Ontology Evolution

Ontology Evolution: State of the Art and Future Directions Rim Djedidi Computer Science Department, Supélec – Campus de Gif, 3 rue Joliot Curie, F- 91 192 Gif-sur-Yvette Cedex, France. [email protected] Marie-Aude Aufaure MAS Laboratory, SAP Business Object Chair –Centrale Paris, Grande Voie des Vignes, F-92295 Châtenay-Malabry Cedex, France [email protected] ABSTRACT Ontologies evolve continuously throughout their lifecycle to respond to different change requirements. Several problems emanate from ontology evolution: capturing change requirements, change representation, change impact analysis and resolution, change validation, change traceability, change propagation to dependant artifacts, versioning, etc. The purpose of this chapter is to gather research and current developments to manage ontology evolution. The authors highlight ontology evolution issues and present a state-of-the-art of ontology evolution approach by describing issues raised and the ontology model considered (ontology representation language), and also the ontology engineering tools supporting ontology evolution and maintenance. Furthermore, they sum up the state-of-the-art review by a comparative study based on general characteristics, evolution functionalities supported, and specificities of the existing ontology evolution approaches. At the end of the chapter, the authors discuss future and emerging trends. KEYWORDS Ontology Evolution Approaches, Change Management, Consistency Maintenance, Ontology Versioning, Ontology Evolution Tools. INTRODUCTION Today, ontologies are finding their way into a wide variety of applications. In addition to the Semantic Web, they are also applied to knowledge management, content and document management, information and model integration, etc. They offer rich and explicit semantic conceptualizations and reasoning capabilities and facilitate query exploitation and system interoperability. However, ontological knowledge cannot be considered as being fixed and static. Just like any structure holding knowledge, it needs to be updated as well. Ontology development is a dynamic process starting with an initial rough ontology, which is later revised, refined and filled in with the details (Noy & McGuinness, 2001). Even during the usage of the ontology, the knowledge of the modeled domain can change and develop. To remain useful, the ontology has to cope with frequent changes in the application environment. The purpose of this chapter is to gather research and current developments to manage ontology evolution and to discuss future direction. The chapter is composed of three main parts. In the first part, we outline ontology evolution requirements, we present a comparative study of ontology, database schema, and knowledge-based system evolution; and we detail ontology change management issues. In the second part, devoted to a state-of-the-art review, we present an overview of existing ontology evolution approaches and highlight the functionalities supported by their process. The study also takes into account the ontology representation language and its consistency constraints. We also describe ontology engineering tools supporting a part of/or a complete ontology evolution process. At the end of this part, we sum up the review by a comparative study based on general characteristics, evolution functionalities supported, and specificities of the existing ontology evolution approaches. The third part focuses on future research directions by gathering the latest research in progress on ontology evolution and giving perspectives on open issues. ONTOLOGY EVOLUTION ISSUES In this section, we try to give a better understanding of the ontology evolution problem by analyzing the context of the problem, comparing it with problems and solutions in related areas and outlining its issues. The increasing number of ontologies in use and their costly adaptation to change requirements make ontology evolution very important. Ontology evolution regards the capability of managing the modification of an ontology in a consistent way. It is defined as being “the timely adaptation of an ontology and consistent propagation of changes to dependent artifacts” (Maedche, Motik, & Stojanovic, 2003, pp.287). Ontology Evolution Requirements Ontology evolution requirements have been discussed in (Blundell & Pettifer, 2004; Noy & Klein, 2004; Stojanovic & Motik, 2002; Stojanovic, 2004; Klein, 2004). Ontology evolution is a complex problem (figure1): Besides identifying change requirements from several sources (modeled domain, usage environment, internal conceptualization, etc.), the management of a change –from a request to the final validation and application– needs to formally specify the required change, to analyze and resolve change effects on ontology, to implement the change, and to validate its final application. In a collaborative or distributed context, it is also necessary to propagate local changes to dependent artifacts and to globally validate changes. Moreover, to justify, explain or cancel a change, and manage ontology versions, change traceability has to be kept. Figure 1. Ontology Evolution Requirements. Before going into depth with ontology change management issues, we compare existing strategies to handle changes in database schemas and knowledge bases with the field of ontology evolution. Comparison with Database Schema and Knowledge Base Evolution Change management is a well-known topic from research on database schema evolution (Roddick, 1996; Breche & Wörner, 1995; Banerjee, Kim, Kim, & Korth, 1987) and knowledge-based system maintenance (Menzis, 1999). Several issues from these fields are relevant to ontology evolution. In the following subsections, we summarize comparative studies of respectively database schema vs. ontology evolution and knowledge-based system vs. ontology evolution. Database Schema vs. Ontology Evolution Ontology evolution is closely related to the area of evolving database schemas (Roddick, 1996) and especially object-oriented databases (Banerjee et al., 1987; Franconi, Grandi, & Mandreoli, 2000). General ideas from version modeling in database evolution are relevant for ontology evolution. However, the scale of the issues around ontology changes is more extended and has to be considered on its own. A comparative study of database schema vs. ontology evolution is summarized in the following table (see Table 1). Table 1. Database Schema vs. Ontology Evolution. Database Schema (DB) Ontology − Quite frequent change operations during DB − Evolution frequency can be substantial especially system lifecycle. if change requirements are captured by users. − Ontology engineer guidance for change detection − Managing evolution is not formally handled; and management. DB administrator has to control consistency maintenance. − Automating change resolution and notifying − Manual adaptation of instances. ontology engineer about change effects. Ontology evolution is more closer to object-oriented DB schema evolution than to relational DB schema as the former schema is semantically richer through inheritance and hierarchy principles. − Object-oriented DB schemas reflect structure − Ontologies reflect a domain structure through of data and codes, and take also into account concepts, relations, and constraints on these object behavior (methods included in the primitives. model). In addition, they integrate physical representation of data (integer, real, etc.). Several ontology evolution approaches adapted existing approaches in object-oriented DB schemas. However, differences between the two models imply that ontology evolution approaches are a kind of extension rather than an adaptation of existing approaches (Noy & Klein, 2004). − Instances (database objects) are not at the − Terminological level (classes and properties) and assertional level (instances) are not delimited. same level as classes. Classes and instances could be manipulated and used together as in querying ontology. Therefore, change effect on queries should be considered (Klein, 2004). − The integration of a schema to another is not − Ontology reuse allows integrating ontologies or possible. part of ontologies. Thus, a change could be an inclusion/exclusion of an ontology in/from another (Stojanovic, 2004). − Changes are defined on the model itself. − Besides representing changes according to ontology model, it is essential to specify change semantic through conditions to verify and actions to apply for maintaining consistency (Stojanovic, 2004). Ontology structure brings more inconsistencies and resolutions are more complex. − Evolution needs the definition of consistency − As ontology models are richer than object model and the explicit specification of models, defining ontology consistency involves changes to apply. more constraints. In addition, possibilities of applying changes are greater than in object or relational models (e.g. in an ontology, an individual can be added without instantiating a class and a property can be defined without being attached to a class). − Schema semantic is not sufficiently explicit − Ontology semantic is more explicit and allows to apply reasoning mechanisms verifying application of reasoning mechanisms to detect consistency. inconsistencies. Solutions