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D2v1.4. Web Service Modeling Ontology (WSMO)

WSMO Working Draft 6 November 2006

This version:
http://www.wsmo.org/TR/d2/v1.4/20061106/
Latest version:
http://www.wsmo.org/TR/d2/v1.4/
Previous version:
http://www.wsmo.org/TR/d2/v1.3/
Editors:
Dumitru Roman
Holger Lausen
Uwe Keller
Co-Authors:
Jos de Bruijn
Christoph Bussler
John Domingue
Dieter Fensel
Martin Hepp
Michael Kifer
Birgitta König-Ries
Jacek Kopecky
Rubén Lara
Eyal Oren
Axel Polleres
James Scicluna
Michael Stollberg

This document is also available in a non-normative PDF version.
Note: This version of the document is compliant with the W3C submission of WSMO in April 2005.



Abstract

This document presents an ontology called Web Service Modeling Ontology (WSMO) for describing various aspects related to Semantic Web Services. Taking the Web Service Modeling Framework (WSMF) as a starting point, we refine and extend this framework, and develop an ontology and a description language.



Table of Contents


1. Introduction

This document presents an ontology called Web Service Modeling Ontology (WSMO) for describing various aspects related to Semantic Web Services. Taking the Web Service Modeling Framework (WSMF) [Fensel & Bussler, 2002] as a starting point, we refine and extend this framework, and develop a formal ontology and language. WSMF [Fensel & Bussler, 2002] consists of four different main elements for describing semantic Web Services: (1) ontologies that provide the terminology used by other elements, (2) goals that define the problems that should be solved by Web Services, (3) Web Services descriptions that define various aspects of a Web Service, and (4) mediators which bypass interoperability problems.

WSMO provides ontological specifications for the core elements of Semantic Web services. In fact, Semantic Web services aim at an integrated technology for the next generation of the Web by combining Semantic Web technologies and Web services, thereby turning the Internet from a information repository for human consumption into a world-wide system for distributed Web computing. Therefore, appropriate frameworks for Semantic Web services need to integrate the basic Web design principles, those defined for the Semantic Web, as well as design principles for distributed, service-orientated computing of the Web. WSMO is therefore based on the following design principles:

This document is organized as follows: Section 2 describes the meta-model structure and language used for defining WSMO based on the Meta-Object Facilities (MOF). We introduce the top level elements of WSMO in Section 3, which are then further refined: Section 4 outlines ontologies, Section 5 discusses Web service descriptions, Section 6 explains WSMO goals, and Section 7 defines mediators. Section 8 defines the syntax of the logical language that is used in WSMO. The semantics and computationally tractable subsets of this logical language are defined and discussed by the WSML working group. Section 9 presents our conclusions and suggestions for further work.

Finally, the appendix provide an overview of the complete WSMO conceptual model in MOF-style (in Appendix A) as well as in the form of UML Class Diagrams (in Appendix B.)

For a more explanatory document on WSMO we refer to the WSMO Primer [Feier, 2004]. For a non-trivial use case demonstrating how to use WSMO in a real-world setting we refer to the WSMO Use Case Modeling and Testing [Stollberg et al., 2004] and for the formal representation languages we refer to The WSML Family of Representation Languages [De Bruijn, 2004].

Besides the WSMO working group there are two more working groups involved in the WSMO initiative: The WSML working group focusing on language issues and developing an adequate Web Service Modeling Language with various sublanguages, and the WSMX working group that is concerned with designing and building a reference implementation of an execution environment for WSMO.

Note: The vocabulary defined by WSMO is fully extensible. It is based on URIs with optional fragment identifiers (URI references, or URIrefs) [Berners-Lee et al, 1998]. URI references are used for naming all kinds of things in WSMO. The default namespace of this document is wsmo:http://www.wsmo.org/2004/d2/. Furthermore this document uses the following namespace abbreviations: dc:http://purl.org/dc/elements/1.1#, xsd:http://www.w3.org/2001/XMLSchema#, and foaf:http://xmlns.com/foaf/0.1/.

Note: "Web Service" and "Service" are used interchangeably in this document; they are meant to represent systems designed to support interoperable machine-to-machine interactions over the Internet.

2. Language for Defining WSMO

In this section we introduce the language used to define WSMO. Besides the Meta-Model Layers (Section 2.1) we explain the use of identifiers (Section 2.2) and data type values (Section 2.3). Section 2.4 describes a set of annotations used in the definition of different WSMO elements.

2.1 The Meta-Model Layers for Defining WSMO

WSMO is a meta-model for Semantic Web Services related aspects. The Meta-Object Facility (MOF) [OMG, 2002] specification is used to specify this model. MOF defines an abstract language and framework for specifying, constructing, and managing technology neutral meta-models.

MOF defines a metadata architecture consisting of four layers, namely:

In terms of the four MOF layers,

The most frequently used MOF meta-modeling construct in the definition of WSMO is the Class construct (and implicitly its class-generalization sub-Class construct), together with its Attributes, the type of the Attributes and their multiplicity specifications. When defining WSMO, the following assumptions are made:

For describing the WSMO conceptual model we will use MOF-style listings throughout this document, using the Class, sub-Class Attributes and type keywords. Note that, for better readability we additionally provide a graphical illustration of the the conceptual model of WSMO in the form of UML class diagrams in Appendix B.

