Evolution in OWL 2 QL & OWL 2 EL Ontologies Dmitriy Zheleznyakov 28 th of January, 2014, Oslo

2 Ontology All popes are clerics Benedict XVI is a pope General rules: Facts:  o To use ontologies in applications, we need special, formal syntax

2 Ontology Explicit knowledge o Do ontologies differ from data bases? o Data bases: explicit knowledge only o Benedict XVI is a pope o Ontologies: explicit & implicit knowledge o Benedict XVI is a pope o Reasoning: Benedict XVI is a cleric reasoning Explicit knowledgeImplicit knowledge

o The focus of this work: ontology languages for the Semantic Web o Web Ontology Language: OWL 2 (W3C Standard) o OWL 2 QL o OWL 2 EL o Good computational properties o Efficient schema and data management o Used in practice 3 Ontology Languages

o Ontology-Based Data Access (OBDA) o provide unified query interface to heterogeneous data sources 4 OWL 2 QL: Ontology-Based Data Access

o Ontology-Based Data Access (OBDA) o provide unified query interface to heterogeneous data sources o EU FP7 project Optique will develop an OBDA system o use-case partners: Statoil, Siemens o Ontologies may change: o new knowledge about domain o new data source is added o Motivation for our work: o to address the dynamicity of OBDA systems by studying evolution of schema and data 4 OWL 2 QL: Ontology-Based Data Access

5 OWL 2 EL: Clinical Science, Bio Ontologies o Ontologies enable communication and knowledge sharing between doctors, scientists, etc. o SNOMED CT: > 311k terms o constantly under development: o 5 modification teams o every 2 weeks the main team integrates changes, o 2002  2008 SNOMED went 278k  311k terms o It is the standard to describe the results of experiments in the US clinical labs o Motivation for our work: o to provide techniques that facilitate ontology development for such a vast community

o To facilitate evolution of ontology-based systems o insertion of knowledge o deletion of knowledge o On two levels: o schema o data o With as little changes as possible 6 Our Goal Original ontology To insert To delete

7 How to Approach the Problem? Original ontologyNew knowledge Resulting ontology 1.Define an operator and understand it a conceptual understanding of how to evolve ontologies checking its computational properties 2.Develop an algorithm to compute the result 3.Implement the algorithm

8 Previous Work AI: 80’s – 90’s Propositional logic, weaker then OWL 2 QL & OWL 2 EL KR: [AGM’85] [Borgida’85] [Dalal’88] [Satoh’88] [Winslett’90] Many evolution operators proposed [Winslett’88] [Katsuno&Mendelzon’91] Model-based operators Formula-based operators [Kang&Lau’04] [Flouris&al’04] [Flouris&al’05] [Qi&al’06] [Liu& al’06] [Qi&Du’09] [DeGiacomo&al’07-09] [Wang&al’10] Adaptation of some operators

9 Model-based operators Formula-based operators General Overview of the Results OWL 2 QL OWL 2 EL Work for restriction of OWL 2 QL Propositional logic OWL 2 QL OWL 2 EL For OWL 2 QL & EL -inexpressibility -counterintuitive results -inexpressibility -counterintuitive results Works for OWL 2 QL Bold operator Works for OWL 2 QL & EL Tunable operator

Works for OWL 2 QL Bold operator Works for OWL 2 QL & EL Tunable operator Model-based operators Formula-based operators Understanding Model-Based Operators Work for restriction of OWL 2 QL Propositional logic 10 OWL 2 QL OWL 2 EL

11 Understanding Model-Based Operators o We have shown: operators are determined by three parameters o this gives a three-dimensional space of operators o Classical operators fit in this space o Novel operators can be easily defined by changing parameters

11 Understanding Model-Based Operators o We noticed: operators are determined by three parameters o this gives a three-dimensional space of operators o Classical operators fit in this space o Novel operators can be easily defined by changing parameters o We can add new values to dimensions! o more operators can be defined!

Works for OWL 2 QL Bold operator Works for OWL 2 QL & EL Tunable operator Model-based operators Formula-based operators Inexpressibility of Model-Based Operators Work for restriction of OWL 2 QL Propositional logic 12 -inexpressibility -counterintuitive results OWL 2 QL OWL 2 EL

13 Inexpressibility of Model-Based Operators Schema: Wives are married to their husbands Priest cannot be husbands Facts: Mary is married to John and Adam and Bob are priests

13 Inexpressibility of Model-Based Operators Priest Adam Bob MaryJohn hasHusband Facts to add: John is a priest Under model-based operators: We incorporate new knowledge directly into models a model:

13 Inexpressibility of Model-Based Operators Priest Adam Bob MaryJohn hasHusband John cannot be a husband of Mary anymore! What happens to her? Three options: 1.She divorced 2.She married some one else 3.She married to a former priest Priest Adam Bob John hasHusband 1.1. Priest Adam Bob John MaryJack hasHusband 2. Priest Adam John MaryBob hasHusband 3.