controlling change effects on schemas are based on rules to satisfy to maintain consistency and inference mechanisms based on axioms (Klein, 2004). Ontology change resolutions combine (depending on ontology language) these two complementary solutions. − To propagate changes to instances, three − For ontology change propagation, two scenarios solutions are adopted (Stojanovic, 2004): (a) are distinguished (Stojanovic, 2004): (a) a data migration and the immediate adaptation centralized scenario based on data migration and of instances to schema; (b) schema-instance (b) a distributed scenario based on delayed synchronization mechanisms (delayed conversion. The scenario choice depends on conversion, propagating change through priority degree assigned to two contradictory delayed conversion objects, or versioning if criteria: global consistency and run-time. no change propagation and if each object is assigned to the different versions); and (c) combination of the two previous solutions. − Change propagation is limited to instances. − Change propagation is extended to all ontology dependant artifacts i.e. instances, annotations, ontologies, and applications. Knowledge-Based System vs. Ontology Evolution To study the problem of ontology evolution, it is interesting to look at existing approaches to handling changes in formal knowledge structures (Coenen & Bench-Capon, 1993) (Menzis, 1999) and compare the evolution of these structures with ontology evolution. The maintenance of a knowledge-based system focuses on the maintenance of its knowledge base (Menzis, 1999). A comparative study of knowledgebased system vs. ontology evolution is summarized in the following table (see Table 2). Table 2. Knowledge-Based System vs. Ontology Evolution. Knowledge-based System (KBS) − The essential characteristic of KBS is the separation of knowledge representation (rules, propositional logic, predicate logic, etc.) and knowledge manipulation. Knowledge manipulation is handled by inference methods provided by reasoners, generally expressed in procedural codes. Ontology − Domain knowledge of a KBS can be represented by an ontology. Thus, KBS maintenance approaches can be adapted to ontology evolution (Stojanovic, 2004). However, in addition to ontology model richness, integration, and reuse of existing ontologies make ontology evolution more complex, especially in a distributed context. Maintenance is classified into four groups (Institute of Electrical and Electronics Engineers [IEEE], 1990) (Stojanovic, 2004): adaptive maintenance, perfective maintenance, corrective maintenance, and preventive maintenance. − Adaptive maintenance results from KBS − Adapting ontology to domain changes and environment changes or a better understanding of new knowledge acquisition. domain knowledge. − Perfective maintenance aims to response to user’s − Capturing ontology change requirements can requirements. be guided by users’ requirements (Völker, Vrandecic, & Sure, 2005; Völker, Vrandecic, Sure, & Hotho, 2007a; Völker, Hitzler, & Cimiano, 2007b). − Corrective maintenance focuses on inappropriate − Ontology evolution has to be driven in a behavior of KBS and aims to resolve errors corrective way to ensure ontology consistency and take into account ontology (syntactic, semantic, or knowledge identification errors). usage (Stojanovic, 2004). − Preventive maintenance aims to anticipate and − Preventive maintenance is adapted to the avoid future problems by analyzing the KB refinement of the ontology. Change discovery is driven by ontology structure and ontology structure to reveal possible errors. usage (Stojanovic, 2004). Ontology Changes The conceptualization modeled in an ontology provides the applications –using the ontology– knowledge about how to use data related to ontology’s concepts. However, neither the data, nor the ontology itself is permanent and stable. Ontology changes have direct effect on the way data should be interpreted and data dynamics have to be reflected to ontology. Thus, a better understanding of ontology change management issues is needed. Change activities Ontology change management can be considered as an umbrella of change activities. The distinction between these activities has been, in some cases, confusing. Considering the role of evolution in the ontology lifecycle and adapting the terminology from the database community (Roddick, 1996), it appears that these activities are performed nearly sequentially to produce the ontology lifeline (O’Brien & Abidi, 2006). Four main activities are distinguished in the literature: ontology revision, ontology versioning, ontology adaptation, and ontology evolution. Ontology Revision Ontology revision consists of operations that change the state of an ontology to adapt its representation characteristics to the modeled domain (Klein & Fensel, 2001). It is based on applying logical manipulation to expand the ontology abstraction level like adding/deleting concepts or refactoring properties (Foo, 1995). Ontology Versioning Ontology versioning consists in creating and managing different versions of an ontology. It addresses the problem of (partial) incompatibility of new ontology versions with the previous one and thus, with ontology’s instances and, applications and dependant ontologies. Two main cases of version generation can be considered: Different versions of an ontology may result when ontologies are independently developed (Klein & Fensel, 2001). The evolved ontology resulted after applying a change can also be considered as a new conceptualization of the domain and, thus, as a new version. However, in general the decision to consider it so, is taken by the ontology engineer. Maintaining multiple versions of an ontology requires a means to differentiate between the different versions and to ensure that instances remain valid. Ideally, we should preserve the different versions of an ontology and keep trace of all information about differences and compatibilities between them. This needs methods for version identification and differentiation (based on the same principles of semantic similarity measure in ontology alignment), specification of relationships between versions, ontology update procedures and access mechanisms to the different versions of an ontology (Klein & Noy, 2003). Ontology Adaptation Ontology adaptation aims to provide facilities to manage frequent changes in the way concepts are represented in the domain, and to automate the process of producing multiple adapters for versions of interest (O’Brien & Abidi, 2006). It is a local adaptation of an ontology to a specific usage or a sub-set of instances of its environment, without any contradiction with the previous version. Adaptation can be seen as practice review in which versions are first adopted and then, adapted for a specific application (O’Brien & Abidi, 2006). Adaptation changes are mostly generated by user-driven change discovery approaches (see section on user-driven change discovery). Ontology Evolution Ontology evolution consists in managing persistent ontology changes to cope with new requirements, and producing new versions. The modification of an ontology is handled by preserving its consistency, tracking and logging the change to provide mapping between subsequent versions (Stojanovic, Maedche, Motik, & Stojanovic, 2002a), and controlling the use of instances (Stojanovic, Stojanovic, & Handschuh, 2002b). Change Effects Changing the behavior of a concept or a relation so that a change requirement comes into effect, one should think about inter-related knowledge and dependant instances; and define mechanisms specifying how knowledge to be change and how to handle consistency. Change effects cannot be considered only by looking at the ontology itself. This also depends on the rationales behind the change and the specificity of the ontology usage. Change effects were considered in the literature from different perspectives. From an ontology-based data warehouse point of view, two categories of change effects are distinguished (Xuan, Bellatreche, & Pierra, 2006): • Normal evolution when the ontology evolves without undermining existing knowledge according to the so-called ontological continuity principle; • Revolution when true axioms can be undermined. According to distributed ontology change management approach (Klein, 04), change effects depend on what we need to preserve in the ontology: • Data to maintain instances between the different versions of the ontology; • The ontology itself to maintain query results i.e. the result of a query q1 on Oi version is included in the result of the same query q1 on the version Oi+1 of the same ontology; • Consequences to maintain inferred facts i.e. the facts inferred from an axiom a1 on Oi version are also inferred from the same axiom a1 on the version Oi+1 of the same ontology; • Consistency to ensure that the new version does not contain logical inconsistencies. With regard to database schema changes, change effects are classified focusing on instances (Noy & Klein, 2004): • Preserving change: Instances are not lost (e.g. adding a concept or a property); • Translating changes: Instances can be preserved as knowledge are translated to another form (e.g. when gathering concepts as a