2.2 Identifiers

Every WSMO element is identified by one of the following identifiers:

2.3 Values and Data Types

In WSMO, literals are used to identify values such as numbers by means of a lexical representation. Literals are either plain literals or typed literals. A literal can be typed by a XML data type (e.g. to xsd:integer). Formally, such a data type d is defined by [Hayes, 2004]:

Furthermore the data type may introduce facets on its value space, such as ordering, and therefore define the axiomatization for the relations <, >, and function symbols like + or -. These special relations and functions are called data type predicates and are defined in more detail in the WSML Family of Representation Languages [De Bruijn, 2004].

2.4. Annotations

Annotations are used in the definition of WSMO elements. We recommend most elements of [Weibel et al., 1998]. The listing below presents a set of general annotations which can be applied to any WSMO element.

Listing 1. Annotations Definition
Class annotation
      hasContributor type dc:contributor
      hasCoverage type dc:coverage
      hasCreator type dc:creator
      hasDate type dc:date
      hasDescription type dc:description
      hasFormat type dc:format
      hasIdentifier type dc:identifier
      hasLanguage type dc:language
      hasOwner type owner
      hasPublisher type dc:publisher
      hasRelation type dc:relation
      hasRights type dc:rights
      hasSource type dc:source
      hasSubject type dc:subject
      hasTitle type dc:title
      hasType type dc:type
      hasVersion type version 
Contributor
An entity responsible for making contributions to the content of the element. Examples of dc:contributor include a person, an organization, or a Web service. The Dublin Core specification recommends that typically the name of a dc:contributor should be used to indicate the entity.
WSMO Recommendation: In order to point unambiguously to a specific resource we recommend the use an instance of foaf:Agent as value type [Brickley & Miller, 2004].
Coverage
The extent or scope of the content of the element. Typically, dc:coverage will include spatial location (a place name or geographic coordinates), temporal period (a period label, date, or date range) or jurisdiction (such as a named administrative entity).
WSMO Recommendation: For more complex applications, consideration should be given to using an encoding scheme that supports appropriate specification of information, such as DCMI Period, DCMI Box or DCMI Point.
Creator
An entity primarily responsible for creating the content of the element. Examples of dc:creator include a person, an organization, or a Web service. The Dublin Core specification recommends that typically the name of a dc:creator should be used to indicate the entity.
WSMO Recommendation: In order to point unambiguously to a specific resource we recommend the use an instance of foaf:Agent as value type [Brickley & Miller, 2004].
Date
A date of an event in the life cycle of the element. Typically, dc:date will be associated with the creation or availability of the element.
WSMO Recommendation: We recommend using an encoding defined in the ISO Standard 8601:2000 [ISO8601, 2004] for date and time notation. A short introduction on the standard can be found here. This standard is also used by the XML Schema Definition (YYYY-MM-DD) [Biron & Malhotra, 2001] and thus one is automatically compliant with XML Schema, too.
Description
An account of the content of the element. Examples of dc:description include, but are not limited to: an abstract, table of contents, reference to a graphical representation of content or a free-text account of the content.
Format
A physical or digital manifestation of the element. Typically, dc:format may include the media-type or dimensions of the element. Format may be used to identify the software, hardware, or other equipment needed to display or operate the element. Examples of dimensions include size and duration.
WSMO Recommendation: We recommend using types defined in the list of Internet Media Types [IANA, 2002] by the IANA (Internet Assigned Numbers Authority)
Identifier
An unambiguous reference to the element within a given context. Recommended best practice is to identify the element by means of a string or number conforming to a formal identification system. In Dublin Core formal identification systems include but are not limited to the Uniform element Identifier (URI) (including the Uniform element Locator (URL)), the Digital Object Identifier (DOI) and the International Standard Book Number (ISBN).
WSMO Recommendation: We recommend using URIs as Identifier, depending on the particular syntax the identity information of an element might already be given, however, it might be repeated in dc:identifier in order to allow Dublin Core meta data aware applications the processing of that information.
Language
A language of the intellectual content of the element.
WSMO Recommendation: We recommend using the language tags defined in the ISO Standard 639 [ISO639, 1988], e.g. "en-GB". In addition, the logical language used to express the content should be mentioned, for example this can be OWL.
Owner
A person or organization to which a particular WSMO element belongs.
Publisher
An entity responsible for making the element available. Examples of dc:publisher include a person, an organization, or a Web service. The Dublin Core specification recommends that typically the name of a dc:publisher should be used to indicate the entity.
WSMO Recommendation: In order to point unambiguously to a specific resource we recommend the use an instance of foaf:Agent as value type [Brickley & Miller, 2004].
Relation
A reference to a related element. Recommended best practice is to identify the referenced element by means of a string or number conforming to a formal identification system.
WSMO Recommendation: We recommend using URIs as Identifier where possible. In particular, this property can be used to define namespaces that can be used in all child elements of the element to which this annotation is assigned.
Rights
Information about rights held in and over the element. Typically, dc:rights will contain a rights management statement for the element or reference a Web service providing such information. Rights information often encompasses Intellectual Property Rights (IPR), Copyright, and various Property Rights. If the Rights element is absent, no assumptions may be made about any rights held in or over the element.
Source
A reference to an element from which the present element is derived. The present element may be derived from the dc:source element in whole or in part. Recommended best practice is to identify the referenced element by means of a string or number conforming to a formal identification system.
WSMO Recommendation: We recommend using URIs as Identifier where possible.
Subject
A topic of the content of the element. Typically, dc:subject will be expressed as keywords, key phrases or classification codes that describe a topic of the element. Recommended best practice is to select a value from a controlled vocabulary or formal classification scheme.
Title
A name given to an element. Typically, dc:title will be a name by which the element is formally known.
Type
The nature or genre of the content of the element. The dc:type includes terms describing general categories, functions, genres, or aggregation levels for content.
WSMO Recommendation: We recommend using an URI encoding to point to the namespace or document describing the type, e.g. for a domain ontology expressed in WSMO, one would use: http://www.wsmo.org/2004/d2/#ontologies.
Version
As many properties of an element might change in time, an identifier of the element at a certain moment in time is needed.
WSMO Recommendation: If applicable we recommend using the revision numbers of a version control system. Such a system could be for example CVS (Concurrent Version System) that automatically keeps track of the different revisions of a document. An example CVS version Tag looks like this "$Revision: 1.127 $".

