Highly-Available Web Service Community Sattanathan Subramanian Department of Computer and Information Science, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway [email protected]

Abstract

services with a common functionality like flight-booking, although these Web services may have distinct nonfunctional properties. Web service community supports different non-functional requirements of user [3], shares the same domain of interest [18], and sustains the high availability of Web service through substitution [24, 15], and provides fault-tolerance in Web service composition [12].

Functionally similar Web services are grouped as a community to facilitate and speed up the process of Web services discovery. This paper presents a solution to keep the Web Service Community highly-available to the user or application. The term highly-available refers that the Web Service Community can continue providing services even when master Web service (i.e., coordinator) fails operationally. Our solution customizes a distributed election algorithm called Fast Bully Algorithm to identify a temporary master Web service when there is any operational failure in existing master Web service of Community. The identified temporary master Web service handles the service provision and management responsibilities of Web service community. Permanent master Web service takes back the mastering responsibilities from temporary master Web service when it resumes. For this, we introduce some additional functions to the existing architectures of master and slave Web services to run the customized Fast Bully Algorithm. Finally, a Weather Community is developed as a prototype example to illustrate our ideas practically. Keywords. Community, Engineering, High-availability, Web service.

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Figure 1. Overall Architecture of WSC The present architecture (Figure 1) [23, 16] of WSC uses a centralized approach to manage its components i.e. Web services. This approach has two roles namely master and slave. In a WSC, only one Web service has the role of master and the remaining Web services has the role of slave. Master Web service manages and controls slave Web services of WSC. Slave Web services follow the requests and orders of master Web service. Master Web service is a centralized coordinator that leads a WSC using special services related to community management such as development (or dismantlement), attraction, and retention of Web services. Master-slave relationship is regulated using Contract Net protocol (CNProtocol, in short), and Web Service Community Development Protocol (WSCDProtocol, in short). CNProtocol is used to provide service according to the users’ request. WSCDProtocol is proposed for the development purposes of WSC like attracting new Web services, registering the potential new Web service as a slave, ejecting the existing Web services, and monitoring the performance of slave Web services. This centralized coordination does not guarantee the availability of WSC when there is any op-

1 Introduction From the past few years, Web services are considered as the technology of choice to develop B2B and B2C applications [17, 25]. A Web service is a URI based accessible application that can be described, advertised, discovered and triggered to satisfy various needs like weather-forecast over a network [3, 21]. Web service technology standards are Web Service Description Language (WSDL), Simple Object Access Protocol (SOAP), and Universal Description Discovery and Integration (UDDI) [26]. Web services that are having similar functionalities are grouped as a community to facilitate and accelerate the process of identifying Web services according to users’ needs [14, 23, 16]. Web Service Community (WSC, in short) is a collection of Web 1

erational failure (time-out fault [7], for instance) in master Web service. The availability of WSC is directly related to the availability of master Web service. This is due to the following responsibilities of master Web service:

tion [13]. The reasons to adopt FBA instead of BA for providing high-availability to WSC are: (i) FBA requires only O(n) messages to identify a new coordinator whereas BA requires O(n2 ) messages, where n is the number of processes in the group, (ii) response time of FBA is quicker (half the time, approximately) than BA, (iii) FBA allows the recovered process to regain the position of coordinator immediately if its priority number is higher than current coordinator. Points (i) and (ii) minimize the communication between Web services of WSC to identify a new master. In [22], Ruberg et al. mentioned the costs involved in making a Web service call. This shows the importance of having less number of messages for providing high-availability to WSC. Point (iii) helps to: - retain the real master-slave relationship of WSC. Any one of the slave Web service can take up the role of master in a temporary basis only during the failuretime of master Web service, and permanent master Web service can regain its controls when it is up again. - keep some operations away from temporarily elected master Web service. Master Web service is not necessary to give all the operations to temporarily elected master Web service. For instance, (a) WS-Registration can be kept available only with permanent master Web service since the financial compensation for staying part of WSC could differ from one Web service to other in a business perspective, (b) Permanent master Web service can retain the functions of WS-Attraction and WS-Withdrawal with itself, in order to avoid temporary master’s personal choices/preferences affecting the introduction/ejection of Web services to/from WSC. The problems of other distributed election algorithms are available in [13]. The messages used in the election procedure of FBA are: election (request to start election), answer (response to election), nomination (notifies the highest numbered process as a potential candidate for coordinator), coordinator (claims the sender as coordinator), IamUp (sent by recovered process), and view (response to IamUp, contains processes list of group). In FBA, the process that first detects the failure of coordinator controls the election procedure. An example of this is shown in the Case-1 of Figure 2, in terms of WSC. Figure 2 has one master and four slave Web services. Master and slave Web services are correlated to the distributed processes of FBA. Master is the coordinator and it has the highest priority number always. Each slave has its own priority number, given by master during WS-Registration. For the illustration purposes, we consider that the Slave-1 has least priority among all Web services of WSC, Slave-2 has higher priority than Slave1, Salve-3 has higher priority than Slave-2, and Slave-4 has higher priority than Slave-3. Slave-1 detects the fail-