union of their super-concepts, sub-concepts, and properties; instances can be preserved.); • Losing changes: Instances are lost (e.g. deleting a property causes the loss of all its instance values). Handling change effects involves not only checking ontology consistency but also maintaining it. The maintenance activity consists in generating and proposing/applying a set of additional changes to resolve inconsistencies. Quite often, this is a manual procedure where the expert revises the ontology using an ontology editor (Stojanovic & Motik, 2002; Sure, 2002). Change Classification The purpose of change classification is to define taxonomy of changes specifying classes of ontology changes and their properties for a specific ontology language representation. The main classifications defined in literature deal with KAON i (Maedche, Stojanovic, Studer, & Volz, 2002; Stojanovic, 2004) and OWLii (Klein, 2004) languages. The ontology of KAON changes classifies KAON changes through three levels of abstraction (Stojanovic, 2004): • Elementary changes applying modifications to one single ontology entity; • Composite changes applying modifications to the direct neighborhood of an ontology entity; • Complex changes applying modifications to an arbitrary subset of ontology entities. In addition, two change types are considered (Stojanovic, 2004): Additive changes adding new entities to an ontology without altering the existing ones, and subtractive changes removing some –piece of– entities. Thought of as a complete and minimal set of changes, the change ontology does not include entity modifications. This kind of change is interpreted as a “rename” change altering only lexical information about entities or a “set” change depending on the entity to modify. A similar taxonomy for OWL ontology is presented in (Klein, 2004). However, unlike KAON change ontology, OWL change ontology contains Modify operations as well as Set and Unset operations for properties characteristics (e.g., symmetry). Klein (2004) distinguishes basic and complex change operations: • Basic changes are simple and atomic changes that can be specified by using the structure of the ontology only, and modify only one specific feature of the OWL knowledge model (e.g. add a class, delete a “is-a” relation.); • Complex changes correspond to composite and rich changes grouping logical sequences of basic changes and incorporating information about their implication on the logical model of the ontology (e.g. raising sub-classes, enlarging the range of an object property to its super-class, merging two classes.). Complexity also deals with change effects. If the effects of a basic change are minor, the cumulative effect of all intermediate changes realizing a complex change can be huge. Basic changes are exhaustively derived from the underlying ontology language. Complex changes are infinite as new compositions of changes can always be defined. ONTOLOGY EVOLUTION APPROACHES Many researches have discussed the characteristics of an ontology evolution process (Klein, 2004; Stojanovic et al., 2002a; Stojanovic et al., 2002b) and several ontology evolution approaches have been proposed in the literature. Some focus on specific change management issues like capturing change requirements (Stojanovic, Stojanovic, Gonzalez, & Studer, 2003a; Cimiano & Völker, 2005; Bloehdorn, Haase, Sure, & Voelker, 2006), change detection and version logging (Klein, Fensel, Kiryakov, & Ognyanov, 2002a; Noy, Kunnatur, Klein, & Musen, 2004; Plessers & De Troyer, 2005; Eder & Wiggisser, 2007), formal change specification (Stojanovic, Stojanovic, & Volz, 2002c; Klein, 2004; Plessers De Troyer, & Casteleyn, 2007), change implementation (Stojanovic, Maedche, Stojanovic, & Studer, 2003b; Stojanovic, 2004; Flouris, 2006), consistency maintenance (Stojanovic, 2004 ; Haase & Stojanovic, 2005; Haase & Völker, 2005; Plessers & De Troyer, 2006), ontology versioning (Klein & Fensel, 2001; Klein et al., 2002a; Klein, 2004), and others propose a more or less global evolution process including change impact analysis and resolution as well as change propagation to dependant artifacts (objects, ontologies and applications referenced by the ontology) (Stojanovic, 2004; Klein, 2004; Bloehdorn et al., 2006). In this section, we present the main approaches and we analyze the functionalities that they support. Ontology Learning Approach Based on Change Requirement Discovery An important research area is to explore change sources and capture change requirements. Change requirements could be initiated because of environment dynamics, the evolution of the modeled domain, the users’ needs may change, adding new knowledge previously unknown or unavailable, correction and refinement of the conceptualization, or ontology reuse for other applications. In a distributed context, if an ontology changes for any of the preceding reasons, dependant ontologies might also need to be modified to reflect potential changes in terminology or representation (Heflin, Hendler, & Luke, 1999). It has to be said that detecting changes and capturing change requirements are two distinguished approaches. The former aims to discover ontology changes –already applied– for versioning purposes for example. The latter aims to generate ontology changes from explicit and implicit requirements. Explicit requirements can be defined by ontology engineers to adapt the ontology to new requirements or by users based on ontology usage feedbacks. Changes resulting from explicit requirements are called top-down changes (Bloehdorn et al., 2006). Implicit requirements are induced by analyzing the system’s behavior and are called bottom-up changes (Bloehdorn et al., 2006). In the approach presented in this section, ontology evolution is considered as dynamics in ontology learning process focusing on change generation (Cimiano, 2007). Learning process is applied as an ontology refinement and, addresses the evolution of data and knowledge. Changes requirements are datadriven based on the corpus (term frequency, matches of lexico-syntactic patterns) or user-driven. Change traceability (which change, by whom, why, etc.) is also considered to keep a repository of explanations and even references to the segment in a corpus which triggered the change. Data-driven Change Discovery Data-driven change discovery consists in deriving changes from modifications to the knowledge from which the ontology has been constructed (Bloehdorn et al., 2006). Data-driven change discovery can be organized through four phases (Cimiano & Völker, 2005): 1. Corpus change processing to formulate change request: Many algorithms are implemented (term extraction, taxonomy learning, learning relations etc.), their execution is guided by a change reference store and a change evidence; 2. Change strategy phase studies how corpus changes affect the different types of ontology elements; 3. Change generation and management phase proposes different change possibilities that are stored as a Model of Possible Ontologies (POM) used to add or remove object, update relevance and confidence, and to explain each possible change. The phase supports incremental ontology learning for efficiency purposes (to update evidence for ontology elements based on observed corpus changes and generate suggestions and explanations for ontology changes based on new evidence) and includes explicit change management (see section on change management below) ensuring the logical consistency of the learned ontology (Haase & Völker, 2005); 4. Ontology change application by adding or removing ontology elements. User-driven Change Discovery In a user-driven change discovery perspective, ontology learning –applied as refinement– is based on: • Procedures for formal self-evaluation (Völker et al., 2005); • Conceptual preciseness based on Learning Disjointness Axioms (LeDA) (Völker et al. 2007a) and Learning Expressive Ontologies (LExO) (Völker et al., 2007b) in OWL DL language. Learning Consistent Ontologies Knowledge extracted from text could be uncertain and potentially contradicting which leads, consequently, to logical inconsistencies in the ontologies learned that the learning process resolve (Haase & Volker, 2008). Ontology learning generates first, ontologies based on a language independent Model Learned Ontology Model (LOM). Knowledge uncertainty is expressed as confidence annotations associated to ontology elements (Haase & Volker, 2008). Then, logical semantics are introduced by transforming the LOM model to a formal ontology expressed in OWL. Transformation algorithm takes into account confidence annotation to avoid generating logical inconsistencies. The purpose is to obtain a consistent ontology capturing the most certain information among different consistent ontologies (Haase & Volker, 2008). Ontology selection is based on evaluation function taking into account the rating annotations. Consistency maintenance is also based on resolution principles presented in (Haase & Stojanovic, 2005). When confident axioms transformed from LOM to OWL lead to an inconsistent ontology, the inconsistency is localized. Then, the most uncertain axiom (lowest confidence value) is identified and removed to resolve inconsistency. Multimedia Ontology Evolution Approach BOEMIE