Change operators are the building blocks of the ontology evolution. The changes in an evolving ontology are performed using change operators. In order to explicitly provide semantics of the ontology changes, researchers have emphasized on classifying the on- tology changes into a number of categories. The purpose of such categorisation is to define a layered taxonomy of change operators in order to provide adequate support for ontology users, having different types of background knowledge and reducing the effort (in terms of time and consistency) required in ontology evolution process.
3.2.1 Elementary, composite and complex change operations
The most prominent categorization of ontology changes is given by Stojanovic [Sto- janovic, 2004] in the KAON project where changes are separated into three levels of abstraction, i.e. elementary, composite and complex changes.
- Elementary Changes - An elementary change performs an atomic change on a single entity of the ontology. The examples of elementary change operations would be Add class, Add individual, Delete subclassOfAxiom etc. Stojanovic argues that such elementary level change representation is not suitable at all times. In most often cases, the intent of change is represented at a higher level. If we represent changes as a sequence of elementary level changes, the intent of change can be interpreted in different ways and can mislead. Moreover, there is a mismatch in the objective of change and how the objective is actually achieved. For example in Figure 3.1, the goal is to split a class Research Student into two, i.e. PhD Student and MSByResearchStudent, however, the goal can be interpreted differently at different time slots during the change operations (Table 3.1). The combination of the first 2 (and the first 4) change operations in the table describes a different intent of change operations, which is to add a new sibling to class ResearchStudent. Similarly, different other intents of change operations can be acknowledged through the process of a composite ontology change.
ResearchStudent Student
Javed Abgaz Zubair
studentId
PhD_Student Student
Javed Abgaz Zubair
studentId
MSByResearchStudent assigned
Course assigned
Course
Figure 3.1: Composite change operation ‘‘Split class’’
To represent the intent of ontology change more explicitly at a higher level, Sto- janovic proposed composite change operations.
Change Operations Intent of Change 1 Add class (PhD Student)
Add sibling to “Research Student” 2 Add subclassOfAxiom (Phd Student, Student)
3 Add class (MSByResearchStudent)
Add sibling to “Research Student” 4 Add subclassOfAxiom (MSByResearchStudent,
Student)
5 Add classAssertionAxiom (Javed, PhD Student) Move instance (Javed, ResearchStudent, PhD Student)
6 Delete classAssertionAxiom (Javed, ResearchStu- dent)
7 Add classAssertionAxiom (Abgaz, PhD Student) Move instance (Abgaz, ResearchStudent, PhD Student)
8 Delete classAssertionAxiom (Abgaz, ResearchStu- dent)
9 Add classAssertionAxiom (Zubair, MSByRe- searchStudent)
Move instance (Zubair, ResearchStudent, MS- ByResearchStudent)
10 Delete classAssertionAxiom (Zubair, ResearchStu- dent)
11 Delete subclassOfAxiom (ResearchStudent, Stu- dent)
12 Delete class (ResearchStudent) Split class “ResearchStudent” Table 3.1: Change operations and the intent
its neighborhood. As described by Stojanovic, The neighborhood of a class consists of its subclasses, superclasses, properties, for which it is specified as a domain or as a range class, and instances defined for that class. The neighborhood of a property contains its domain class, range classes, subproperties, superpropeties, instances it is defined for as well as instances it points to. The neighborhood of an individual includes its (rdf:type) classes, properties that are instantiated for it as well as properties that point to it.
Examples of composite ontology changes (related to the class-class relationship) include Merge classes, Split class, Move up class, Group classes etc. It is not fea- sible to present a comprehensive list of useful composite change operations, as in future, different combinations of elementary change operations may lead to new composite change operations. For example, splitting a class into two and making
individuals as instances of both split classes may be extended into splitting a class into n (more than two) ontology classes and individuals may be split into all n on- tology classes. Stojanovic argues that some extensions can also be domain-specific, for example, grouping of subclasses, which are parent of a concrete individual in an ontology model. Therefore, there exist another higher layer of abstraction of ontology changes, i.e. complex changes.
- Complex Changes - apply a change that is an arbitrary combination of at least two elementary and composite ontology changes.
In addition to the classification of ontology changes given above, the author catego- rized the ontology changes into Additive and Subtractive changes. The additive changes are those changes which add a new element in the ontology without altering the existing ontology elements. Whereas, the subtractive ontology change involve the deletion of few of the existing ontology elements.
3.2.2 Atomic and composite change operations - basic and complex A similar categorisation of changes for OWL ontologies is given by Klein [Klein, 2004]. The author classifies the OWL ontology changes into atomic and composite types.
- Atomic change operations are similar to elementary change operations that can modify one single entity of OWL ontology model (e.g. Delete subclassOfAxiom, Add class). The author states that such atomic change operations can further be classified as simple or rich in content.
- An atomic simple change operation is a basic change operation that can be determined from the ontology structure. Adding a new class, properties, indi- viduals or creating relationships between classes (i.e. subclass, equivalent classes, disjoint classes) etc. are examples of atomic simple change operations.
- An atomic rich change operation is a complex change operation that expresses the implications of the applied changes. For example, an atomic rich change oper- ation may specify that the range of property is enlarged (i.e. the range class of a particular property is changed to the superclass of the original range class). - Composite change operations are composed of several atomic change operations
and are of complex category. Such composite change operations can also be simple or rich in content, incorporating the implications of the change operations on the ontology model (e.g. add subtree, move siblings, restrict domain, merge multiple siblings, split into multiple siblings etc.).
The major difference between the KAON ontology change classification (given by Stojanovic) and the OWL ontology change classification (given by Klein) is, that Klein considers modification as a distinct type of change operation in order to provide complete specification that allows reversing the changes and is often available in the logs of changes provided by the tools. Change operation modify takes two arguments, one the old value and the other the new value. The old value is replaced by a new value.
3.2.3 Atomic, entity and complex change operations
Compared to the above given classification of ontology changes, Palma proposed a slightly different taxonomy of ontology changes comprising of atomic, entity and com- posite changes in his proposed generic change ontology [Palma et al., 2009].
- Atomic changes - Palma proposed a lower layer below the elementary change op- eration layer (as proposed by Stojanovic and Klein) and argues that elementary (atomic) change operations had been introduced as operations that cannot be sub- divided into smaller operations; however, such change operations are all at the entity level. In his change ontology, an atomic change includes the applied ax- ioms which later at the entity level can be associated to a specific ontology entity.
Addition of the class axioms, assertions, declaration, object property axioms are examples of atomic change operations.
- Entity changes - The next level at the top of atomic changes is the entity level. En- tity level associates ontology changes to the ontology elements. The changes such as Add subclassOf, Add disjointClasses, Add inverseObjectProperty are examples of the entity level change operations as they are linked to a particular ontology element, which can be class, object property, data type property or individual. - Complex changes - Similar to the previous approaches, the final level is comprised
of complex changes. Complex changes are groups of entity changes that are applied together and constitute a logical entity, e.g. merge a set of siblings, group a set of classes etc. Similar to the past literature, s/he also mentioned that providing an exhaustive list of composite change operations is impracticable as entity and com- posite changes can be combined together in different ways to create new composite changes.