(i) Entry/exit point for all service requests and responses. Master Web service is the only external contact point of WSC. None of the user or application can get the service from WSC when master Web service fails from its operation. In other terms, WSC cannot continue providing the committed services or new commitments to the user or application when master Web service is unavailable from its operation. (ii) Management Responsibility of WSC. Master Web service controls and coordinates the slave Web services of WSC. Slave Web services will be totally out of control from WSC when master Web service fails from its operation. This out of control may lead to a chaos situation in WSC. For instance, slave Web service can move out from WSC without any prior notice. The present architecture does not provide any way to replace (either temporarily or permanently) the failed master Web service with a new Web service to take charge of master’s functionality. WSC provides a solution to the highavailability of Web service [15], but not for itself. This is due to the static and design time identification of master Web service. In this paper, we provide a dynamic solution to identify a new Web service as a temporary master in runtime when there is any failure in existing master Web service. Temporary master Web service is identified from the slave Web services of WSC by taking advantage of Fast Bully Algorithm [13] which is an extension of classical Bully Algorithm [19]. Further, we customize Fast Bully Algorithm to minimize the number of messages that are required to identify a new Web service as temporary master. Permanent master Web service retakes the role of master from temporary master Web service when it becomes active again. This paper is organized as follows. Section 2 customizes Fast Bully Algorithm according to the requirements of WSC. Architecture and its operations of highly-available WSC are given in Section 3. The details of prototype implementation are reported in Section 4. Related works are discussed in Section 5 before providing our conclusion and future works in Section 6.

2 Customization of Fast Bully Algorithm Lee et al. [13] proposed Fast Bully Algorithm (FBA, in short) as an extension to Bully Algorithm (BA, in short) suggested by Molina in [19] for identifying distributed coordinator among distributed processes when existing coordinator fails. Due to the simplicity and fault tolerance, BA is commonly used algorithm for distributed process elec2

Slave-4 sends the IamUp message directly to Master. (ii) when permanent master becomes active. As like previous case, it is unnecessary to send IamUp message to all slaves because permanent master always has the highest priority number in WSC. So, we customize FBA to send coordinator message directly for all slaves instead of sending IamUp message and continuing further by permanent master Web service. This customization reduces the total number of messages from n3 to n for retaining the position of master. An example is shown in Case-2 of Figure 2, where the permanent master sends coordinator message directly to all Slaves i.e. 1 to 4. Apart from the above mentioned two cases, customized FBA (cFBA, in short) removes the necessity of sending election message to permanent master Web service during the failure cases of FBA given in [13]. This is because, cFBA provides the rights to permanent master for sending coordinator message directly to all slaves when it becomes active. This reduces one message from the respective failure case of FBA.

ure of Master in the Case-1 of Figure 2. So, Slave-1 initiates the election by sending election message to every slaves which has higher priority number than Slave-1. Slaves 2 to 4 receive election message in this case. Slave-1 receives answer messages from Slave 2 and 3. No response from Slave-4 since it is inactive. After seeing the answer messages, Slave-1 sends the nomination message to Slave-3 since it has the highest priority number among the received answer messages. After that, Slave-3 sends coordinator message to other slaves which has lesser priority number than Slave-3 to declare itself as temporary master. Master

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3 Highly-Available Web Service Community Active