iii BOEMIE approach (Bootstrapping Ontology Evolution with Multimedia Information Extraction) aims to automate the process of knowledge acquisition from multimedia content. Evolving multimedia ontologies are used as background knowledge to guide the extraction of information from multimedia content in networked sources. Besides, reasoning mechanisms are applied on the fused semantic information extracted to populate and enrich the ontologies. Ontology Population and Enrichment Patterns A pattern-driven approach was adopted for ontology evolution. Evolution patterns characterize the input of the evolution process –corresponding to the extracted information presented in form of ABox (concept and relation instances)– and determine the evolution operation to be performed (Castano et al., 2007; Petasis, 2007): population (adding new instances) or enrichment (extension by new concepts, relations, properties) . Four evolution patterns are distinguished. Population patterns are used when the interpretation of a multimedia resource (input) can be explained by single (P1) or multiple (P2) ontology concepts (Castano et al., 2007). Enrichment patterns are used when there are no ontology concepts explaining the resource – with (P3) or without (P4) metadata information (Castano et al., 2007). The enrichment is then performed to acquire the missing knowledge. Each pattern is articulated into a set of activities implementing all the required changes as instance matching in population and concept learning in enrichment. Ontology population aims to identify instances referring to the same real object (or event) based on instances matching and nonstandard clustering techniques (Castano et al., 2007). Ontology enrichment aims to learn new concepts (or relations) by applying clustering techniques and, in some cases (pattern P3), it includes concept enhancement by considering external sources (e.g. external domain ontologies or taxonomies). Consistency Maintenance Population and enrichment operations both integrate a consistency validation activity. In population process, consistency maintenance consists in identifying and eliminating redundant information and checking that the instance does not cause any contradicting information using standard reasoning services. In enrichment process, consistency maintenance is limited to inconsistency detection with respect to applied modifications. Change Versioning BOEMIE approach also includes a versioning and change audit phase that coordinates the updated ontology to reflect newly inserted knowledge; logs performed changes, and generates an evolved ontology version (Castano, 2006). Change Management Approach for Distributed Ontology (Heflin et al., 1999; Klein & Fensel, 2001; Klein et al., 2002a; Klein, Kiryakov, Ognyanov, & Fensel, 2002b; Klein & Noy, 2003; Maedche et al., 2003; Klein, 2004) research focuses on mechanisms and methods required to cope with ontology change in dynamic and distributed environments. They propose a formalism to represent change between ontologies based on a taxonomy of change operations (considering particularly OWL meta-model), and a method for interpreting data from different ontology versions. Change management is considered within a global framework that includes generating change information, deriving additional change information, and also solving specific problems in specific situations (Klein, 2004). It is assumed that there is not one general procedure for ontology change management, but a need for a variety of methods to consider different types of change information and to support particular tasks. Methods and techniques proposed are (Klein, 2004): comparison algorithms, ontology mapping, reasoning services, human validation, change visualization, effect prediction heuristics, and guidelines for some change-related tasks. Distributed Change Management The study brings out three change management characteristics specific to distributed context but related only to the nature of ontology (i.e. independent of distributed settings) (Klein et al., 2002b; Klein, 2004): • The propagation of a change to other levels of interpretation depends on whether it modifies the specification or the conceptualization of the ontology; • As distributed ontologies can be used for different tasks, consistency maintenance cannot focus on one specific feature to preserve. Thus, change consequences should be considered for specific ontology use-cases; • The expressivity and the semantics of the ontology language can sometimes be exploited to solve specific change management problems as for example in matching ontology versions. An Ontology of Change Operations and a Change Specification Language The importance of change specification in exchanging information about change between users, tools, or independent processes was also highlighted. Change specification consists of a set of change operations derived from the meta-model of the ontology language and/or specialized and composite operations prescribing the required follow-up steps to transform an old ontology version into a new one (Klein, 2004). It includes conceptual and evolution relations between old and new versions of constructs, metadata about the change, and change consequences. An ontology of change operations was proposed as well as a change specification language (RDFbased syntax) based on this ontology. The Ontology of change operations is an ontology change model extending the taxonomy of OWL change operations (see change classification section) to a general change specification language. The change specification language aims to specify transformations of one ontology version into another. In addition, it is proposed to facilitate change reverting or re-execution, change effect analysis, tool interaction based on unambiguous and formal change information, expressiveness in capturing scope of modifications, minimality in change description (i.e. concise format), and description in different granularity levels (Klein, 2004). A Global Evolution Process for KAON Ontology A global ontology evolution process is described in (Stojanovic, 2004). The process ensures change semantic specification, consistency maintaining, and change propagation for KAON ontologies. Ontology evolution is defined as the formal interpretation of all change requirements captured from different sources, the application of changes to the ontology, and their propagation to dependent artifacts while preserving consistency (Stojanovic, 2004). Dependent artifacts include objects referenced by the ontology and, dependent ontologies and applications. A six-phase evolution methodology has been implemented within the KAON ontology management infrastructure, targeted for business-oriented ontology management (Stojanovic, 2004; Oberle, Volz, Motik, & Staab, 2004). Change Capturing The starting phase of the process consists in capturing the ontology changes to apply. It is based on explicit requirements and the application of data and usage-driven change discovery methods. Data-driven changes are derived from the ontology instances by applying techniques like data-mining, Formal Concept Analysis (FCA), and many heuristics (Stojanovic, 2004; Maedche et al., 2003). Usage-driven change discovery is based on ontology usage patterns derived by analyzing the behavior of users and tracking queries in ontology applications to discover the most used part of an ontology and to identify user interests (Stojanovic et al., 2003a). Change Representation In this phase, the identified changes are described according to the specification of KAON language. Change can be represented on three granularity levels: elementary change, composite change, and complex change (see section on change classification). Change Semantics Evolving an ontology assumes preserving its consistency so that it is still relevant to the modeled domain and to the applications using it (Haase & Völker, 2005). The phase of change semantics aims to evaluate and resolve change effects in a systematic manner by ensuring the consistency of the whole ontology (Stojanovic et al., 2002a). Ontology consistency is defined as following: “A single ontology […] is defined to be consistent with the respect to its model if, and only if, it preserves the constraints defined for underlying ontology model” (Stojanovic, 2004, pp.30). The global approach focuses on KAON ontology. In addition, in (Haase & Stojanovic, 2005; Haase, Van Harmelen, Huang, Stuckenschmidt, & Sure, 2005), the authors described the semantics of changes for the consistent evolution of OWL ontologies. Consistency Maintenance of KAON Ontology KAON is a constraint-centered language based on closed-world assumption. Its consistency model is defined as a set of constraints describing KAON model invariants (e.g. identity distinction, concept hierarchy, concept closure, concept hierarchy closure, etc.), soft-constraints, and user-defined constraints (Stojanovic et al., 2003b; Stojanovic, 2004). It is mandatory that invariants be satisfied. Soft-constraints could be temporarily invalid to facilitate ontology population, for example. User-defined constraints are rather directives for well-formed ontology construction. Two type of inconsistency are distinguished: structural and semantic