3. WSMO Top-Level Elements

WSMO refers to the concepts it defines as "elements". The listing below presents a general definition which applies to every element defined in WSMO. As we shall see in the subsequent sections, each element refines this definition.

Listing 2. WSMO Element Definition
Class wsmoElement
      hasAnnotation type annotation
Annotations
Each WSMO element has an attached set of annotations. Section 2.4 provides a description of annotations .

Following the main elements identified in the Web Service Modeling Framework, WSMO identifies four top level elements as the main concepts which have to be described in order to describe Semantic Web services.

Ontologies provide the terminology used by other WSMO elements to describe the relevant aspects of the domains of discourse; they are described in detail in Section 4.

Web services describes the computational entity providing access to services that provide some value in a domain. These descriptions comprise the capabilities, interfaces and internal working of the Web service (as further described in Section 5). All these aspects of a Web Service are described using the terminology defined by the ontologies.

Goals represent user desires, for which fulfillment could be sought by executing a Web service. Ontologies can be used for the domain terminology to describe the relevant aspects. Goals model the user view in the Web service usage process and are therefore a separate top-level entity in WSMO described in detail in Section 6.

Finally, Mediators describe elements that overcome interoperability problems between different WSMO elements. Mediators are the core concept to resolve incompatibilities on the data, process and protocol level, i.e. in order to resolve mismatches between different used terminologies (data level), in how to communicate between Web services (protocol level) and on the level of combining Web services (and goals) (process level). These are described in detail in Section 7.


4. Ontologies

In WSMO, Ontologies are the key to linking conceptual real-world semantics defined and agreed upon by communities of users. An ontology is a formal explicit specification of a shared conceptualization [Gruber, 1993]. From this rather conceptual definition we want to extract the essential components which define an ontology. Ontologies define an agreed common terminology by providing concepts, and relationships between the concepts. In order to capture semantic properties of relations and concepts, an ontology generally also provides a set of axioms, which are expressions in some logical language. The following listing consists of the definition of the WSMO ontology element and the following sub-sections describe in more detail the attributes of the WSMO ontology element.

Listing 3. Ontology Definition
Class ontology sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type ooMediator
      hasConcept type concept
      hasRelation type relation
      hasFunction type function
      hasInstance type instance
      hasRelationInstance type relationInstance
      hasAxiom type axiom

4.1 Importing Ontologies

Building an ontology for some particular problem domain can be a rather cumbersome and complex task. One standard way of dealing with the complexity is modularization. Importing Ontologies allows a modular approach for ontology design. Importing can be used as long as no conflicts need to be resolved, otherwise an ooMediator needs to be used.

4.2 Using Mediators

When importing ontologies, most likely some steps for aligning, merging, and transforming imported ontologies have to be performed. For this reason and in line with the basic design principles underlying the WSMF ontology mediators (ooMediator) are used when an alignment of the imported ontology is necessary. Mediators are described in in more detail in Section 7.

4.3 Concepts

Concepts constitute the basic elements of the agreed terminology for some problem domain. From a high-level perspective, a concept – described by a concept definition – provides attributes with names and types. Furthermore, a concept can be a subconcept of several (possibly none) direct superconcepts as specified by the isA-relation.

Listing 4. Concept Definition
Class concept sub-Class wsmoElement
      hasSuperConcept type concept
      hasAttribute type attribute
      hasDefinition type logicalExpression multiplicity = single-valued
Superconcept
There is a finite number of concepts that serve as a superconcept for some concept. Being a sub-concept of some other concept in particular means that a concept inherits the signature of this superconcept and the corresponding constraints. Furthermore, all instances of a concept are also instances of each of its superconcepts.
Attribute
Each concept provides a (possibly empty) set of attributes that represent named slots for data values for instances. They can be filled at the instance level. An attribute specifies a slot of a concept by fixing the name of the slot as well as a logical constraint on the possible values filling that slot. Hence, this logical expression can be interpreted as a typing constraint.
Listing 5. Attribute Definition
Class attribute sub-Class wsmoElement
   hasRange type concept  multiplicity = single-valued
Range
A concept that serves as an integrity constraint on the values of the attribute.
Definition
The definition is a logical expression (see Section 8) which can be used to define formally the semantics of the concept. More precisely, the logical expression defines (or restricts, respectively) the extension (i.e. the set of instances) of the concept. If C is the identifier denoting the concept then the logical expression takes one of the following forms

where l-expr(?x) is a logical expression with precisely one free variable ?x.

The first example expresses that there is a necessary condition for membership in the extension of the concept. The second example expresses that there is a sufficient condition, and the third case means that there is a sufficient and necessary condition for an object being an element of the extension of the concept.

