FinalSWT Presentation.ppt

8/14/2019 FinalSWT Presentation.ppt

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User-driven Ontology Evolution

Management


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Introduction

Ontology evolution is the timely adaptation of an ontology to

changed business requirements, to trends in ontology instances

and patterns of usage .

A modification in one part of the ontology may generate subtleinconsistencies in other parts of the same ontology, in the

ontology based instances as well as in depending ontologies and

applications.

We specifically focus on the problem that ontology has to remain

consistent under complex changes during evolution.


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Ontology Evolution Requirements

ontology evolution require following set of design requirements

The first requirement is the essential one for any ontology evolutionapproach after applying a change to a consistent ontology, the ontologyshould remain in consistent state. It has to enable resolving the givenontology changes and ensure the consistency of the underlying ontology and

all dependent artifacts.

The second requirement complements the first one by presenting the userwith information needed to control changes and make appropriate decisionsIt should be supervised allowing the user to manage changes more easily.

The last one states that potential changes improving the ontology may bediscovered semi automatically from ontology-based data and throughanalysis of users behavior It should offer advice to user for continualontology refinement.


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Ontology evolution process


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Ontology evolution process

It consist of Six Phases as follows

(1) Change Capturing

(2) Change Representation

(3) Semantics of Change

(4) Change Implmentation

(5) Change Propagation

(6) Change Validation.


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Change Capturing

The process of ontology evolution starts with capturing changes

either from explicit requirements or from the result of change

discovery methods, which induce changes from patterns in data

and usage.

It has two types of change Discovery usage-driven and data-

driven change discovery.

Usage-driven changes result from the usage patterns created over

a period of time

Data-driven change discovery as the problem of deriving

ontological changes from the ontology instances by applyingtechniques such as data-mining, Formal Concept Analysis (FCA)

or various heuristics.


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Consistency requirement states that after applying and resolving

changes in an ontology already in a consistent state, the ontology,

its instances and dependent ontologies/applications must remain

in the consistent state.

Data-driven change discovery can be defined as the task of

deriving ontology changes from modifications to the knowledge

from which the ontology has been constructed.

Change Capturing


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To resolve changes, they have to be identified and represented in

a suitable format which means that the change representation

needs to be defined for a given ontology model.

The set of ontology change operations depends heavily on the

underlying ontology model

A lot of unnecessary changes may be performed if each change isapplied alone.

There is an impedance mismatch between the intent of the

request and the way the intent is achieved.

To avoid these drawbacks, it should be possible to expresschanges on a more coarse level, with the intent of change directly

visible.

Change Representation


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Semantics of Change

Application of an elementary change in the ontology can induce

inconsistencies in other parts of the ontology.

Syntax inconsistency arises when undefined entities at the

ontology or instance level are used or ontology model constraints

are invalidated.

Semantic inconsistency arises when meaning of an ontologyentity is changed due to changes performed in the ontology

If an ontology is large, it may be difficult to fully comprehend

the extent and meaning of each induced change.

The task of semantics of change phase is to enable resolution ofinduced changes in a systematic manner, ensuring consistency of

the whole ontology.


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Semantics of Change The authors describe the semantics of change for the consistent

evolution of OWL ontologies, considering the structural, logical,

and user-defined consistency conditions

Structural Consistency ensures that the ontology obeys the

constraints of the ontology language with respect to how the

constructs of the ontology language are used. consistent.

Logical Consistency regards the formal semantics of the

ontology: viewing the ontology as a logical theory, an ontology

as logically consistent if it is satisfiable, meaning that it does not

contain contradicting information.


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Semantics of Change

User-defined Consistency: Finally, there may be definitions of

consistency that are not captured by the underlying ontology

language itself, but rather given by some application or usage

context. The conditions are explicitly defined by the user andthey must be met in order for the ontology to be considered


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Semantics of Change

Author describes and compares two approaches to verify

ontology consistency:

1) A posteriori verification, where first the changes are executed,

and then the updated ontology is checked to determine whether it

satisfies the consistency constraints.

2) A priori verification, which defines a respective set of

preconditions for each change. It must be proven that, for each

change, the consistency will be maintained if an ontology isconsistent prior to an update and the preconditions are satisfied.

Ch I l i


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Change Implementation

The role of the change implementation phase of the ontology

evolution process is

(i) to inform an ontology engineer about all consequences of a

change request

(ii) to apply all the (required and derived) changes and

(iii) to keep track of performed changes

Change Notification: In order to avoid performing undesired

changes, a list of all implications for the ontology and dependent

artefacts should be generated and presented to the ontology

engineer, who should then be able to accept or abort these

changes.


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Change Implementation

Change Application: The application of a change should have

transactional properties, that is (A) Atomicity, (C) Consistency,

(I) Isolation, and (D) Durability. This approach realizes the

requirement by strict separation between the request specification

and the change implementation

Change Logging: There are various ways to keep track of the

performed changes The evolution ontology covers the various

types of changes, dependencies between changes as well as the

decision-making process.

Ch P ti


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Change Propagation

When the ontology is modified, ontology instances need to be

changed to preserve consistency with the ontology.

This can be performed in three steps

The instances are on the Web, collected from the knowledge base

In the second step, modification of instances is performed

according to the changes in the ontology

In the last step out-of-date instances on the Web are replaced

with corresponding up-to-date instances.


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Change Propagation

So when an ontology is changed, applications based on the

changed ontology may not work correctly

An ontology evolution approach has to recognize which change

in the ontology can affect the functionality of dependent

applications and to react correspondingly

Ch V lid ti


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Change Validation

There are numerous circumstances where it can be desirable to

reverse the effects of the ontology evolution

It is the task of the change validation phase to recover from these

situations

Change validation enables justification of performed changes

or undoing them at users request


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Ontology Evolution Process


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Ontology Evolution Process The overall ontology evolution process derived from our

discussion of ontology evolution requirements is presented in

Figure 3.

It has a cyclic structure, since validation of realized changes may(automatically) induce new changes in order to obtain model

consistency or to satisfy users expectations.


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Conclusion

In this way we have studied the approach of Ontologyevolution which is based on a six-phase evolution

process.

Which systematically analyses the causes and the

consequences of the changes and ensures the

consistency of the ontology and depending artifacts

after resolving these changes


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Thank You

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