A Holistic Method for Selecting Web Services in Design of Composite Applications

Transcription

1 A Holisti Method for Seleting Web Servies in Design of Composite Appliations Mārtiņš Bonders, Jānis Grabis Institute of Information Tehnology, Riga Tehnial University, 1 Kalu Street, Riga, LV 1658, Latvia, martins@iti.rtu.lv Abstrat. Seletion of appropriate web servies is an important step in development of omposite appliations. Quality of Servie (QoS) data haraterizing nonfuntional properties of andidate web servies are usually used in web servie seletion. Funtional harateristis, whih are diffiult to measure, and omposite appliation s lifeyle management fators are equally important. The paper develops a omprehensive approah for seleting web servies in design of omposite appliations. This approah aounts for fators related to non-funtional and funtional requirements and to omposite appliation s lifeyle management. The servie seletion is performed using a generi mathematial optimization model. Establishing relationships between omposite appliation s lifeyle management proesses and web servie seletion is the ey hallenge in elaboration of the seletion model. Keywords. Web servies, QoS, ITIL, Servie Lifeyle Management 1 Introdution Composite appliations use external servies to gain aess to vast data and proessing resoures. Web servies relying on suh open tehnologies as XML, SOAP, WSDL, REST and others are often used as building blos for omposite appliations. That allows developing standards-based, salable and flexible appliations. Properties of omposite appliations diretly depend upon harateristis of external servies used and environmental fators, whih in the ase of publi networs, exhibit high degree of variability. Therefore, seletion of appropriate and reliable servies is of major importane. Multiple methods have been elaborate for seletion of suh servies from the set of andidate servies providing similar funtionality [1,2,3]. However, majority of these methods fous on non-funtional attributes of web servies as a main fators influening web servie seletion. [4] shows that funtional aspets have an equally signifiant impat. Additionally, the omposite appliation inorporating the seleted servies should be supported throughout its lifeyle, and the seleted servies also have impat on lifeyle management and osts of omposite appliations. The objetive of this paper is to develop a omprehensive web servie seletion model for design of omposite appliations. The model should aount for both nonfuntional and funtional requirements as well as for lifeyle management fators of

2 the omposite appliation. The web servie seletion problem is formulated as an optimization problem. This optimization problem is an inherently multi-objetive problem. In order to aount for this harateristi, impat of servie seletion is measured in monetary terms. The paper presents the general holisti approah to web servies seletion and develops a generi optimization model for servie seletion. These developments will serve as a basis for future researh on elaboration of the omprehensive optimization model and on development of a platform for prototyping and experimenting with omposite appliations. Setion 2 desribes the general approah for web servie seletion. A generi mathematial programming model for servie seletion is given in Setion 3. Setion 4 outlines future researh ativities. 2 Approah Web servie eletion methods often use Quality-of-Servie (QoS) measurements to evaluate servies. These measurements inluding reliability, seurity, trust and exeution ost are mainly onerned with nonfuntional harateristis of servies. However, servie onsumers are equally onerned about both funtional and nonfuntional harateristis of servies and suessfully management of developed omposite appliation. There have been attempts to expand the QoS onept in the ase of web servie seletion by defining it as: The degree to whih a system, omponent or proess meets ustomer or user needs or expetations [1]. This definition inludes evaluation of both funtional and nonfuntional requirements. Unfortunately, formal evaluation of funtional harateristis in the framewor of web servie seletion is more diffiult than evaluation of nonfuntional harateristis. Funtional requirements are represented either in a simplified form or require judgmental appraisal of degree of their satisfation. From the perspetive of managing omposite appliations throughout their lifeyle, the seleted web servies have diret impat on development and maintenane. This issue has been largely ignored in servie seletion literature. The propose web servie seletion approah attempts to aount for both nonfuntional and funtional requirements as well as for omposite appliation s management aspet (Fig. 1). The non-funtional requirements are aounted for using QoS measurements suh as response time and reliability. The funtional requirements are aounted for using quantitative performane measures of the omposite appliation. Finding quantitative performane measures haraterizing funtional requirements is diffiult. One approah ould be measuring appliation s impat on business ey performane indiators though these indiators give only indiret measurements, they are affeted by different side-effets and an be obtained only after the appliation has been deployed and used for some time. Deision-maing appliations form one lass of appliations for whih quantitative performane measurements an be determined beause funtionally these appliations are required to produe speifi performane indiators. For instane, if omposite appliation is used in vehile routing then the main funtional requirement is finding a route

