show that every such query can be translated into a polynomially-sized (disjunctive) Datalog program if the maximal number of variables in the (disjunctive) TGDs is bounded by a constant. To overcome the challenge that Datalog has no direct means to express the existential quantification present in TGDs, we define a two-player game that characterizes the satisfaction of the dependencies, and design a Datalog query that can decide the existence of a winning strategy for the game. For guarded disjunctive TGDs, we can obtain Datalog rules with disjunction in the heads. However, the use of disjunction is limited, and the resulting rules fall into a fragment that can be evaluated in deterministic single exponential time. We proceed quite differently for the case when the TGDs are not disjunctive and we show that we can obtain a plain Datalog query. Notably, unlike previous translations for related fragments, our translation requires only polynomial time if the maximal number of variables in the (disjunctive) TGDs is bounded by a constant.
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Shqiponja Ahmetaj, Magdalena Ortiz, and Mantas Simkus. Rewriting Guarded Existential Rules into Small Datalog Programs. In 21st International Conference on Database Theory (ICDT 2018). Leibniz International Proceedings in Informatics (LIPIcs), Volume 98, pp. 4:1-4:24, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2018)

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@InProceedings{ahmetaj_et_al:LIPIcs.ICDT.2018.4,
  author =	{Ahmetaj, Shqiponja and Ortiz, Magdalena and Simkus, Mantas},
  title =	{{Rewriting Guarded Existential Rules into Small Datalog Programs}},
  booktitle =	{21st International Conference on Database Theory (ICDT 2018)},
  pages =	{4:1--4:24},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-063-7},
  ISSN =	{1868-8969},
  year =	{2018},
  volume =	{98},
  editor =	{Kimelfeld, Benny and Amsterdamer, Yael},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2018.4},
  URN =		{urn:nbn:de:0030-drops-85950},
  doi =		{10.4230/LIPIcs.ICDT.2018.4},
  annote =	{Keywords: Existential rules, Expressiveness, Descriptive Complexity, Query Rewriting}
}
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