Presentation Structure

Background What a semantic network is What an ontology is

Ontology languages XML, RDF, DAML+OIL

Use of Semantic in Web Services SOAP, WSDL, UDDI, DAML-S

Semantic Networks

Main consepts and motivation

Semantic Net: Motivation

The main idea: the meaning of a concepts comes from the ways in which it is connected to other concepts.

Psychology: human memory is a network of associations between different pieces of knowledge

Semantic Net: Starting point

Starting point: associationism – traces back to Aristotle in ancient Greek

Required: A satisfactory knowledge representation language should reflect the high degree of interconnectivity between the pieces of information contained in the human memory.

This requirement made researchers to develop the idea of semantic networks ===>

Semantic Net: Idea

Semantic Networks as:

Models of human memory and meaning representation

Components of language understanding and reasoning systems

Semantic networks capture not only definitions of concepts but also inherently provide links to other concepts

The term ”Semantic Network”

The term ”Semantic Network” encompasses a family of graph-based representations.

This means...

A Semantic ”Bird-example”

The term ”Semantic Network”

Basically a Semantic Net consists of a set of nodes and links:

Nodes represent object and descriptive information abut those objects

Links describe the relationships between the nodes

Objects in a Semantic Net

Objects can be any physical item such as a book, car, desk, or even a person.

Objects can also be concepts, events, or actions.

Attributes of an object can also be used as nodes. These might represent size, color,class,age…

Links in a Semantic Net

Basic types of links-relationship:

isa link: represnting the inclusion relationship of an object in another (Bird isa Animal)

has-part link: an object is described by another object (Bird has-part Wings and Bird has-part Feather)

instance link between a type and a token (Tweety is an instance of bird)

More domain-specific relations: e.g., has-color, triggers, etc

Advantages of Semantic net

Hierarchical organisation of knowledge

People store information at its most abstract level and thereby economy storage by inheritance

Exception knowledge is stored on the most directly related node, while default information higher up in the hierarchy

Advantages of Semantic net

In particular, Semantic Networks –

realized as massively parallell networks or agents may provide the appropiate framework for modelling reflexive reasoning... (isa-link)

...this is very useful, when constructing dynamic and self-aware system (Model Driven Systems with Meta-Data)

Disadvantages of Semantic net

Wood’s paper ’What’s in a link’ (1975) shows ambiguities and unclarities in Semantic Networks:

No semantics forsemantic networks

No semantics for semantic networks ’What’s in a link’

Different interpretations for the same network

Possible interpretations

A definition of the concept of a black telephone A specific black telephone (both description of a

specific entity in the world and the assertion on the existence of a black telephone)

Assertional: some or maybe all telephones are black

telephone

blackt54

instance

modify

What is needed...

Need a common understanding of the semantic in a semantic network

This leeds to...

What an Ontology is

An ontology is a specification of a conceptualization

An Ontology is then...

A description (like a formal specification of a program)

of the concepts and relationships that can exist for an agent/user or a community of agents/users that use the program.

The purpose of an ontology

The purpose of an ontology is to enable knowledge sharing and reuse.

Common understanding of a domain Used for communication and reasoning across

people and applications

In this context, an ontology is a specification used for making ontological commitments...

Ontological commitments

A set of ontological commitments, is the answer to the question:

In what terms should I think about the world?

An IKT Example Ontology

Common use of ontologies

We use common ontologies to describe ontological commitments for a set of agents/users

...so that they can communicate about a domain of discourse without necessarily operating on a globally shared theory.

We say that an agent/user commits to an ontology if its observable actions are consistent (but not complete) with respect to the theory specified by an ontology

The term Ontology

The term is borrowed from philosophy, where an Ontology is a systematic account of Existence

What "exists" is that which can be represented

An ontology is thus a declarative formalism of a semantic network

Declarative formalism means...

When the knowledge of a domain is represented in a declarative formalism,

The set of objects that can be represented is called the universe of discourse.

This set of objects, and the describable relationships among them, are reflected in the representational vocabulary (dictionary) which a knowledge-based program represents knowledge

Knowledge-based repository:

An Ontology is then...

Formally, an ontology is the statement of a logical theory and is specified by Description Logics....

Description Logics

Based on concepts (classes) and roles Concepts (classes) are interpreted as sets of objects Roles are interpreted as binary relations on objects

Decidable fragments of First Order Logic Closely related to propositional modal logics

Key features of Description Logics are Well defined semantics (they are logics) Provision of inference services

Description Logics

Description Logic is characterised by set of constructors provided for building complex concepts and roles from simpler ones. Usually include at least:

Conjunction, disjunction, negation Restricted (guarded) forms of quantification

DL Syntax and Semantics

Set of operators/axioms restricted so that reasoning is decidable

Describing an Ontology

We can then describe the ontology of a semantic net by defining a set of representational terms.

In such an ontology, definitions

associate the names of entities in the universe of discourse (e.g., classes, relations, functions, or other objects)

with human-readable text describing what the names mean,

and formal axioms that constrain the interpretation and well-formed use of these terms.