inconsistency. Structural inconsistency relates to a noncompliance with the KAON ontology model constraints. Semantic inconsistency alters the meaning of ontology entities. The method focuses only on structural inconsistency as it enables ontology engineer assistance whereas; semantic inconsistency is heavily dependent on specific semantic information that is not explicitly expressed in standard ontology model (Stojanovic, 2004). Consistency maintenance is handled in three steps: 1) localizing inconsistency (minimally inconsistent subsets), 2) determining possible resolutions, and 3) choosing and applying the best change. Consistency Checking. Two approaches are distinguished for consistency checking (Stojanovic, 2004): • A posteriori verification: Only one check is performed for all applied changes; • A priori verification: The check is performed before a change application. For each change, a respective set of preconditions is associated and must be satisfied so that the change can be applied. Besides, it is assumed that the ontology was previously consistent. A posteriori verification is a costly approach, since it is applied to the whole ontology and the resolution needs roll back mechanisms. Besides, it is not possible to explain change impacts and find out which change caused the detected inconsistency after applying changes in batch. To limit the checking to the local range of a change and avoid reverting the ontology into a consistent state, the second approach was adopted (Stojanovic, 2004). A priori verification is based on the specification of the necessary preconditions to satisfy to enable the applicability of a change. Besides, it requires the definition of sufficient post-conditions to satisfy after a change application to enable its validation. Inconsistency Resolution. Two approaches are proposed for automatic inconsistency resolution (Stojanovic et al., 2003b; Stojanovic et al., 2002a; Stojanovic, 2004): 1. Procedural approach: Consistency is maintained by considering the constraints of the consistency model and the definite rules that have to be performed to satisfy them. The approach is organized through two main phases: • Handling the semantics of change: The change request is represented as a sequence of changes processed one by one. Preconditions associated to each change are checked and then, inconsistency resolutions are generated. Different resolution possibilities called evolution strategies are generated for a change. Each one proposes a set of additional changes resolving the inconsistency in a way that meets particular needs of an ontology engineer. The purpose of generating several strategies is to adapt evolution policies to different ontology applications. The process is repeated until there is no change that should be handled, • Change application: All the changes are applied to the ontology by considering their respective post-conditions. 2. Declarative approach: Consistency is maintained by considering a comprehensive set of inferred axioms formalizing the evolution. The approach is organized through three phases: • Request formalization: The ontology engineer expresses a change request in a declarative manner as a collection of supported ontology changes split into two sets of changes: must be formed changes (e.g. remove the concept a) and must not be formed changes (e.g. do not remove the concept b). • Change resolution: Only the first set of changes is applied so that inconsistencies caused can be detected. Then, resolution generation is performed by considering the two sets of changes. All possible changes eliminating inconsistencies are generated. The resolution is reduced to a graph searching problem where nodes correspond to evolving ontologies and edges to applied changes. The search is guided by ontology engineer constraints, consistency model rules, and annotations associated to edges and nodes, • Solutions ordering: All possible consistent states of the ontology are ranked according to meta-information given by ontology engineer. Both approaches are able to perform the same set of changes and offer the same possibilities for controlling and customizing the change resolution. Their comparison has to be based on subjective criteria as efficiency of the evolution system (Stojanovic, 2004). Consistency Maintenance of OWL Ontology OWL is an axiom-centered language based on open-world assumption. Four different approaches were proposed to handle OWL inconsistencies (Haase et al., 2005): consistent ontology evolution, repairing inconsistencies, reasoning in the presence of inconsistencies, and multi-version reasoning. In (Haase & Stojanovic, 2005), a formal model was proposed for consistent OWL ontology evolution. Consistency is defined as a set of conditions to satisfy and is classified into three levels: structural, logical, and userdefined consistencies. • Structural consistency refers to ontology language constraints. • Logical consistency refers to the formal semantic of the ontology and to its satisfiability. It ensures that the ontology is semantically correct and does not present any logical contradiction. User-defined consistency refers to ontology usage and application context. Constraints are explicitly • defined by users. Two user-defined conditions are considered (Haase & Stojanovic, 2005): • Generic consistency specifying qualitative criteria for ontology modeling as applying OntoClean meta-properties (Guarino & Welty, 2002); • Domain dependant consistency specifying particular conditions related to the domain of discourse and depending on user domain expertise. The definition of OWL semantics is based on a model theory associating OWL syntax to the model of the ontology domain: Satisfying an ontology within an interpretation is constraint by the satisfaction of all its axioms (Haase & Stojanovic, 2005). Consistency Checking. Consistency is checked only for change operations adding axioms. The rationale behind this choice is based on the monotonic logic of OWL. The purpose is to localize the minimal inconsistent sub-ontology i.e. a minimal set of contradicting axioms (Haase & Stojanovic, 2005). Inconsistencies Resolution. Rewriting rules was proposed to make axioms compatible with OWL Lite model and resolve structural inconsistencies (Haase & Stojanovic, 2005). For logical inconsistencies, resolution strategies were introduced based on OWL Lite constraints (Haase & Stojanovic, 2005). Each consistency condition is mapped to a resolution function specifying the additional changes to apply. These changes correspond to a set of axioms that have to be removed in order to obtain a logically consistent ontology with ‘minimal impact’ on the existing ontology. A maximal consistent sub-ontology is obtained (Haase & Stojanovic, 2005). Additional changes are presented to the expert so that he can determine which changes should be generated. Change Propagation The propagation phase aims to propagate ontology changes to the possible dependent artifacts in order to preserve the overall consistency. These artifacts can be ontologies reused or extended by the evolved ontology, or distributed applications. Propagation consists in tracking and broadcasting applied changes. Synchronization approaches proposed are described in detail in (Maedche et al., 2003; Stojanovic, 2004). Change Implementation This phase consists in the physical implementation of the required change and the derived changes resolving it. First, changes are notified to the ontology engineer to be approved and then, applied. Besides, all performed changes are logged in order to support recovery facilities. KAON Change logging is based on two specific notions (Stojanovic, 2004): evolution ontology and evolution log. Evolution ontology defines a model of applicable changes on an ontology, facilitating the management of these changes. Evolution log describes applied-change historic through chronological information sequences about each change. It holds knowledge about the ontology development and maintenance. Modeling change (evolution ontology) and their application historic (evolution log) help in synchronizing the ontology evolution. If a change needs to be cancelled, evolution log based on a formal change model, helps in guiding revoke operations. Change Validation This phase consists in the final validation of the applied changes. It ensures the reversibility of the changes if they are finally disapproved by users (may be due to no convincing impacts, divergent points of view in collaborative context, etc.), the rationale explanation of changes, and their usability (Stojanovic, 2004). At this stage, other problems can be identified, inferring new change requirements in a cyclic change management process. Ontology Evolution Approach Based on Belief Change Principles The motivation idea of this work (Flouris, Plexousakis, & Antoniou, 2005; Flouris & Plexousakis, 2006; Flouris, Plexousakis, & Antoniou, 2006; Flouris, 2006) was that the mature field of belief change can provide the necessary formalizations that can be exploited in the ontology evolution research. Belief change deals with the automatic adaptation of a knowledge base to new knowledge, without human participation in the process (Flouris et al., 2006). The starting assumption of this