3 between two loations. In this ase, alternative servies an be evaluated by omparing travel times returned by the servies. From the omposite appliation s lifeyle management perspetive, the omposite appliation itself is pereived a servie, whih requires appropriate maintenane and support. Standards lie Information Tehnology Library (ITIL) [5] govern management of suh servies throughout their lifeyle. They inlude a number of management proesses and not all of them are affeted by servie seletion. Proesses affeted by web servie seletion inlude hange management (external servies should be ontinuously monitored for hanges in servie definition and performane) and information tehnology arhiteture management (impat of using different web servie integration methods). Some other proesses diretly depend upon QoS harateristis of web servies, for instane, availability management. The web servie seletion model inorporates all fators mentioned above and is used to selet servies to be used in development of the omposite appliation. A generi form of the servie seletion model is presented in the following setion. Web servies... Web servies Appliation lifeyle fators Funtional fators (quantitative performane measures) Non-funtional fators (QoS) Web servie seletion model Seleted web servie Fig. 1. The web servie seletion method

4 3 Mathematial Model It is assumed that requirements for a omposite appliation relevant to web servie seletion are given by N funtional requirements referened by index i and M nonfuntional requirements referened by index j. Additionally, there are R parameters referened by index r haraterizing lifeyle properties of the omposite appliation depending upon servies used in its design. There are K alternative web servies S s,..., 1 sk satisfying the requirements to some degree. QoS data G,..., g1 gm haraterize how well the th servie performs aording to the jth nonfuntional requirement. Quantitative performane measures H h h,..., 1 N haraterize how well the th servie performs aording to the ith funtional requirement. Lifeyle properties L,..., l1 lr haraterize impat of the th servie on rth property of the omposite appliation. A web servie seletion model taes G, H and L as input data and finds the best servie using anappropriate seletion algorithm. A mathematial programming is used as the seletion method. In order to ombine different funtional and nonfuntional riteria, a universal optimization metri is used. This metri haraterizes the ost of building and operating the omposite appliation depending upon web servies used in its development. It is omputed as N f M nf M lf min 1 i1 i i j1 j j j1 j j, TC h g l X where X l indiates whether the th servie is seleted and f i, nf i and lf i are parameters haraterizing osts assoiated with funtional requirements, nonfuntional requirements and lifeyle management properties, respetively. The optimization is performed subjet to relevant onstraints. The mathematial model is given in its generi form and should be augmented with appropriate onstraints. However, the main hallenge is estimation of the model s parameters. G an be determined aording to data provided by the servie provider or using simulation studies. In order to evaluate H, a prototype omposite appliation is built. A variant of the prototype P is developed for eah andidate web servie. Values of H are determined by exeuting the orresponding prototype P. Values of L are omputed by taing into aount appliation lifeyle management ey performane indiators used at the enterprise. 4 Future Wor and Conlusion The paper has proposed a general approah to seleting web servies for design of omposite appliations. The ey distintive feature of this approah is ombination of fators related to non-funtional and funtional requirements and to omposite appliation s lifeyle management. The approah uses the mathematial

5 programming model for servie seletion. The main subjet of further researh is estimation of parameters of this model. Aounting for funtional requirements ass for development of prototype omposite appliations for eah ombination of servies to be evaluated. That is feasible only if web servies an be substituted with relative ease (i.e., hanging servies should not affet the ore of the omposite appliation). Additionally, that would require implementation of a speialized framewor for experimenting with omposite appliations. The proposed approah for estimating impat of funtional requirements is restrited to deision-maing appliations, whih have lear quantitative performane measures. Possibilities to formally quantify satisfation of funtional requirements for other types of appliations also should be explored. Current web servie seletion methods are able to deal with aounting for nonfuntional fators, and [4] demonstrates inorporation of funtional fators in the web servie seletion model while estimation of lifeyle properties of omposite appliations subjet to seleted web servies is the subjet of further researh. In general, relationships between web servie seletion and development and operation of omposite appliations should be identified and formally defined. Referenes 1. Huang Angus F.M., Ci-Wei Lan, Stephen J.H. Yang, An optimal QoS-based Web servie seletion sheme, Information Sienes: an International Journal arhivevolume 179, Issue 19, Pages: , Buhwan J., Hyunbo C., Choonghyun L., On the funtional quality of servie (FQoS) to disover and ompose interoperable web serviesexpert Systems with Appliations 36, , Dimitrios T., Ioanna R., Efstathios S., QoS-aware servie evaluation and seletion, European Journal of Operational Researh Volume 191, Issue 3, Pages , Bonders M., Grabis J., Kampars J. Combining Funtional and Nonfuntional Attributes for Cost Driven Web Servie Seletion Frontiers in Artifiial Intelligene and Appliations // Databases and Information Systems VI - Seleted Papers from the Ninth International Balti Conferene: DB&IS Vol.224, Latvia, Riga, July, Offiial ITIL Website,