Review of Semantic Web Service Composition Based on OWL-S

MA Xiuli,DING Yingying,WANG Hongxia

(Shenyang Ligong University,Shenyang 110159,China)

With the Web service related standards continuously improves and perfect,more and more enterprises will be their business functions and processes encapsulated into a standard Web service release out and provide application development based on Internet with abundant resources.

In applications,corporate IT systems to be able to cope with the complicated application environment,most of the system is built on a Service-Oriented Architecture(SOA) foundation,and then according to changes in the environment quickly adjust applications to adapt to business needs,enterprise application system development can also be increasingly sophisticated technology to build Web services[1].But traditional Web services protocol stack is not for service discovery,composition and execution provide a generic mechanism,and therefore cannot support the dynamic interaction and combinations among Web services.The Semantic Web and Web services combine to make it possible to solve this problem.After the Semantic Web service appears,academia and industry on how to make use of Semantic Web technology,through the services combinations reuse of existing and rich Web service resources,and quickly to generate a new service resources of meeting the complex application need,carried on the thorough study,then proposed a series of semantic Web service composition method.

Semantic Web Services is the Semantic Web markup language used to describe the semantics of the services,so that a computer can understand Web services entities to support automatic service discovery,automatic invocation and automatic composition[2].Semantic Web services is the semantic extension of Web services,so that the computer can understand and automatic deal with Web service.Semantic Web Services is to show the Semantic information expression of Web Services by using definite and clear the concept of ontology in the semantic Web to resolve the problem of ambiguity and heterogeneity among Web services.The advantage of Semantic Web Services can improve the efficiency of service discovery,and service discovery or information integrated automation can be possible.In the numerous studies field of Semantic Web Services,the service description language and registration publishing framework are the core of the study and the basic problem of Semantic Web Services.Current Semantic Web Services using semantic models are generally the ontology[3].Ontology provides the machine-readable semantic model for application,then through these semantic information describing Web services to achieve automatic service discovery,invocation and composition.

Semantic Web Services are a key step in the Web service semantic description.OWL-S is the latest ontology language most widely used in describing Web services.Using OWL-S describes the Web service as a variety of semantic Web services technology research and development work on the basis that it enables service providers and service requester can share common semantics in order to achieve interoperability between machines for automated processing of information to provide a basis so choose,match with some intelligence,initially reflect the needs of the service requested,the realization of semantic retrieval practical significance.

Web service composition is the ability to provide value-added services through the combination of basic Web services.Service composition is not only an important way of service reuse,but also is a new way of building complex application and system software based on Web.On the one hand,the Web service standard constant maturity and the constant improvement of infrastructure promote the development of application development based on Web services.On the other hand,Web services are developed by different service providers,therefore has the heterogeneity in the development of language,running platform,this makes the traditional service matching based on grammar hard to meet the application needs of the complicated network environment.In this environment,how to make use of semantic Web technology to describe Web services,and to realize intelligent and automation of semantic Web service composition,become a research focus in SOA.

1 ONTOLOGY AND RELATED CONCEPTS

1.1 Ontology Overview

In recent years,the concept of ontology is more and more used in the computer engineering field of knowledge,so far widely accepted definition of ontology is proposed in 1994 by Gruber:Ontology is a shared conceptual model of the field of formal and explicit type specification.Intuitively speaking,the ontology is an entity that is the analysis and modeling result using the method of ontology for a field.It abstractly describes a field of the real world as a set of concept(such as entities,attributes,processes,etc.) and the relationship between,so as to construct the ontology of the field and make the information processing of this field more convenient[4].

Ontology concept includes four main aspects:①conceptualization:the objective world phenomena abstract model;②explicit:concepts and the links between them are precisely defined;③formal:precise mathematical description;④share:the knowledge reflected by ontology is recognized by the common.

1.2 Ontology elements

Ontology can be divided into four types:Domain Ontology,Application Ontology,General Ontology and Presentational Ontology.

Perez who organized the ontology with taxonomy summed up the 5 basic modeling elements of the ontology containing:Classes,Relations,Functions,Axioms and Instances[5].

1) Classes:Also knew concepts.Semantically,it represents a collection of objects,which are defined generally with frame structure,including the name of the concept,the set of relationship between the concept and the other,and a natural language description of the concept.

2) Relations:The basic relationship of the ontology mainly includes four kinds:part-of,instance-of,is-a and attribute-of.part-of expressing the relationship of the part and the whole among the concepts;instance-of expressing the relationship of instances among the concepts;is-a expressing the relationship of inheritance among the concepts;attribute-of expressing the relationship of property among the concepts.