4.4 Relations

Relations are used in order to model interdependencies between several concepts (respectively instances of these concepts).

Listing 6. Relation Definition
Class relation sub-Class wsmoElement
     hasSuperRelation type relation
     hasParameter type parameter
     hasDefinition type logicalExpression multiplicity = single-valued
Superrelation
A finite set of relations of which the defined relation is declared as being a subrelation. Being a subrelation of some other relation in particular means that the relation inherits the signature of this superrelation and the corresponding constraints. Furthermore, the set of tuples belonging to the relation (the extension of the relation, respectively) is a subset of each of the extensions of the superrelations.
Parameter
The list of parameters.
Listing 7. Parameter Definition
Class parameter sub-Class wsmoElement
     hasDomain type concept multiplicity = single-valued
Domain
A concept constraining the possible values that the parameter can take.
Definition
A logical expression (see Section 8) defining the set of instances (n-ary tuples, if n is the arity of the relation) of the relation. If the parameters are specified, the relation is represented by an n-ary predicate symbol with named arguments (see Section 8) (where n is the number of parameters of the relation) and the identifier of the relation is used as the name of the predicate symbol. If R is the identifier denoting the relation and the parameters are specified, then the logical expression takes one of the following forms:

If the parameters are not specified, then the relation is represented by a predicate symbol (see Section 8) where the identifier of the relation is used as the name of the predicate symbol. If R is the identifier denoting the relation and the parameters are not specified, then the logical expression takes one of the following forms:

l-expr(?v1,...,?vn) is a logical expression with precisely ?v1,...,?vn as its free variables and p1,...,pn are the names of the parameters of the relation.

Using implies means there is a necessary condition for instances ?v1,...,?vn to be related. Using impliedBy means that there is a sufficient condition and using equivalent means that there is a sufficient and necessary condition for instances ?v1,...,?vn being related.

4.5 Functions

A function is a special relation, with an unary range and a n-ary domain (parameters inherited from the relation), where the range value is functionally dependent on the domain values. In particular, the following constraint must hold (where F is the name of the function and != stands for inequality):

forAll ?x1,...,?xn,?r1,?r2 (false impliedBy F(?x1,...,?xn,?r1) and F(?x1,...,?xn,?r2) and ?r1 != ?r2)

In contrast to a function symbol, a function is not only a syntactical entity but has a defined semantics that allows to actually evaluate the function if concrete input values for the parameters are given. That means that we can actually substitute the (ground) function term in some expression by its concrete value. Functions for example can be used to represent and exploit built-in predicates of common datatypes. Their semantics can be captured externally by means of an oracle, or can be formalized by assigning a logical expression to the hasDefinition property inherited from relation.

Listing 8. Function Definition
Class function sub-Class relation
      hasRange type concept multiplicity = single-valued
Range
A concept constraining the possible return values of the function.

The logical representation of a function is almost the same as that of relations, whereby the result value of a function (respectively the value of a function term) has to be represented explicitly: the function is represented by a (n+1)-ary predicate symbol with named arguments (see Section 8) (where n is the number of arguments of the function) and the identifier of the function is used as the name of the predicate. In particular, the names of the parameters of the corresponding relation symbol are the names of the parameters of the function as well as one additional parameter range for denoting the value of the function term with the given parameter values. If the parameters are not specified, the function is represented by a predicate symbol with ordered arguments, and by convention the first argument specifies the value of the function term with given argument values.

If F is the identifier denoting the function and p1,..., pn is the set of parameters of the function then the logical expression for defining the semantics of the function (inherited from relation) can for example take the form

forAll ?v1,...,?vn,?range ( F[p1 hasValue ?v1,...,pn hasValue ?vn, range hasValue ?range]
                            equivalent l-expr(?v1,...,?vn,?range) )

where l-expr(?v1,...,?vn,?range) is a logical expression with precisely ?v1,...,?vn,?range as its free variables and p1,...,pn are the names of the parameters of the function. Clearly, in order to prevent ambiguities, range may not be used as the name for a parameter of a function in order to prevent ambiguities.

4.6 Instances

Instances are either defined explicitly or by a link to an instance store, i.e., an external storage of instances and their values.

An explicit definition of instances of concepts is as follows:

Listing 9. Instance Definition
Class instance sub-Class wsmoElement
      hasType type concept
      hasAttributeValues type attributeValue  
Type
The concept of which this instance is an instance.
Attribute Value
The attribute values for the single attributes defined in the concept. For each attribute of the concept, this instance is assigned so there can be one or more corresponding attribute values. These values have to be compatible with the corresponding type declaration in the concept definition.
Listing 10. Attribute Value Definition
Class attributeValue sub-Class wsmoElement
      hasAttribute type attribute multiplicity = single-valued
      hasValue type {instance, literal, anonymousId} 
Attribute
The attribute this value refers to.
Value
An instance, literal or anonymous ID representing the actual value of an instance for a specific attribute.

Instances of relations (with arity n) can be seen as n-tuples of instances of the concepts which are specified as the parameters of the relation. Thus we use the following definition for instances of relations:

Listing 11. Relation Instance Definition
Class relationInstance sub-Class wsmoElement
      hasType type relation
      hasParameterValue type parameterValue
Type
The relation this instance belongs to.
Parameter Value
A set of parameter values specifying the single instances that are related according to this relation instance. The list of parameter values of the instance has to be compatible wrt. names and range constraints that are specified in the corresponding relation.
Listing 12. Parameter Value Definition
Class parameterValue sub-Class wsmoElement
      hasParameter type parameter multiplicity = single-valued
      hasValue type {instance, literal, anonymousId} multiplicity = single-valued
Parameter
The parameter this value refers to.
Value
An instance, literal or anonymous ID representing the actual value of an instance for a specific parameter.