work was that adopting belief change principles and algorithms may reduce knowledge engineer dependence in the ontology evolution process. Indeed, the approach is addressed to applications based on frequently changing ontologies and autonomous applications like software agents i.e. in case it is highly difficult to handle ontology changes by a manual or semi-automatic process. Belief change theory considered in this study is the AGM theory initiated by the three authors Alchourron, Gärdenfors and Makinson (1985), and applied to belief revision as a method giving minimal properties a revision process should have. Application Scope The study shows that AGM theory can be applied only to classical logics like Propositional Logic (PL) and First-Order Logic (FOL) but not (directly) to ontology representation standards like Description Logics DL and OWL (Flouris, 2006). Indeed, belief change techniques are not applicable to DLs under a closed-world assumption. In case of open-world assumption, they are applicable on some DLs but not OWL (Flouris et al., 2005). Ontology Evolution Operations Four different ontology evolution operations are distinguished considering belief change literature (Flouris & Plexousakis, 2006): ontology revision, ontology contraction, ontology update, and ontology erasure. Ontology revision and contraction occur when the perception of the domain changes i.e. its conceptualization. They are applied regarding a static state of the world. Ontology update and erasure reflect changes in the domain itself. These four operations do not really match with change operations used in the ontology evolution literature as they are based on different paradigms. Inspired from belief change principles, they have a different viewpoint on how a change should be interpreted and managed. They are fact-centered (Flouris, 2006): Each new fact represents a certain need for ontology evolution. By identifying the type of the new fact (static/dynamic world state) and its impact (add/remove knowledge), the type of operation involved can be determined and thus, the system identifies modifications needed and performs them automatically. Standard approaches however, are modification-centered (Flouris, 2006): They focus on modifications that should be physically performed in response to a new fact which makes change management process less complicated and gives ontology engineers more control. Ontology Evolution Algorithm The ontology evolution process is performed as an evolution algorithm mapping an ontology and a change to an ontology where both the ontology and the change are represented by sets of axioms. The algorithm is close to the axiomatic approach described in (Haase & Stojanovic, 2005). The evolution operations, namely revision, contraction, update, and erasure; are implemented as four evolution functions. In (Flouris, 2006) a first fully automatic contraction algorithm for ontologies based on AGM compliant DLs was introduced. However, the algorithm is based only on syntactic considerations. Comprehending ontology evolution from a belief revision perspective is actually different from current approaches. The researchers concede that foundations are different (Flouris & Plexousakis, 2006; Flouris, 2006): Belief revision is based on postulation methods, whereas ontology evolution approaches focus on explicit construction involving ontology engineer participation –which cannot be postulated– to cope with technical and practical issues related to change management problem. Applying AGM theory to ontology evolution is presented as a complementary approach to overcome the lack of efficient formalizations of the processes behind ontology evolution in current research. Consistency Maintenance Consistency maintenance is also considered in this approach. Two notions are distinguished (Flouris, 2006): consistency and coherence. Consistency is related to the satisfaction of all the ontology DL axioms, whereas coherence deals with satisfaction of predefined constraints or invariants related to efficient ontology design. Ontology coherence is not considered in the evolution algorithm as it deals with design problems. Consistency maintenance is handled according to belief change principles (Flouris, 2006). Change Detection Approach Using a Version Log In (Plessers & De Troyer, 2005; Plessers et al., 2006), a change detection approach was proposed within an OWL DL ontology evolution framework. The approach aims to detect changes that were not explicitly requested by an ontology engineer and automatically generate a detailed overview of changes that have occurred based on a set of change definitions. Different overviews of the changes can be provided for the same evolution as each user can have its own set of change definitions. A Change Definition Language CDL was proposed proving several levels of abstraction. Changes are expressed as temporal queries applied to a version log. The version log keeps track of all the different versions of all concepts ever defined in an ontology, while the CDL allows users to define the meaning of changes in a formal way (Plessers et al., 2007). Evolution Framework Overview The framework focuses on two kinds of evolution tasks, each one targeting a user role: Evolution-onrequest for ontology engineers modifying the ontology and Evolution-in-response for maintainers of depending artifacts looking for information about the change made. Change detection approach is applied in the two tasks but at different steps. Evolution-on-Request This task is organized through five phases: (1) First, the ontology engineer expresses his change request in CDL (see section change definition language below). (2) The consistency maintenance phase deals with inconsistency localization and resolution (see section consistency maintenance below). (3) The change detection phase aims to detect which changes occurred as a consequence of the applied modifications. (4) In the change recovery phase, all unnecessary intermediate changes can be recovered (Plessers, 2006). (5) Finally, in the change implementation phase, the change applied to a local copy of the ontology is implemented in the public version. Evolution-in-Response Considering that maintainers of dependant artifacts can have a different viewpoint on the change definitions applied by ontology engineer, this task allows them to approve or not the applied modifications and to decide about their propagation to dependant artifacts. It is organized through three phases: (1) It starts with the change detection phase to obtain evolution log (see section evolution log below). (2) The cost of the evolution phase aims to evaluate the cost of updating dependant artifacts. (3) Finally, if the update is approved, the version consistency phase ensures the consistency of the dependent artifact with the evolved version of the ontology; otherwise, the depending artifact remains consistent with the old version of the ontology. Change Definition Language Change Definition Language CDL specifies change definitions in a formal and declarative (differences between past and current versions) way. It is a temporal logic based language defining an ontology change in terms of preconditions and post-conditions. The syntax and the semantics of this language are presented in (Plessers et al., 2007). Evolution Log Model Evolution logs aims to express different users’ interpretations of an ontology evolution. By applying a temporal query on a version log through a change detection phase, a collection of change definition occurrences –expressed in CDL– is obtained. It corresponds to an evolution log. Version log stores evolution historic of each entity defined in the ontology starting from its creation, over its modifications, until the end of its lifecycle. To represent version log, a snapshot approach was adopted. It captures the different states of an ontology over time and keeps track of the evolution of each individual ontology concept (Plessers et al., 2007). Consistency Maintenance In (Plessers & De Troyer, 2006), authors defined an approach and an algorithm localizing axioms causing inconsistencies and proposed a set of rules that ontology engineers can use to resolve inconsistencies. The consistency model refers to OWL DL constraints. Consistency checking copes with two change scenarios (Plessers & De Troyer, 2006): • Adding/modifying axioms in the terminological level of OWL DL –the TBox (classes and properties): The verification starts with the satisfiability of the TBox concepts and then, the consistency of the ABox with respect to the modified TBox; • Adding/modifying axioms in the assertional level of OWL DL –the ABox (instances): The verification in this case is applied only to the ABox. Consistency checking is not applied after a delete operation based on the logic monotonicity of OWL DL. The algorithm proposed to select axioms causing inconsistencies is based on two types of tracking trees (Plessers & De Troyer, 2006): Axiom transformation trees keeping track of axiom transformations that occur in the preprocessing step and, concept dependency trees keeping track of axioms leading to a clash during algorithm execution. Based on the clash information, concept dependency trees, and axiom transformation trees, the algorithm