EVALUATING WEB SERVICE
COMPOSITION METHODS WITH THE
HELP OF A BUSINESS APPLICATION
R. JAYA PRAKASH
Department of Computer Science, Pondicherry University, Puducherry, India email@example.com
R. VIMAL RAJA
Lecturer, Department of Information Technology, E.S. College of Engineering & Technology, Villupuram, TamilNadu, India.
This paper is an attempt to study and understand the Web Service Composition approaches for developing dynamic business applications and making a evaluation based on some perspectives (like QoS, scalability, and correctness). With the tremendous increase in the number of web services available today, searching for a specific web service has become very difficult. A single service may not be able to serve our needs, but a combination of web services can serve the purpose. It is an important task to solve the problem of finding such a combination, called Web Service Composition. Numerous attempts were made to tackle Web Service Composition, Workflow based, XML based and Ontology based techniques are being the dominant ones. For this purpose a dynamic E-Business application – Online Book Shop was developed using the dominant techniques specified and the comparative evaluation is made.
Keywords: Web Service composition; WSC; Ontology; XML; Semantic Workflow.
Web service is a software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine processable format (specifically WSDL – Web Service Description Language). Other systems interact with the Web service in a manner prescribed by its description using SOAP messages, typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards . In a simple way Web Service is defined as: a piece of business logic, located somewhere on the Internet, that is accessible through standard-based Internet protocols such as HTTP or SMTP .
The Web Services are interacting computer applications capable of running on different platforms, managed by different organizations. The Web Service technology includes three open standards named SOAP (Simple Object Access Protocol), WSDL and UDDI (Universal Description Discovery and Integration). To make the Web Service accessible, it must be based on at least two standards named SOAP and WSDL. By using Web services, business applications can publish its function or message to the rest of the world. Web services use XML to code and to decode data, and SOAP to transport it (using open protocols).
2. Related Work
The service provider provides the Web services. The service requesters consume information or services offered by service providers. The system also contains: translator, process generator, evaluator, execution engine and service repository. The translator translates between the external language used by the participants and the internal languages used by the process generator. For each request, the process generator tries to generate a plan that composes the available services in the service repository to fulfill the request. If more than one plan is found, the evaluator evaluates all plans and proposes the best one for execution. The execution engine executes the plan and returns the result to the service provider.
Web service composition (WSC) approaches are classified here according the popular approaches such as Work-flow based Web Service Composition approaches, XML-based Web Service Composition approaches and Ontology based Web Service Composition approaches.
2.1. Workflow-based WSC approach
In the workflow-based composition approaches the classification is made as static and dynamic workflow generation. The static one means that the requester should build an abstract process model before the composition planning starts. On the other hand, the dynamic composition both creates process model and selects atomic services automatically. This requires the requester to specify several constraints, including the dependency of atomic, the user’s preference and so on. In Short these can be explained as: In Static Composition, the requester should plan to build an abstract process model before starting the composition. In Dynamic Composition, the requester has to specify user’s preference.
E-Services are typically delivered as point-to-point. However, the e-service environment creates the opportunity for providing value-added, integrated services, which are delivered by composing existing e-services. In order to enable organizations to pursue this business opportunity, eFlow  has been introduced. E-flow uses static workflow based approach; it is a system that supports the specification, enactment, and management of composite e-services, modeled as processes that are enacted by a service process engine. In eFlow, a composite service is described as a process schema that composes other basic or composite services. A composite service is modeled by a graph (the flow structure), which defines the order of execution among the nodes in the process. Thus eFlow platform has the required characteristics and functionality to satisfy the need of Internet-based service providers.
Polymorphic Process Model (PPM)
Polymorphic Process Model (PPM)  uses a different approach that combines the static and dynamic service composition. The static setting is supported by reference process-based multi-enterprise processes, the processes that consist of abstract sub processes, i.e., sub processes that have functionality description but lack implementation. The abstract sub processes are implemented by service at runtime. This is similar to the service binding in EFlow. The dynamic part of PPM is supported by service-based processes. Here, a service is modeled by a state machine that specifies that possible states of a service and their transitions. Transitions are caused by service operation (also known as service activity) invocations or internal service transitions. Based on the state machine, in the setting, the dynamic service composition is enabled by the reasoning.
