E-Science AHM, Nottingham. September 20 th, 2005. Glen Dobson: [email protected] An OWL Ontology...

e-Science AHM, Nottingham. September 20th, 2005. Glen Dobson: [email protected]

An OWL Ontology for QoS

Glen Dobson(Russell Lock, Ian Sommerville)

Lancaster [email protected]


Overview

QoSOnt is an OWL ontology for Quality of Service (QoS)

I will attempt to answer: What is an ontology? What is OWL? What is QoS? Why is a QoS ontology needed? How should one go about designing such an

ontology? What are the possible approaches? What are the difficulties?


What is an ontology?

Standard answer: “A specification of a conceptualization” (Gruber)

Pragmatically: A description of the concepts and relationships

which exist in some domain using a formal language.

An ontology is an engineering artefact for machine understanding

Its purpose is important. It should represent shared conceptualisations.

A shared vocabulary is the fundamental component of an ontology

Domain rules are also important


What is OWL?

OWL is the Web Ontology Language Supports sharing ontologies via the web Built on top of RDF (and XML in turn) Aim is to enable machine “interpretation” of

terms and their relationships It is a Description Logic

Primary constructs are Classes and their Properties

A Class defines a set of Individuals by precisely stating a set of membership conditions.

Main form of inference is subsumption i.e. is Class B a complete subset of Class A? + Classification: What Classes is Invidual I a member of?


OWL in the Semantic Web

OWL


Class Definitions in OWL

Classes can be described As named resources (as in RDF) As an enumeration By constraints on their Properties By combining other Classes using set operators

Descriptions be combined to give a Class definition using OWL’s: subClassOf equivalentClass disjointWith


OWL and Inference

A Dog could be asserted to be a Mammal. Or this classification could be inferred

based upon the Class Dog’s Properties (and Property restrictions) E.g. warm blooded, feeds young with milk,

internal fertilisation, etc. Problem of maintaining a polyhierarchy

manually a Dog is a Mammal, an Animal, a Pet, etc. Therefore assert a “monohierarchy” and have

multiple classifications inferred


What is your definition of QoS?

Any non-functional aspect of a system that someone may use to judge quality Extends the definition in distributed multimedia

where QoS is primarily concerned with the network (and performance in particular)

In practice we have concentrated primarily on dependability – but the concepts apply beyond this.

What QoS concepts are modelled? We are primarily concerned with the core

concepts of QoS (e.g. attributes, metrics) Also some consideration to QoS requirements


Why an ontology for QoS?

To provide a shared vocabulary For use primarily by machines – but perhaps also

in human-readable documents (e.g. requirements documents, SLAs).

To embody machine interpretable “knowledge” e.g. QoS brokers may need to translate between

terms/infer aggregate values/convert units, etc. Also the provision of QoS description and

reasoning capabilities to the semantic web


QoS Sub-Systems

Service Discovery

Service negotiation

Service Mediation

Service Monitoring

Service Agreements

Service Payment

Service Operation

Banking systems

Service Differentiation

Law

Re-negotiation

QoSPrediction

WorkflowPlanning


What “added value” could a QoS ontology provide?

Translation based upon machine “understanding” Translation of units, computation of composite

metrics, inference of aggregate QoS for workflows

Leeway in syntax matching i.e. multiples terms can refer to the same thing

An interlingua for translation between other QoS languages

A means for agents to communicate


QoSOnt Structure

At the core of QoSOnt is a taxonomy of Attributes and Metrics i.e. two trees formed using the subClassOf

construct An attribute is e.g. reliability, performance A metric is e.g. Probability of Failure on Demand,

Transactional Throughput This becomes a (complex) directed graph

once properties are considered e.g. The Property hasMetric (and its inverse

isMetricOf) is the basic link between the attribute and metric trees


Danger of Ontology Creep

Should we provide a model to represent: Time

Currently we do – but we should instead use the OWL-Time ontology.

Ways of composing metrics, Mathematical constructs that don’t exist in OWL

This originally put us off and thus we have a separate XML language as well as the ontology.

Ways of composing services We currently use a very shallow model – but perhaps this

is all that is needed?


QoSOnt High-Level Structure

Time

Performance Dependability Etc ….Attribute Layer

Low level concepts Base concepts

MetricsMetric Layer

Underlying OWL

Metric Instances


IFIP 10.4 Dependability Taxonomy

Our example of an attribute layer ontology Familiar Fault-Error-Failure model Main point of linkage is DependabilityAttribute is

a subclass of QoSAttribute Shows how a detailed model of certain attributes

can help E.g. without the definition of Failure, Failure Domain it is

impossible to be specific about what a Probability of Failure On Demand refers to


Overview of Metric Definition


Representing QoS Requirements

As OWL Classes using built-in OWL constructs Datatype support is poor

No consistent way of using custom XML types Reasoning support for quantification over datatypes (e.g.

allValuesFrom 0-100) is poor. Level of datatype support mandated by OWL spec is poor

Using QoSOnt defined Classes, Properties, Restrictions, etc.

As a separate (XML) language referencing the ontology


SQRM Tool


Requirements Matching in SQRM


Evaluation/Future (1)

An ontology is a good idea – but a large-scale standardisation effort is required Need external input in order to evolve

Two interested parties are now involved

Requirements representation and matching using built-in OWL features would be nice Need to wait for OWL to develop

Need to look at SWRL (Semantic Web Rules Language) E.g. would provide a neater way to express unit

conversions


Evaluation/Future (2)

Need to work on tools that make use of QoSOnt (and also enhance SQRM) Difficult to evaluate otherwise since the purpose

is machine-machine understanding But are there really a lot of QoS

“semantics” to model? Service Composition/Workflow

Integrating existing work with ontology


Questions

For more information:http://digs.sourceforge.net

E-Science AHM, Nottingham. September 20 th, 2005. Glen Dobson: [email protected] An OWL Ontology...

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