generates a set of axioms causing the inconsistency. The assumption stated for inconsistency resolution is that inconsistency is a consequence of overrestrictive axioms (contradicting each others) that have to be weakened (Plessers & De Troyer, 2006). Authors propose a collection of rules guiding ontology engineers in inconsistency resolution. A rule can either call another rule or apply a change to an axiom. Some rules can be applied for weakening axioms (e.g. how to weaken a concept definition or a concept inclusion) others for weakening or strengthening concepts (e.g. disjunction relation, existential quantification) (Plessers & De Troyer, 2006). After describing existing ontology evolution approaches, we present, in the following section, the main tools supporting ontology evolution functionalities and implementing some of these approaches. TOOLS SUPPORTING ONTOLOGY EVOLUTION As described in the previous sections, handling ontology evolution manually is not a trivial task. Ontology engineers can not comprehend all side-effects of the changes, resolve caused inconsistencies, and evaluate change impacts on the ontology. Therefore, appropriate tools providing technical means for supporting ontology evolution are required. Ontology evolution should be a part of the functionalities of an ontology editor to drive ontology development in an iterative and dynamic process. However, requirements regarding ontology evolution process are not supported by all existing ontology editors. These requirements are mainly (Stojanovic, 2004): functionality, customisation, transparency, reversibility, auditing, refinement, and usability. In (Noy, Chugh, Liu, & Musen, 2006), functional requirements considered particularly for collaborative environment are: change annotation, change history for a concept, change representation from one version to the next, definition of access privileges, querying an old version by using the vocabulary of the new version, and printed summary of changes. Other functional requirements specific to asynchronous collaborative editing, continuous editing, curated (laconic) editing, and non-monitored editing are also described. Besides some existing ontology editors supporting certain evolution features, current researches on ontology evolution –including some of the approaches described in the previous sections– have proposed more specialized tools whose aim is to guide users to perform the change(s) manually or to perform the change(s) automatically. Some of these tools allow collaborative edits (Duineveld, Stoter, Weiden, Kenepa, & Benjamins, 2000; Noy et al., 2006), others support transactional changes (Haase & Sure, 2004), and others support features related to ontology versioning (Duineveld et al., 2000; Klein et al., 2002a; Noy & Musen, 2002; Noy et al., 2006). KAON iv Tool An ontology evolution system is proposed within the KAON framework (Stojanovic, 2004). Besides automating the evolution process, the KAON tool suite guides ontology engineers in formulating their change request by providing additional information and suggestions for ontology improvement. It includes data-driven change discovery functionality (Maedche et al., 2003), allows users to define some “evolution strategies” that control how changes will be made and helps in adapting the ontology towards needs of end-users that are discovered from the usage of this ontology (Stojanovic et al., 2002a). v Protégé Editor Protégé is a free, open source ontology editor and knowledge-base framework; developed by the Stanford Medical Informatics group (SMI) at Stanford University. It provides a graphical and interactive ontologydesign and knowledge-acquisition environment. Protégé architecture is component-based allowing the enrichment of the editor’s functionalities by adding specialized plug-ins (e.g. Protégé-OWL plug-in). Some plug-ins are proposed to support specific evolution features and are presented below. Ontology Versioning Tools In the change management framework proposed for distributed ontology (see section on change management for distributed ontology), several specialized prototypes were developed (Noy & Klein, 2003; Noy et al., 2004; Klein, 2004): • OntoView tool implements a change detection procedure for RDF-based ontologies. It uses rules to find specific operations and produces transformation sets between ontology versions (Klein et al., 2002a). • Two extensions to the PROMPTdiff tool –developed as a plug-in for Protégé finding mappings between frames by means of heuristics (Noy & Musen, 2002)– was proposed in (Klein, 2004). Their role is to produce the evolution relation between the elements of two ontology versions. The user interface allows visualizing some complex changes between ontology versions. A more comprehensive ontology-evolution system was described in (Noy et al., 2006). The core of the system is the Change and Annotation Ontology (CHAO). The instances of CHAO ontology represent changes between two versions of an ontology and user annotations related to these changes. The system is implemented as two related Protégé plug-ins: • Change Management Plug-in providing access to a list of changes and enabling users to add annotations to individual or grouped changes and to see concept history; PROMPT Plug-in providing comparisons of two versions of an ontology, information on users who • performed changes and facilitating change acceptance and rejection (Noy & Musen, 2003). Change Detection and Logging Tools In (Plessers et al., 2007), two extension prototypes for Protégé ontology editor are presented (for the approach, see section change detection approach using a version log): • Version Log Generator Plug-in automatically creating a version log by tracking all the changes applied to an ontology. Applied change are caught as events thrown by Protégé, and the version log is updated by setting the end time of the latest concept-versions of the concepts involved in the change, and by creating the appropriate new concept versions representing the new state of the changed concepts; Change Detection Plug-in taking as input a set of change definitions and a version log; and • providing as output an evolution log by evaluating the given change definitions on the given version log. Ontology Learning and Data-Driven Change Discovery Tool Text2Onto vi Text2Onto an ontology learning system for semi- or fully automatic ontology creating process was presented in (Cimiano & Völker, 2005). Text2Onto was developed as a successor of TextToOnto (Mäedche & Volz, 2001). It supports data-driven change discovery and includes three main components (see section on change requirement discovery): vii • GATE (General Architecture for Text Engineering) as an NLP (Natural Language Processing) tool; • POM (Model of Possible Ontologies) to store the different generated changes proposed and their explanation; • Change management component to catch change impact on the ontology and choose from the possible changes proposed those who fit the corpus. Change impact management is driven through an incremental learning process. Text2Onto follows a translation-based approach translating instantiated modeling primitives into OWL (Haase & Volker, 2008). SYNTHESIS In this section, we sum up the state-of-the-art presented in the previous sections through a comparative table based on general characteristics, evolution functionalities supported, and specificities of the existing ontology evolution approaches (see Table 3). Table 3. Synthesis of ontology evolution approaches and tools. Ontology Learning Approach Based on Change Requirement Discovery BOEMIE Approach (Castano et al., 2007) (Petasis, 2007) Functionalities General Characteristics (Cimiano, 2007) (Cimiano & Völker, 2005) Change Management Approach for Distributed Ontology (Klein, 2004) (Maedche et al., 2003) (Noy et al., 2006) A process of creating change specification A Global Evolution Process for KAON Ontology Ontology Evolution Approach based on Belief Change Principles (Stojanovic, 2004) (Flouris, 2006) (Flouris & Plexousakis, 2006) (Flouris et al., 2006) Evolution Process Some phases Some phases Global Process Ontology Language OWL DL OWL OWL KAON Tool / Prototype Text2Onto Ontology Evolution Toolkit, a component of BOEMIE prototype - OntoView - PROMPTdiff - Protégé plug-ins KAON Tool Suite Change Requirement Identification - Data-driven - User-driven Change Specification Learning Ontology Model (LOM) By reasoning on multimedia sources and external knowledge sources (existing ontologies or taxonomies, etc.) ABox (concept and - Ontology of relation instances) change operations and version transformation (extended OWL An algorithm that takes a set of axioms as input (ontology + change) and applies them Some DL in openworld assumption but not OWL Change Detection Approach Using a Version Log (Plessers & De Troyer, 2005) (Plessers et al., 2007) - Evolution on request - Evolution in response OWL DL Two Protégé plugins: - Version log generator - Change detection - Data-driven - Usage-driven By new fact identification (factcentered change operations) -KAON change specification - Evolution ontology Axioms based on AGM theory Change definition language CDL (declarative and formal definition based on temporal meta-model) - Change