2.2.XML-based WSC approach
In XML-based Web service composition approach there are two main approaches they are Web service Orchestration and Web service Choreography.
Web Service Orchestration
which the service is automated. Without orchestration of the interactions between the four actors in the system, they would have to be co-ordinated manually. The service provider can reduce operational costs by using an automated means of co-ordination. There are two types of orchestration:Web Service Choreography Interface (WSCI), Business Process Execution Language for Web Services (BPEL4WS or BPEL).
The Web Service Choreography Interface (WSCI) is an XML-based interface description language that describes the flow of messages exchanged by a Web Service interacting with other Web Services.
WS-BPEL is an acronym for Web Services Business Process Execution Language. WS-BPEL 2.0 is a revision of the original acronym BPEL4WS (Business Process Execution Language for Web Services) 1.0 and 1.1. WS-BPEL is an XML based language enabling users to describe business process activities as Web services and define how they can be connected to accomplish specific tasks. WS-BPEL is designed to specify business processes that are both composed of, and exposed as, Web Services. WSBPEL2.0 is an orchestration language.
2.3.Ontology-based WSC approach
Ontologies  are a key enabling technology for the Semantic Web. They interweave human understanding of symbols with their machine process ability. The reason Ontologies are becoming popular is largely due to what they promise: a shared and common understanding of a domain that can be communicated between people and application systems. As such, the use of Ontologies and supporting tools offers an opportunity to significantly improve knowledge management capabilities in large organizations and it is their use in this particular area. Querying and browsing semantically enriched information sources. We describe semantically enriched search engines, browsing and knowledge sharing support that makes use of machine process able semantics of data. In this approach, Ontologies are used as data models. Mainly this approach has Ontology Web Language (OWL-S) and Web Service Modeling Ontology (WSMO).
3. Proposed Work
The Dynamic E-Business system considered here is Online- Book-Shop. The proposed system is developed using JAVA, and for purpose of developing Ontologies we also used Resource Descriptive Framework (RDF). The proposed system which supports Web Service composition approach is developed with the various techniques mentioned above.
Figure 2. Overview of Online Book Shop
Figure 3. Full details of a book with another service “Banking Service” is also called here.
4. Comparative Evaluation
Here the Web service composition approaches which have discussed so far are compared with each other based on some perspectives such as Quality of Service (QoS), connectivity, Exception handling, Scalability, Correctness and Compensations. Results are indicated as Good, Average and Low. Good is mentioned if the approach can provide all aspects of the given perspectives. Average is mentioned if the approach can provide part of what the perspectives given. Low is mentioned if the approach cannot provide any aspects of the given perspectives. The result is indicated in Table 1.
In Web Service composition reliable connectivity is expected more. If there is no reliable connectivity, the web service composition cannot guarantee the continuity of Web service delivery after composition.
Exception handling is a one of benchmark of Web service composition approaches because it must deal with what happens in case of an error occur and how to undo the already completed activities.
Scalability represents the ability of the Web service to process multiple requests in a certain time interval. It can be measured by number of requests resolved in a certain time interval.
One of the crucial factors in Web service composition is to select component services such that they fulfill appropriate QoS level such as price, reliability, trust, reputation, execution cost and efficiency. Unfortunately, there are no standard QoS criteria for Web services selection  as different users/applications have different requirements.
Table 1. Comparison of Web Service Composition Approaches
BENCHMARKS Approaches CONNECTIVITY EXCEPTION
SCALABILITY CORRECTNESS QOS
E-Flow Low Low Low Low Low
PPM Low Low Low Low Low
BPEL4WS Good Good Average Low Average
OWL-S Good Average Good Low Good
WSMO Good Good Good Low Good
This paper focused on general overview of some specific dynamic web service composition approaches and compared each with others with some perspectives using a dynamic E-business application. While there exist several papers that compare and analyze Web Service Composition languages, this paper compared against a set of characteristics that any approach should aim to support to facilitate Web Service composition. Although the different methods provide different level of automation in service composition, we can not say the higher automation the better. Because the Web service environment is highly complex and it is not feasible to generate everything in an automatic way. Usually, the highly automated methods are suitable for generating the implementation skeletons that can be refined into formal specification. Further work will include a more thorough analysis of the field in addition to practical testing of and experiments with the methods.
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