13 Inexpressibility of Model-Based Operators We showed: all these options cannot be captured in OWL 2 QL and OWL 2 EL We need at least disjunction which is not in OWL 2 QL and OWL 2 EL Priest Adam Bob John hasHusband 1.1. Priest Adam Bob John MaryJack hasHusband 2. Priest Adam John MaryBob hasHusband 3. OR Priest Adam Bob MaryJohn hasHusband

Works for OWL 2 QL Bold operator Works for OWL 2 QL & EL Tunable operator Model-based operators Formula-based operators Bad Behaviour of Model-Based Operators Work for restriction of OWL 2 QL Propositional logic OWL 2 QL OWL 2 EL 14 -inexpressibility -counterintuitive results

15 Bad Behaviour of Model-Based Operators Facts: Adam and Bob are priests Facts to add: John is a priest No schema Some of model-based operators behave as follows:

15 Bad Behaviour of Model-Based Operators Priest Adam Bob Priest Adam Bob John Expected result: Priest John Actual result: Such behaviour is not useful for any application Some of model-based operators behave as follows:

Works for OWL 2 QL Bold operator Works for OWL 2 QL & EL Tunable operator Model-based operators Formula-based operators Restriction of OWL 2 QL Propositional logic OWL 2 QL OWL 2 EL 16 Work for restriction of OWL 2 QL

17 Restriction of OWL 2 QL o We found the reason of the bad behaviour of model-base operators: A binary relation participates in disjointness o Priest cannot be husbands o What if we forbid this bad interaction? o We showed: most of model-based operators work! o this fragment captures (FO part of) RDFS (another W3C standard) Priest Adam Bob MaryJohn hasHusband disjoint with

18 Summing up on Model-Based Operators o Model-based operators o suffer from inexpressibility o tend to lose too much of information o counterintuitive behaviour o Our verdict: o model-based operators are not suitable for the case of OWL 2 QL or OWL 2 EL o We turned to Formula-based operators!

Works for OWL 2 QL Bold operator Works for OWL 2 QL & EL Tunable operator Model-based operators Formula-Based Operators Propositional logic OWL 2 QL OWL 2 EL 19 Formula-based operators Work for restriction of OWL 2 QL -inexpressibility -counterintuitive results

20 Formula-Based Operators Explicit schema Implicit schema o Preserve all the knowledge: both explicit and implicit o Example: delete Priests are Males o We do not want to lose info that Adam is Male Explicit data Implicit data

20 Formula-Based Operators Explicit schema Implicit schema o Preserve all the knowledge: both explicit and implicit o Example: delete Priests are Males o How to delete it in such a way that it will not appear even implicitly? o Delete o either Priests are Clerics o or Clerics are Males

20 Formula-Based Operators o Preserve all the knowledge: both explicit and implicit o Example: delete Priests are Males o How to delete it in such a way that it will not appear even implicitly? o Delete o either Priests are Clerics o or Clerics are Males o What to do with a multiple choice? Classical approaches: o Keeping both – impossible o Combining them o too much of information is lost o we proved: it is computationally hard The resulted schema: either or

Works for OWL 2 QL & EL Tunable operator Formula-based operators Model-based operators Bold Operator Propositional logic OWL 2 QL OWL 2 EL 21 Work for restriction of OWL 2 QL Works for OWL 2 QL Bold operator

22 Bold Operator o Example: delete Priests are Males o How to delete it in such a way that it will not appear even implicitly? o Delete o either Priests are Clerics o or Clerics are Males

22 Bold Operator o Example: delete Priests are Males o How to delete it in such a way that it will not appear even implicitly? o Delete o either Priests are Clerics o or Clerics are Males o What to do with a multiple choice? o We propose: Bold operator. It picks up one of them o The result is non-deterministic… o … But can be computed in polynomial time (for OWL 2 QL) o In the case of OWL 2 EL: o Implicit knowledge can be infinite o Bold operator does not work The resulted schema: either or There is no way to decide which result is better! This is application dependent and should be up to the user

Formula-based operators Model-based operators Tunable Operator Propositional logic OWL 2 QL OWL 2 EL 23 Work for restriction of OWL 2 QL Works for OWL 2 QL Bold operator Works for OWL 2 QL & EL Tunable operator

24 Tunable Operator Explicit schema Implicit schema o Tunable operator o allows to choose what part of implicit knowledge will be preserved

24 Tunable Operator o Tunable operator o allows to choose what part of implicit knowledge will be preserved Explicit schema Implicit schema

25 Tunable Operator No implicit part Whole implicit part

26 Summing up on Formula-Based Operators o Classical Formula-based operators o suffer from inexpressibility o tend to lose too much of information o Bold operator: o works for OWL 2 QL o fails for OWL 2 EL o Tunable operator: o works for both OWL 2 QL and OWL 2 EL

27 Current Work o Applying our results to the Optique project o in progress o Incorporating evolution in transition systems o IJCAI’2013 o Information hiding & Controlled query evaluation o ISWC 2013 o submitted to an international conference