specification language (in RDF) - Change and annotation ontology CHAO Consistency Maintenance Change Propagation Level Logical Structural and logical Checking Based on confidence Yes (for annotation enrichment and population) Compatibility between the different versions Proposition of Resolution Several solutions ordered by an evaluation function Derive additional changes Automatic Resolution Deletion of some uncertain axioms If population: Elimination of redundancy and logical inconsistencies Resolve specific problems (ontology-related tasks) Target - Data sources - Data sources - Ontologies Type Definition and update of knowledge mappings between domain ontology and the external knowledge sources involved - Analysis of compatibilities between ontology versions and data sources and partly translating data - Proposition for ontology logic) - Structural (KAON) - Structural and logical (OWL Lite) (Haase & Stojanovic, 2005) - A priori (KAON) - Localization of the minimal inconsistent sub-ontology after axiom adding operations (OWL Lite) (Haase & Stojanovic, 2005) - Resolution strategies proposing axioms to delete (logical level, OWL Lite) - Declarative and procedural approaches (KAON) - Rewriting rules (structural level, OWL Lite) - Ontologies - Applications - Instances Different kind of synchronizations with dependant artifacts Logical Logical Axiom satisfaction revision Localization algorithm Proposition of rules weakening restrictive axioms Based on belief change principles - Ontologies - Applications Artifact maintainers decide synchronization - Conceptual and evolution relations between versions - Generating change information (RDF ontologies) Change Detection Versioning Evolution Log Evolved Version Version Comparison Managing Several Versions Applied Changes Trace of management Operations Specificities Generation of an evolved version Yes Saving the evolved version Saving the evolved version Yes Saving detected changes: - Version log of each ontology entity - Evolution log of the ontology Yes Yes Yes Yes Change explanation traceability Traceability of all the process (evolution journal) Ontology A framework of A global process and a population and distributed ontology dedicated system enrichment guided and version by patterns synchronization Evolution integrated to ontology learning Yes (detected changes) Applying AGM theory as a complementary approach to have a formal formalism and an automatic process Several interpretations on changes according to the different users FUTURE RESEARCH DIRECTIONS The purpose of this section is to give an overview on current work in progress in ontology evolution and to discuss open issues and future research directions. Ontology Debugging and Evolution Inconsistency resolution cannot be handled without a formal consistency checking, delimiting the detected inconsistencies and giving rational explanations. Existing reasoners are more or less precise in their analysis and do not give sufficient details. Ontology debugging is therefore a promising field to cope with the problem. In (Moguillansky, Rotstein, & Falappa, 2008), a theoretical approach to handle ontology debugging through a dynamic argumentation framework based on description logics was presented. The purpose of the methodology is to bridge ontology-specific concepts to argumentation notions and to employ argumentation acceptability semantics to restore consistency to ontologies. A Dynamic Argumentation Framework for DLs was detailed. Other interesting researches focusing on ontology debugging are presented in (Parsia, Sirin, & Kalyanpur, 2005; Wang, Horridge, Rector, Drummond, & Seidenberg, 2005). The purpose is to provide more comprehensible explanations of the inconsistency than standard reasoners’ results. Two techniques are distinguished: • Black-box techniques considering the reasoner as a black box and applying inferences to localize inconsistencies; • Glass-box techniques modifying the internal mechanism of the reasoner to explain inconsistencies and complete reasoners’ results which really improve consistency maintenance. A glass-box approach was discussed in (Sirin & Parsia, 2004). It provides information about the contradictions found and axioms causing the inconsistency but does not propose inconsistency resolutions. Integrating Ontology Evaluation in Ontology Evolution Evaluation is an important issue for ontology evolution. Ontology evaluation is already employed in capturing change requirements. Based on quality metric assessment, several changes can be proposed for ontology refinement and improvement. Moreover, ontology evaluation can be employed to control the evolution process and even to guide the resolution of change impacts. In (Dividino & Sonntag, 2008), a controlled evolution of ontologies through semiotic-based evaluation methods was presented. A tool S-OntoEval assessing ontology quality by implementing existing quality metrics was described. It carries out a complete evaluation combining several evaluation metrics categorized into the semiotic levels. In (Djedidi & Aufaure, 2008; 2010) an ontological knowledge maintenance methodology was proposed. The goal of the methodology is to manage ontology evolution while maintaining consistency and evaluating change impact on ontology quality. A hierarchical model describing and measuring ontology quality through several criteria and metrics was defined. The model is employed to assess the impact of the different inconsistency resolutions proposed for a change, to guide the choice of the most appropriate one regarding ontology quality. Towards Ontology Evolution Guidelines The approach described in (Djedidi & Aufaure, 2008), has been enriched by the definition of ontology change management patterns (CMPs). CMPs are proposed as a solution looking for invariances in change management that repeatedly appear when evolving ontologies. Three categories of patterns are distinguished: Change Patterns, Inconsistency Patterns and Alternative Patterns. The goal of CMP modeling is to offer different levels of abstraction, to establish conceptual links between these three categories of patterns determining the inconsistencies that could be potentially caused by a type of change and the alternatives that may resolve a kind of inconsistency and, thus, to guide an automated change management process (Djedidi & Aufaure, 2009; 2010). Future Issues By analyzing typical problems in managing ontology changes and studying existing and current research on ontology evolution, the following observations arise: • An ontology evolution process has to handle the application of a given ontology change by deriving intermediate changes required, capturing change impacts, and ensuring the consistency of the underlying ontology and all dependent artifacts; • Evolution process should be automated and optimized but also sufficiently flexible to allow the user to easily manage changes and validate or revoke a change application or resolution; • Evolution process should offer possibilities to catch useful changes for ontology refinement and improvement based on ontology domain, ontology usage and application, and the quality of the ontology itself. Although the successful realizations that have been performed in the field of ontology evolution, there are still open issues that can be considered more deeply. From our perspective, these issues are based on the following observations: • Many of the implementations of the described approaches seem to work better for relatively small changes than for complex ones. Complex changes cannot be predefined exhaustively; the question is how to provide, in practice, guidelines for handling their application in a more automated and optimized way? Analyzing ontology evolution use-cases is required to come up with additional specific procedures and guidance for complex change management. • Distributed and collaborative dimensions of ontology environment have to be considered more deeply because, in practice, ontologies are still maintained in a centralized way and dependant artifacts are often concerned by their evolution. Research in change propagation, global validation, conflict resolution, and even reasoning on inconsistent ontologies where mutual agreements cannot be reached are required. Ontology evolution system should be enriched by a meta-model or a kind of generic layer language• independent to be able to propose generic change management guidelines that can be used for different evolution process. Moreover, this can guide change propagation to dependant ontologies developed in different ontology languages. CONCLUSION Ontology evolution is an essential research area for the widespread use of ontologies in industrial and academic applications. In this chapter, we have outlined ontology evolution issues and requirements, and presented a comparative study of ontology, database schema, and knowledge-based system evolution. A state-of-the-art review considering the main existing ontology evolution approaches and tools was presented. Furthermore, a comparative study of this review is given. Before concluding, we have discussed some research in progress on ontology evolution and given some perspectives on future research directions. 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