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We customize FBA in the following two cases for reducing the number of messages further in the process of identifying temporary master Web service: (i) when a slave becomes active newly. As per FBA, if a process comes up then it has to send IamUp message to all other processes to find the coordinator. This is unnecessary for WSC, because permanent master always has the highest priority number than others. So, we customize FBA to send IamUp message only to permanent master initially when a slave becomes active. If permanent master is active then the newly active slave receives the view message to confirm the master. Otherwise, the newly active slave sends IamUp message to all other slaves, and chooses the coordinator based on the received view messages. In the best case (i.e. when there is a response from permanent master Web service), this customization reduces the total number of messages from n2 to 2 to identify the master, where n is the number of Web services in WSC. An example is shown in Case-3 of Figure 2, where the newly active

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Legend: New interactions to provide high-availability Existing interactions

Figure 3. Arch. of Highly-Available WSC Figure 3 shows the overall architecture of highlyavailable WSC, it has one master and n slave Web services. This architecture introduces two items newly to WSC, in comparison with the existing architecture which is shown in Figure 1. Those are: (i) Master Web service shares its WSC-database details with Slave Web services. Slave Web service makes use of this shared WSC-database when master Web service fails operationally. Master Web service shares its WSC-database information completely or partially with the Slave Web services. The amount of WSCdatabase sharing is based on the business requirements and objectives of a WSC. For instance, complete WSC-database can be shared with slave Web services when the community is meant for nonmonetary 3

In Slave Web Service (SWS, in short), we introduce the following functions newly: SWS-Election, SWS-Nomination, SWS-Coordinator, and SWS-IamUp. These are placed in a new module called SWS-HighAvailability. The updated architecture of slave Web service is shown in Figure 4. In that, existing functions are shown in shaded parts and newly introduced functions are non-shaded. The details of newly introduced functions are: • Answer ← SWS-Election(ElectionDetails) initiates the election after detecting the failure of master Web service. – SWS-Election is the name of the function that permit slave Web services to express the election requirement of WSC. – ElectionDetails is the incoming message from a slave Web service to other. Data type of ElectionDetails is complex that includes the discovery time of master’s failure. Application designer determines the complete structure of this message. – Answer is the outgoing message from a slave Web service to other slave Web service. Data type of Answer is boolean i.e. true or false.

services. For instance, book-enquiry of government library. Whereas, the WSC-database can be shared only partially when it is meant for monetary services. For instance, commercial flight-booking. This is because, the profit related community’s master could vary the financial compensation from a slave to slave. Updating replicated (or shared) WSC-databases are beyond the scope of this paper. (ii) Slave Web services can interact with each other. This enables the slave Web services to communicate with each other to identify a new temporary master from them when master Web service fails operationally.

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• SWS-Nomination(NominationDetails) nominates a slave Web service as potential temporary master for WSC. – SWS-Nomination is the name of the function that permit a slave Web service to nominate other slave Web service as temporary master. This is carried out after the successful execution of SWSElection function. – NominationDetails is the incoming message from a slave Web service to other slave Web service. Data type of NominationDetails is complex that includes the reason for nomination. Application designer determines the complete structure of this message. • SWS-Coordinator(MasterDetails) intimates the slave Web service about its new master (either temporary or permanent). – SWS-Coordinator is the name of the function that permit a slave/master Web service to intimate itself as temporary/permanent master of WSC. This is carried out after the successful execution of SWS-Election, SWS-Nomination functions in the case of slave Web service. However, permanent master Web service can carry out this function without those functions. – MasterDetails is the incoming message from a slave/master Web service to other slave Web service. Data type of MasterDetails is complex that includes the type of master (i.e., temporary or

SWS-RequestHandler SWS-Request

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Figure 4. Updated Arch. of Slave Web service We introduce some new functions to the existing architectures [16] of master and slave Web service. These new functions provide the support to run cFBA in WSC for providing high-availability. In Master Web Service (MWS, in short), we introduce a new function called MWS-IamUp. This is placed in a new module called MWSHighAvailability. The details of MWS-IamUp function are: • View ← MWS-IamUp(SelfDetails) informs the rebirth of a slave Web service to master Web service. – MWS-IamUp is the name of the function that permit slave Web services to express its rebirth. – SelfDetails is the incoming message from a slave Web service to master Web service. Data type of SelfDetails is complex that includes the details of rebirth time. Application designer determines the complete structure of this message. – View is the outgoing message from master Web service to slave Web service. Data type of View is complex that includes the details of recent updates in WSC. 4

permanent) and nomination details in the case of the coordinator is temporary master. Application designer determines the complete structure of this message.