Ontology Languages

For the Semantic Web

Language Requirements

Well designed Intuitive to human users Adequate expressive power

Well defined Clearly specified syntax (obviously) Formal semantics (equally important) Adequate expressive power

Compatible with existing (web) standards

Semantic Web (3rd generation)

First generation: handwritten HTML pages

Second generation: machine generated and/or active pages Still intended for direct human processing Reading, browsing, form-filling, etc.

Third generation: machine understandable/processable pages Will enable intelligent services Requires further levels of interoperability Standards for semantics as well as syntax

W3C standard schema language

Proposals for ontology languages already exist for W3C standard schema languages XMLS (XML Schema) and RDFS (RDF Schema)

Both have been touted as (standard) web ontology languages

However, both suffer from Expressive inadequacy — lack of basic modelling

primitives Poorly (un) defined semantics

Semantic Web languages

Semantic Web languages

Semantic Web’s layered architecture

XML provides syntax transport layer RDF provides basic relational language RDFS provides basic ontological primitives DAML+OIL provides (decidable) logical layer Further layers (e.g., rules) will extend

DAML+OIL. Extensions will almost certainly be undecidable

DAML+OIL

Ontology Inference Language (OIL)

DAML+OIL Ontology language

Describes structure of the domain RDF used to describe specific instances

Structure described in terms of classes (concepts) and properties (roles), just like Description Logic

DAML+OIL Ontology language

Ontology consists of set of axioms E.g., asserting class subsumption/equivalence

Classes can be names or expressions Various constructors provided for building class

expressions Expressive power determined by

Kinds of axiom supported Kinds of class (and property) constructor

supported

DAML+OIL: Class Constructors

Arbitrarily complex nesting of constructors is allowed

DAML+OIL: Axioms

Axioms (mostly) reducible to subClass/PropertyOf

Web Services

Semantics in Web Service Languages

SOAP and WSDL

SOAP is designed to provide descriptions of message transport

protocols

WSDL is designed to provide descriptions of the interface used by

each service

Limitations in SOAP and WSDL

However neither SOAP or WSDL are of any help for the automatic location of web services on basis of their capabilites

UDDI language

Provides a registry of business and web services

UDDI describes business by their physical attributes such name, address and the services that they provide.

In addition, UDDI descriptions are augmented by a set of attributes, called TModels, which describe additional features such as classification of services within taxonomies (such as NAICS)

UDDI limitation...

Because UDDI does not represent service capabilities, it is of no help to search for services on the basis of what they provide.

…many proposals for doing this

Shared limitations...

A limitation shared by all the XML based standards described to now is their lack of explicit semantics

Two identical XML descriptions may mean very different things depending on the context of their use.

This proves to be a major limitation for capability matching: In fact, one crucial aspect of capability matching is that it can be done only at the semantic level.

Capability matching

Can be done only at the semantic level.

This is the case because the requestor does not know what services are provided at any given time, otherwise it could contact the providers directly without need to search them;

Furthermore, advertisers and requestors have very different perspectives and different knowledge about the same service.

Capability matching

The major problem with capability matching is that it is unrealistic

to expect advertisements and requests to be equivalent

Need an ontology commitment of the semantics in a Web Service

DAML-S: Ontology language

Build upon the well-defined semantics of DAML+OIL

Is expected to provide a common understanding of the semantic in a web-service

By spesifing an ”Upper Ontology for Services”

An Upper Ontology for Services

An Upper Ontology for Services

Three essential types of knowledge about a service, each characterized by the question it answers:

What does the service require of the user(s),and provide for them?

How does it work? How is it used?

DAML-S: Ontology language

DAML-S is expected to provide five Automatic Web service tasks

Discovery and invocation. Composition and interoperation. Execution monitoring.

The Service Profile

The service profile tells “what the service does”; that is, it gives the type of information needed

by a service-seeking agent to determine whether the service meets its needs

(typically such things as input and output types, preconditions and postconditions, and binding patterns).

The Service Model

The service model tells “how the service works”; that is, it describes what happens when the service is carried out.

A more detailed perspective on services is that a service can be viewed as a process.

DAML-S defines a particular subclass of ServiceModel, the ProcessModel which draws upon well established work in a variety of fields, such as AI planning and workflow automation (Petri-Nets etc)

The Process Ontology

The Service Grounding

A service grounding (“grounding” for short) specifies the details of how an agent can access a service.

Typically a grounding will specify a communications protocol (e.g., RPC, HTTP-FORM, CORBA IDL, SOAP, Java RMI, OAA ACL), and service-specific details such as port numbers used in contacting the service.

In addition, the grounding must specify, for each abstract type specified in the ServiceModel, an unambiguous way of exchanging data elements of that type with the service (that is, the marshaling/serialization techniques employed).

DAML-S Summary

Taken together, the ServiceModel and ServiceGrounding-objects associated with a service provide enough information for an agent to make use of a service.

...and realise the vision of the Semantic Web and Dynamic Web Services