A detailed discussion and a concrete proposal for how to integrate large sets of instance data in an ontology model can be found in the DIP Deliverable D2.2 [Kiryakov et. al., 2004]. Basically, the approach taken there is to integrate large sets of instances which already exist on some storage device by means of sending queries to external storage devices or oracles.

4.7 Axioms

An axiom is a logical expression together with its annotations.

Listing 13. Axiom Definition
Class axiom sub-Class wsmoElement
      hasDefinition type logicalExpression
Definition
The actual statement captured by the axiom is defined by a formula in a logical language as described in Section 8.

5. Web Service Descriptions

WSMO Web service descriptions consist of non-functional, functional, and the behavioral aspects of a Web service. A Web service is a computational entity which is able (by invocation) to achieve a users goal. A service in contrast is the actual value provided by this invocation. Thereby a Web service might provide different services, such as for example Amazon can be used for acquiring books as well as to find out an ISBN number of a book.

In the following, we describe the elements of a Web service description:

Listing 14. Web Service Description Definition
Class webService sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type {ooMediator, wwMediator}
      hasNonFunctionalProperties type nonFunctionalProperty
      hasCapability type capability multiplicity = single-valued
      hasInterface type interface
Importing Ontology
Used to import ontologies as long as no conflicts need to be resolved.
Using Mediator
A Web service can import ontologies using ontology mediators (ooMediator) when steps for aligning, merging, and transforming imported ontologies are needed. A Web service can use wwMediators to deal with process and protocol mediation.

5.1 Non-Functional Properties

The listing below defines a class for describing Web service specific non-functional properties. Examples of such non-functional properties include cost-related and charging-related properties of a Web service [O`Sullivan et al., 2002], or properties like like accuracy, network-related QoS, performance, reliability, robustness, scalability, security, etc [Rajesh & Arulazi, 2003]. In opposite to simple annotation non-functional properties are not only represented by simple key value pairs, but can be expressed using a logical expression. WSMO does not pose any restrictions on the set of valid formulae that can be used.

Listing 15. Non-Functional Properties Definition
Class nonFunctionalProperty sub-Class wsmoElement
      hasDefinition type logicalExpression
Definition
A logical expression that specifies non-functional information. WSMO does not pose restrictions on how non-functional information is encoded into logical expressions. All expressions that are valid according to the specification in Section 8 can be used. To ease interoperability it is recommended to use standard terms where appropriate, WSMO recommends: accuracy, financial, networkRelatedQoS, performance, reliability, robustness, scalability, security, transactional, trust. In the following we give intuitive definitions of the meaning of the recommended terms:
Accuracy
It represents the error rate generated by the service. It can be measured by the numbers of errors generated in a certain time interval.
Financial
It represents the cost-related and charging-related properties of a service [O`Sullivan et al., 2002]. This property is a complex property, which includes charging styles (e.g. per request or delivery, per unit of measure or granularity etc.), aspects of settlement like the settlement model (transactional vs. rental) and a settlement contract, payment obligations and payment instruments.
Network-related QoS
They represent the QoS mechanisms operating in the transport network which are independent of the service. They can be measured by network delay, delay variation and/or message loss.
Performance
It represents how fast a service request can be completed. According to [Rajesh & Arulazi, 2003] performance can be measured in terms of throughput, latency, execution time, and transaction time. The response time of a service can also be a measure of the performance. High quality services should provide higher throughput, lower latency, lower execution time, faster transaction time and faster response time.
Reliability
It represents the ability of a service to perform its functions (to maintain its service quality). It can be measured by the number of failures of the service in a certain time internal.
Robustness
It represents the ability of the service to function correctly in the presence of incomplete or invalid inputs. It can be measured by the number of incomplete or invalid inputs for which the service still function correctly.
Scalability
It represents the ability of the service to process more requests in a certain time interval. It can be measured by the number of solved requests in a certain time interval.
Security
It represents the ability of a service to provide authentication (entities - users or other services - who can access service and data should be authenticated), authorization (entities should be authorized so that they only can access the protected services), confidentiality (data should be treated properly so that only authorized entities can access or modify the data), traceability/auditability (it should be possible to trace the history of a service when a request was serviced), data encryption (data should be encrypted), and non-repudiation (an entity cannot deny requesting a service or data after the fact).
Transactional
It represents the transactional properties of the service.
Trust
It represents the trust worthiness of the service.

5.2 Capability

A capability defines the Web service by means of its functionality.

Listing 16. Capability Definition
Class capability sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type {ooMediator, wgMediator}
      hasNonFunctionalProperties type nonFunctionalProperty
      hasSharedVariables type sharedVariables
      hasPrecondition type axiom
      hasAssumption type axiom
      hasPostcondition type axiom
      hasEffect type axiom
Importing Ontology
Used to import ontologies as long as no conflicts need to be resolved.
Using Mediator
A capability can import ontologies using ontology mediators (ooMediator) when steps for aligning, merging, and transforming imported ontologies are needed. It can be linked to a goal using a wgMediator.
Non-functional Properties
The set of properties strictly belonging to a capability, defined according to Section 5.1.
Shared Variables
Shared Variables represent the variables that are shared between preconditions, postconditons, assumptions and effects. They are all quantified variables in the formula that concatenates assumptions, preconditions, postconditions, and effects.

