11-05-2011, 04:38 PM
TQoS: Transactional and QoS-Aware Selection Algorithm for Automatic Web Service Composition
Abstract
Web Services are the most famous implementation of service-oriented architectures that has brought some challengingresearch issues. One of these is the composition, i.e., the capability to recursively construct a composite Web service as a workflow ofother existing Web services, which are developed by different organizations and offer diverse functionalities (e.g., ticket purchase,payment), transactional properties (e.g., compensatable or not), and Quality of Service (QoS) values (e.g., execution price, successrate). The selection of a Web service, for each activity of the workflow, meeting the user’s requirements, is still an important challenge.Indeed, the selection of one Web service among a set of them that fulfill some functionalities is a critical task, generally depending on acombined evaluation of QoS. However, the conventional QoS-aware composition approaches do not consider the transactionalconstraints during the composition process. This paper addresses the issue of selecting and composing Web services not onlyaccording to their functional requirements but also to their transactional properties and QoS characteristics. We propose a selectionalgorithm that satisfies user’s preferences, expressed as weights over QoS criteria and as risk levels defining semantically thetransactional requirements. Proofs and experimental results are presented.Index Terms—Web service selection, automatic composition, transactional Web service, local optimization of Quality of Service,workflow patterns.
1 INTRODUCTION
WEB Services (WSs) are the most famous implementationof service-oriented architectures allowing the constructionand the sharing of independent and autonomoussoftwares. Web service composition consists in combiningWeb services, developed by different organizations andoffering diverse functional (e.g., ticket purchase, payment),behavioral (e.g., compensatable or not), and nonfunctionalproperties (i.e., Quality of Service values—e.g., executionprice, success rate), to offer more complex services.The Web service composition can be viewed as a threestepprocess: 1) composite Web service specification,2) selection of the component Web services, and 3) executionof the composite Web services. At the first step, the usersubmits the goal he/she wants the composite serviceachieves, along with some constraints and preferences thatneed to be satisfied [1]. Workflows can be used to model thecomposite Web service specification. During the secondstep, component Web services fulfilling the user’s goal areselected among a set of available services. This WS selectioncould be done by hand (in this case, steps specification andselection are integrated) or could be automatically decidedby the system. When component WSs are selected at designtime, the third step of the composition process consists in executing the selected component WS. At runtime, selectionand execution of component WS are integrated and theselection is described as dynamic. In this paper, we focus ondesign-time WS selection, and particularly, on automaticselection, where the user is relieved as much as possiblefrom the composition process. We do not focus on theexecution step or on the recovery or replanning problems.While many works have been done for Web serviceselection, designing a composite Web service to ensure notonly correct and reliable execution but also optimal QoSremains an important challenge [2]. Indeed, WSs compositionbased on transactional properties ensures a reliableexecution, however, an optimal QoS composite Web serviceis not guaranteed. Moreover, composing optimal QoS Webservices does not guarantee a reliable execution of theresulting composite Web service. Thus, QoS-aware andtransactional-aware should be integrated. However, theproblem is generally addressed from the QoS side or fromthe transactional side separately. The conventional QoSawarecomposition approaches [3], [4], [5], [6], [7] do notconsider the transactional constraints during the compositionprocess, likewise transactional-aware ones [8], [9], [10],[11], [12] do not consider QoS. As far as we know, only Liuet al. [2] propose a composition model in design time, whichcaptures both aspects in order to evaluate the QoS of acomposite WS with various transactional requirements.However, the authors do not consider the automaticselection step and only analyze the impact of the transactionalrequirements on the QoS of the composite WS.Our research objective is to propose a design-timeselection algorithm for automatic WS composition, wheretransactional and QoS requirements are both integrated inthe selection process.
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