3) Functions:a special relationship.

4) Axioms:represents the true assertions,such as the concept of B belongs to the scope of the concept of A.

5) Instances:represents the object.

In the actual process of building the ontology,not necessarily strictly in accordance with the above five elements to build ontology,while the relationship between the concepts is also not limited to the above listed four basic relationships,and can be defined according to the specific circumstances of the field to meet the needs of the application.

2 OWL LANGUAGE AND ITS APPLICATION

2.1 The OWL ontology language

Ontology is the foundation of service discovery based on semantic.Representation and reasoning of ontology has an important impact on the services discovery based on semantic.OWL (Web Ontology Language,referred to as OWL) is a Web oriented ontology description language which is proposed by W3C on the basis of the DAML,is the most commonly used ontology description language at present.

OWL has three seeds Language:OWL Lite,OWL DL and OWL Full,expression ability in ascending order.OWL Lite is used to provide for the service requester which just need a classification hierarchy and simple attributes constraints,including a narrow cardinality constraints mechanism to express binary relations;OWL DL defines a class separation,that is,a class cannot be used as individual and nature,nature can not be used as classes and individuals;OWL Full is no such restrictions,the semantic description of a more liberal and flexible.OWL-S version clearly stipulates that OWL-S ontology[6]is described by using OWL DL.

2.2 The OWL language instance

OWL ontology language semantics has rich and its application is wide.Today in e-commerce field of ontology purchasing books field as an example,the paper analyzes the application of the OWL language.

The purchasing books field contains books,payment,various data changes and logistics,and several other categories.Each class will contain many sub-classes,and among the classes have above the four relations,here summarized as affiliation,parallel relationship,mutually exclusive relationships and instances relationship.Such as the book-class includes the type of books and books information.There is parallel relationship between the type of books and books information.There are affiliation relationship between books and the type of books as well as books and books information.Then for example,the data structure belonging to computer science,among them is subordinate relationship;in computer science,including operating systems,databases,data structures,computer composition principle,etc.,which are parallel relationship,but they are not identical to each other,they are also different from the same philosophy,geography,art,etc.,so they are mutually exclusive;and version of a data structure with Yan Weimin authors compiled and written by other authors,they belong to the specific instance of the data structure,namely instance relationship.Comprehensive above analysis,the ontology of purchasing books is concluded.figure 1 shows the purchasing books domain ontology structure diagrams.

Figure1Purchasingbooksdomainontologystructurediagrams

3 OWL-S DESCRIPTION LANGUAGE AND ITS APPLICATION

3.1 OWL-S Services Description Language

OWL can be used to explicitly represent the meaning of terms in vocabularies and the relationships between terms,compared to XML and RDF/RDFS,it provides more means to express the meaning of these terms,so in the representation of machine understandable Web information resources,it is more powerful.With OWL as OWL Web ontology language standard recommended by the W3C,DAML-S also evolved into the corresponding OWL-S[7].OWL-S is based on OWL ontology language.It has a well-defined semantics,according to the object and the complex relationships between them to define the Web service ontology,and can contain XML data type information.Research on semantic Web services is currently mainly around the OWL-S,mainly used in digital libraries,enterprise integration,knowledge search fields[8].

OWL-S provides a markup language.Thus the attributes and capabilities of Web services can be precisely described in machine-readable forms.Based on the definition of OWL-S,a Web service is described by three properties:Service Profiles,Service Model and Service Grounding.

1) Service Profile describes what the service does,that is,service query agent used to judge whether services could satisfy the requirement of a set of attributes information,such as Service functions,Service Input,Output,Precondition,Effect,etc.,used for automatic discovery of services.

2) Service Model describes how to work,that is,service process model (using the services involved in the control flow and data flow).A service is regarded as a process.Defines the Service Model a subclass of OWL-S,the Process Model.Process of ontology is divided into three kinds,respectively is:Atomic Process,Simple Process and Composite Process.Process of ontology through Input,Output,Precondition and Effect to describe the Process.Cooperate with Service Grounding be used for the automatic combination and execution services.

3) Service Grounding describes how to access a service,and to link the process model,the WSDL description of the communication protocol and message description[9].Figure2 shows OWL-S service description ontology.

Figure 2 OWL-S service description ontology

3.2 OWL-S Description Example

Using ontology modeling tool Protégé3.2 and OWL-S Editor plug-in,through the domain ontology and OWL-S,this section achieves an service of e-commerce purchase books.

For example:someone wants to buy a book about data structure on the Internet,he wanted to pay by alipay,if not enough money,with e-bank,if the e-bank is not enough,with the bank card payment.In the last section,we have described the purchase books domain ontology,the next step for the service description.