If ?v1,...,?vn are the shared variables defined in a capability, and pre(?v1,...,?vn), ass(?v1,...,?vn), post(?v1,...,?vn) and eff(?v1,...,?vn), are used to denote the formulae defined by the preconditions, assumptions, postconditions, and effects respectively, then the following holds:

forAll ?v1,...,?vn ( pre(?v1,...,?vn) and ass(?v1,...,?vn)
     implies post(?v1,...,?vn) and eff(?v1,...,?vn)).

Precondition
Preconditions specify the information space of the Web service before its execution.
Assumption
Assumptions describe the state of the world before the execution of the Web service.
Postcondition
Postconditions describe the information space of the Web service after the execution of the Web service.
Effect
Effects describe the state of the world after the execution of the Web service.

5.3 Interface

An interface describes how the functionality of the Web service can be achieved (i.e. how the capability of a Web service can be fulfilled) by providing a twofold view on the operational competence of the Web service:

This distinction reflects the difference between communication and cooperation. The choreography defines how to communicate with the Web service in order to consume its functionality. The orchestration defines how the overall functionality is achieved by the cooperation of more elementary Web service providers [1].

An interface is defined by the following properties:

Listing 17. Interface Definition
Class interface sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type ooMediator
      hasNonFunctionalProperties type nonFunctionalProperty
      hasChoreography type choreography
      hasOrchestration type orchestration
Importing Ontology
Used to import ontologies as long as no conflicts need to be resolved.
Using Mediator
An interface can import ontologies using ontology mediators (ooMediator) when steps for aligning, merging, and transforming imported ontologies are needed.
Non-functional Properties
The set of properties strictly belonging to an interface, defined according to Section 5.1.
Choreography
Choreography provides the necessary information to enable communication with the service from the client point of view [Roman et al., 2005].
Orchestration
Orchestration describes how the service makes use of other services in order to achieve its capability [Roman et al., 2005] .

6. Goals

Goals are representations of an objective for which fulfillment is sought through the execution of a Web service. Goals can be descriptions of Web services that would potentially satisfy the user desires. The following listing presents the goal definition:

Listing 18. Goal Definition
Class goal sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type {ooMediator, ggMediator}
      hasNonFunctionalProperties type nonFunctionalProperty
      requestsCapability type capability multiplicity = single-valued
      requestsInterface type interface
Importing Ontology
Used to import ontologies as long as no conflicts need to be resolved.
Using Mediator
A goal can import ontologies by using ontology mediators (ooMediator) in case assistance for aligning, merging, and transforming imported ontologies are needed. A goal may be defined by reusing one or several already-existing goals. This is achieved by using goal mediators (ggMediator). For a detailed account on mediators we refer to Section 7.
Non-functional Properties
The set of properties strictly belonging to a goal, defined according to Section 5.1.
Capability
The capability of the Web services the user would like to have.
Interface
The interface of the Web service the user would like to have and interact with.

7. Mediators

In this section, we introduce the notion of mediators and define the elements that are used in the description of a mediator.

We distinguish four different types of mediators:

The mediators are defined as follows:

Listing 19. Mediators Definition
Class mediator sub-Class wsmoElement
      importsOntology type ontology
      hasNonFunctionalProperties type nonFunctionalProperty
      hasSource type {ontology, goal, webService, mediator}
      hasTarget type {ontology, goal, webService, mediator}
      hasMediationService type {goal, webService, wwMediator}

Class ooMediator sub-Class mediator
      hasSource type {ontology, ooMediator}

Class ggMediator sub-Class mediator
      usesMediator type ooMediator
      hasSource type {goal, ggMediator}
      hasTarget type {goal, ggMediator}

Class wgMediator sub-Class mediator
      usesMediator type ooMediator
      hasSource type {webService, goal, wgMediator, ggMediator}
      hasTarget type {webService, goal, ggMediator, wgMediator}

Class wwMediator sub-Class mediator
      usesMediator type ooMediator
      hasSource type {webService, wwMediator}
      hasTarget type {webService, wwMediator}
Importing Ontology
Used to import ontologies as long as no conflicts need to be resolved.
Non-functional Properties
The set of properties strictly belonging to a mediator, defined according to Section 5.1.
Source
The source components define entities that are the sources of the mediator.
Target
The target component defines the entity that is the target of the mediator.
Mediation Service
The mediation service points to a goal that declarative describes the mapping or to a Web service that actually implements the mapping or to a wwMediator that links to a Web service that actually implements the mapping.
Using Mediator
Some specific types of mediators, i.e. ggMediator, wgMediator, and wwMediator, use a set of ooMediators in order to map between different vocabularies used in the description of goals and Web service capabilities and align different heterogeneous ontologies.

A further remark on wgMediators is in place: A wgMediator can - depending on which is the source and target of the mediator - have two different functions: (1) A goal is linked to a Web service via its choreography interface meaning that the Web service (totally or partially) fulfills the goal to which it is linked (2) A Web service links to a goal via its orchestration interface meaning that the Web service needs this goal to be resolved in order to fulfill the functionality described in its capability.

Notice that there are two principal ways of relating mediators with other entities in the WSMO model: (1) an entity can specify a relation with a mediator through the has usesMediator attribute and (2) entities can be related with mediators through the source and target attributes of the mediator. We expect cases in which a mediator needs to be referenced directly from an entity, for example for importing a particular ontology necessary for the descriptions in the entity. We also expect cases in which not the definition of the entity itself, but rather the use of entities in a particular scenario (e.g. Web service invocation) requires the use of mediators. In such a case, a mediator needs to be selected, which provides mediation Web services between these particular entities. WSMO does not prescribe the type of use of mediators and therefore provides maximum flexibility in the use of mediators and thus allows for loose coupling between Web services, goals, and ontologies.

8. Logical Language for Defining Formal Statements in WSMO

As the major component of axiom logical expressions are used almost everywhere in the WSMO model to capture specific nuances of meaning of modeling elements or their constituent parts in a formal and unambiguous way. In the following, we give a definition of the syntax of the formal language that is used for specifying logical expressions. The semantics of this language is defined formally by the WSML working group in a separate document.

Section 8.1 introduces the identifiers recommended for variables in WSMO. Section 8.2 gives the definition of the basic vocabulary and the set of terms for building logical expression. Section 8.3 defines the most basic formulas (i.e. atomic formulae) which allows us to eventually define the set of logical expressions.

8.1 Variable Identifiers

Apart from the identifiers (URIs and anonymous) defined in Section 2.1 and values defined in Section 2.2, logical expressions in WSMO can also identify variables. Variable names are strings that start with a question mark '?', followed by any positive number of symbols in {a-z, A-Z, 0-9, _, -}, for example ?var or ?lastValue_Of.

8.2 Basic Vocabulary and Terms

Let URI be the set of all valid uniform resource identifiers. This set will be used for the naming (or identifying, respectively) various entities in a WSMO description.

Definition 1. The vocabulary V of our language L(V) consists of the following symbols:

As usual, 0-ary function symbols are called constants. 0-ary predicate symbols correspond to propositional variables in classical propositional logic.

Definition 2. Given a vocabulary V, we can define the set of terms Term(V) (over vocabulary V) as follows:

As usual, the set of ground terms GroundTerm(V) is the subset of terms in Term(V) which do not contain any variables.

Terms can be used in general to describe computations (in some domain). One important additional interpretation of terms is that they denote objects in some universe and thus provide names for entities in some domain of discourse.

8.3 Logical Expressions

We extend the previous definition (Definition 2) to the set of (complex) logical expressions (or formulae, respectively) L(V) (over vocabulary V) as follows:

Definition 3. A simple logical expression in L(V) (or atomic formula) is inductively defined as follows:

The intuitive semantics for simple logical expressions (wrt. an interpretation) is as follows:

Definition 4. Definition 3 is extended to complex logical expressions in L(V) as follows

The intuitive semantics for complex logical expressions (wrt. to in interpretation) is as follows:

Notational conventions:

There is a precedence order defined for the logical connectives as follows, where op1 < op2 means that op2 binds stronger than op1: implies, equivalent,impliedBy< or, and < not.

The precedence order can be exploited when writing logical expressions in order to avoid extensive use of parenthesis. If there are ambiguities in evaluating an expression, parenthesis must be used to resolve the ambiguities.

The terms O[ATT ofTypeSet (T)] and O[ATT hasValues {V}] (that means for the case n = 1 in the respective clauses above) can be written more simple by omitting the parenthesis.

A logical expression of the form false impliedBy L (commonly used in Logic Programming systems for defining integrity constraints) can be written using the following syntactical shortcut: constraint L.

We allow the following syntactic composition of atomic formulas as a syntactic abbreviation for two separate atomic formulas: C1 subConceptOf C2 and C1[ATT op V] can be syntactically combined to C1[ATT op V] subConceptOf C2. Additionally, for the sake of backwards compatibility with F-Logic, we also allow the following notation for the combination of the two atomic formulae: C1 subConceptOf C2 [ATT op V]. Both abbreviations stand for the set of the two single atomic formulae. The first abbreviation is considered to be the standard abbreviation for combining these two kinds of atomics formulae.

Furthermore, we allow path expressions as a syntactical shortcut for navigation related expressions: p.q stands for the element which can be reached by navigating from p via property q. The property q has to be a non-set-valued property (hasValue). For navigation over set-valued properties (hasValues), we use a different expression p..q. Such path expressions can be used like a term wherever a term is expected in a logical expression.

Note: Note that this definition for our language L(V) is extensible by extending the basic vocabulary V. In this way, the language for expressing logical expressions can be customized to the needs of some application domain.

Semantically, the various modeling elements of ontologies can be represented as follows: concepts can be represented as terms, relations as predicates with named arguments, functions as predicates with named arguments, instances as terms and axioms as logical expressions.

9. Conclusions and Further Directions

This document presented the Web Service Modeling Ontology (WSMO) for describing several aspects related to services on the Web, by refining the Web Service Modeling Framework (WSMF). The definition of the missing elements (choreography and orchestration) will be provided in separate deliverables of the WSMO working group, and future versions of this document will contain refinements of the mediators.

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Acknowledgments

This work is funded by the European Commission under the projects DIP, Knowledge Web, InfraWebs, SEKT, SWWS, ASG and Esperonto; by Science Foundation Ireland under the DERI-Lion project; by the Vienna city government under the CoOperate programme and by the FIT-IT (Forschung, Innovation, Technologie - Informationstechnologie) under the projects RW² and TSC.

The editors would like to thank to all the members of the WSMO, WSML, and WSMX working groups for their advice and input into this document.