First analyzing the atomic processes of existence in the cases,including:select the required books,payment services including alipay payment,e-bank payment,credit card payment,deposit card payment and other atomic process,finally completing the purchase books and ending the service.There is a choice in the process of payment.Consumers will accord to pay the balance sufficient to choose the means of payment.Here to select the required books process as an example,the Input is data structure,the Output is price,the Precondition is effective for the book title,the Effect is book information.OWL-S description of choosing books is shown in Figure 3.

Figure 3 Book selection OWL-S description

In describing the atomic processes,service is based on the previously ontology.This is one of the advantages of OWL-S description language.In the atomic process inputting,pointing button will appear the input block diagram,in the parameterType line of the bottom left of input block diagram there is a yellow button (figure the ring black),which is the link of service and ontology,pressing the button will pop up the diagram of the right side.Figure 4 shows the input description ontology,ring black place is OWL-S description language and the ontology connection.

Figure 4 The input description ontology

4 BASED ON SITUATION CALCULUS AUTOMATIC SERVICE COMPOSITION

Previously described the OWL-S can be described services can also be combined services,but not for automatic service composition,this chapter gives a method of using Situation Calculus for automatic service composition based on OWL-S model.

Situation Calculus is a kind of formalized planning method.It is a kind of first-order logical language,mainly used to represent the dynamic changes in the world,is a kind of about the state,action and the action result of formal predicate calculus.In the situation calculus,all the change process in the world are the results of the action.We put knowledge express as formula about the status and action,and then use the deduction system of first-order predicate calculus to solve this problem.

In the situation calculus,the state of the world can be expressed as some functions and relationships(called predicates stream,fluent) values associated with a particular states.FlowF(x,s) to statesas its last parameter.Course of action from the initial state,it is a constant,denoted by theS0.Function do(a,s) put the status and actions map to a new state,which means a subsequent state of a performing actions in the states.

In the situation calculus,atomic processes are mapped into actiona(y).The input parameters of atomic processes are mapped into the parameteryaction.For example,atomic process BuyBook is mapped into action Buybook (bookName).Logical connectives (priority from high to low):(negative),∧(conjunctive),→(implication);The necessary premise of atomic processes are formalized as formula as follows[10]in the situation calculus:

Poss(a,s)?πi

(1)

Poss (a,s) is a stream,which means that the action of a in the state ofsis likely to be executed.πiis a stream associated with the state ofs,expresses a necessary premise whether to be tenable.For example:

Poss (Buybook(bookname),s)→

Valid (bookname,s)

Vaild (boodname,s) is a stream,said that bookname is effective in the state ofs.

In many conditions,its general form is:

Poss(a,s)→π1∧π2∧…∧πn

(2)

Before performing a service,the execution agent must know each input parameter values,therefore has the following formula:

Poss(a,s)→kref(φ1,s)∧…∧kref(φn,s)

(3)

φ1represents an input parameter,in the flowkref(φ1,s) the valueφ1is known under thes.Based on the above two equations,we can obtain the following action precondition axiom:

Poss(a,s)→π1∧π2∧…∧πn∧…∧kref(φ1,s)∧…∧kref(φn,s)

(4)

A example is:

Poss(Buybook(bookname),s)→

Vaild(bookbame,s)∧kref(bookname,s)

In the situation calculus,the effect of atomic process and output parameters are form into positive and negative effects of the axiom as follows:

(5)

(6)

In the situation calculus,the service is considered to be a state and the change of the state.Through the mapping between the situation calculus and OWL-S items,the paper realizes automatic service composition.The corresponding mapping is presented in Table 1.

Table 1 Situation Calculus and OWL-S Model Mapping

Suppose,choice books is an atomic process,when the state of the input bookname is effective,the next state will be executed.The OWL-S text is shown as follows:

<--Only when bookname valid is true to perform-->

A plurality of atomic processes are combined to educe the purchase books service combination as shown in figure 5.

圖5 Purchase books service combination

5 CONCLUSION

Research on the issue of Web service automatic composition using situation calculus theory has become a hot discussion in the field of international and domestic Web service at present.This paper discusses a service composition technology based on OWL-S platform.In the purchase book as an example,using OWL to describe the ontology needed by services and using OWL-S to describe the service,Although OWL-S can also describe service composition,but it isn′t automatic service composition.State calculus has good ability to describe dynamic state transition.Based on the situation calculus intelligent bottom,can use the semantic aspect of OWL-S platform to solve many problems of traditional service registration system.This framework relates only to the service combination,service discovery and invocation as an important part of the system will be the focus of future work.

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