Appendix A. Conceptual Elements of WSMO

Class annotation 
      hasContributor type dc:contributor 
      hasCoverage type dc:coverage
      hasCreator type dc:creator 
      hasDate type dc:date
      hasDescription type dc:description 
      hasFormat type dc:format 
      hasIdentifier type dc:identifier 
      hasLanguage type dc:language 
      hasOwner type owner 
      hasPublisher type dc:publisher
      hasRelation type dc:relation 
      hasRights type dc:rights 
      hasSource type dc:source 
      hasSubject type dc:subject 
      hasTitle type dc:title
      hasType type dc:type 
      hasVersion type version 
      
Class wsmoElement
      hasAnnotation type annotation

Class ontology sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type ooMediator
      hasConcept type concept
      hasRelation type relation
      hasFunction type function
      hasInstance type instance
      hasRelationInstance type relationInstance
      hasAxiom type axiom

Class concept sub-Class wsmoElement
      hasSuperConcept type concept
      hasAttribute type attribute
      hasDefinition type logicalExpression multiplicity = single-valued

Class attribute sub-Class wsmoElement
      hasRange type concept  multiplicity = single-valued

Class relation sub-Class wsmoElement
      hasSuperRelation type relation
      hasParameter type parameter
      hasDefinition type logicalExpression multiplicity = single-valued

Class parameter sub-Class wsmoElement
      hasDomain type concept multiplicity = single-valued

Class function sub-Class relation
      hasRange type concept multiplicity = single-valued

Class instance sub-Class wsmoElement
      hasType type concept
      hasAttributeValues type attributeValue

Class attributeValue sub-Class wsmoElement
      hasAttribute type attribute multiplicity = single-valued
      hasValue type {instance, literal, anonymousId}

Class relationInstance sub-Class wsmoElement
      hasType type relation
      hasParameterValues type parameterValue

Class parameterValue sub-Class wsmoElement
      hasParameter type parameter multiplicity = single-valued
      hasValue type {instance, literal, anonymousId} multiplicity = single-valued

Class axiom sub-Class wsmoElement
      hasDefinition type logicalExpression

Class webService sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type {ooMediator, wwMediator}
      hasNonFunctionalProperties type nonFunctionalProperty
      hasCapability type capability multiplicity = single-valued
      hasInterface type interface

Class capability sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type {ooMediator, wgMediator}
      hasNonFunctionalProperties type nonFunctionalProperty
      hasSharedVariables type sharedVariables
      hasPrecondition type axiom
      hasAssumption type axiom
      hasPostcondition type axiom
      hasEffect type axiom

Class interface sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type ooMediator
      hasNonFunctionalProperties type nonFunctionalProperty
      hasChoreography type choreography
      hasOrchestration type orchestration

Class goal sub-Class wsmoElement
      importsOntology type ontology
      usesMediator type {ooMediator, ggMediator}
      hasNonFunctionalProperties type nonFunctionalProperty
      requestsCapability type capability multiplicity = single-valued
      requestsInterface type interface

Class mediator sub-Class wsmoElement
      importsOntology type ontology
      hasNonFunctionalProperties type nonFunctionalProperty
      hasSource type {ontology, goal, webService, mediator}
      hasTarget type {ontology, goal, webService, mediator}
      hasMediationService type {goal, webService, wwMediator}

Class ooMediator sub-Class mediator
      hasSource type {ontology, ooMediator}

Class ggMediator sub-Class mediator
      usesMediator type ooMediator
      hasSource type {goal, ggMediator}
      hasTarget type {goal, ggMediator}

Class wgMediator sub-Class mediator
      usesMediator type ooMediator
      hasSource type {webService, goal, wgMediator, ggMediator}
      hasTarget type {webService, goal, ggMediator, wgMediator}

Class wwMediator sub-Class mediator
      usesMediator type ooMediator
      hasSource type {webService, wwMediator}
      hasTarget type {webService, wwMediator}

Class nonFunctionalProperty sub-Class wsmoElement
      hasDefinition type logicalExpression

Appendix B. UML Class Diagrams for WSMO Elements

Upper WSMO Elements

WSMO Elements

Ontology Related Classes

Ontology and Related classes

Goal and Web Service Classes

Goal and Service Ontolgies

Mediator Classes

Mediator Classes


Footnotes:

[1] One could argue that orchestration should not be part of a public interface because it refers to how a Web service is implemented. However, this is a short-sighted view that does not reflect the nature of fully open and flexible eCommerce. Here companies shrink to their core processes which they are really profitable in. All other processes are outsourced and consumed as eServices. eCommerce companies advertise their services in their capability and choreography description and they advertise their needs in the orchestration interfaces. This enables on-the-fly creation of virtual enterprises in reaction to demands from the market place. Even in this dinosaurian phase of eCommerce where large companies still exist, orchestration may be an important aspect. The orchestration of a Web service may not be made public but may be visible to the different departments of a large organization that compete for delivering parts of the overall service. Notice that the actual business intelligence of a service provider is still hidden. It is their capability to provide a certain functionality with a chorography that is very different from the sub services and their orchestration. The ability for a certain type of process management (the overall functionality is decomposed differently in the choreography and the orchestration) is where it comes in as a silver bullet in the process. How they manage the difference between the process decomposition at the choreography and the orchestration level is the business intelligence of the service provider.

[2] Note that [Preist, 2004] also distinguishes between a computational entity in general and Web service, where the former does not necessarily have a Web accessible interface. WSMO does not make this distinction.


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