Web services engaged in composition scenarios [15], reputation to assess a WSC as perceived by users [8], and semantic data mediation for Web services input and output [20]. Unfortunately, none of the above mentioned works address the high-availability of WSC. This is the first work to address the high-availability requirement of WSC. However, we report some of the related works from the perspective of self-healing Web service composition since it appears close to the focus of this paper. Self-healing systems automatically detects, diagnoses, and repairs the localized problems resulting from failures in software and hardware. [11]. Some of the solutions available for self-healing of Web service composition are: (i) separate Web service to monitor the Web service that participates in composition [1], (ii) supervise the composition process through policies and aspects [2], (iii) use logs to track the failure of composition [10], and (iv) access proxy to identify or replace Web service [9]. Solution (i) opens up another problem of handling the failure of monitoring Web service, whereas our solution does not have such problem. Solution (ii) is undemocratic due to the usage of policies for identifying new Web service, whereas our solution is democratic in nature. Solution (iii) and (iv) assume that the coordinator is always available, whereas our paper’s focus starts when the coordinator is not available.

• SWS-IamUp is same as MWS-IamUp but the communication happens between two slave Web services. SWS-AbstractInterface allows the user or application to communicate with the slave Web service directly when that slave Web service takes charge as temporary master. SWS-CommunityInterface permits to communicate with other slave Web services as well as master Web service. SWS-RequestHandler is meant for running CN protocol. This module is activated when slave Web service takes charge as temporary master, and deactivated after handing over its charge to permanent master or other high priority slave. The details of SWS-RequestHandler is same as MWS-RequestHandler which is given in [23].

4 Prototype Implementation The prototype is implemented using Web service Toolkit Project (WTP) of Eclipse, Java Development Kit (JDK) 1.5, and Document Object Module (DOM) application programming interfaces. WTP provided an integrated development environment for developing Web services. JDK is used as a development language for developing Web services of community including its functions i.e. MWS/SWSHighAvailability. DOM is used for XML parsing purposes. The developed Web services are executed in Windows operating system of Java Runtime Environment (JRE). We have developed a WeatherCommunity to illustrate the assignment of temporary master and the acceptance of selected Web service to perform the role of temporary master. WeatherCommunity has four Web services, namely W1 , W2 , W3 , and W4 . W1 is the master, and remaining are slaves. W1 has higher priority than W2 . W2 has higher priority than W3 . W3 has higher priority than W4 . Initially, W4 detects the failure of W1 , and sends election message to W2 and W3 . After that, W4 nominates W2 after receiving answers from W2 and W3 . Finally, W2 takes charge as new temporary master after sending coordinator message to W3 and W4 . This includes SWS-Highavailability functions of slave Web service (W2 , in this case) of WSC. After sometime, W1 comes alive and retakes the role of master from W2 by sending coordinator message to W2 , W3 , and W4 .

6 Conclusion and Future Works This paper provides a solution for keeping a Web Service Community highly-available to the user or application. We have customized a distributed process election algorithm called Fast Bully Algorithm for identifying new temporary master Web service when the existing master Web service fails operationally. Temporary master Web service is identified from the slave Web services of a community. Also, we have given some new functionalities to the existing architectures of master and slave Web services of community for running customized Fast Bully Algorithm. This enables a Web Service Community to continue providing services and manage themselves in all situations. As the future work, we plan to analyze the performance of cFBA in large scale communities. Also, we plan to investigate the software agents for minimizing the Web service communication overhead during community management and high-availability related activities.

5 Related Works

Acknowledgements

Web service community is relatively a new research topic which is very active in last few years. The topics addressed so far are: definition of WSC [14], approach to engineer a WSC [23, 16], argumentation to improve the performance of WSC [5, 4, 6], sustain the high-availability of

This work was carried out during the tenure of an ERCIM “Alain Bensoussan” Fellowship Programme. Author would like to thank Vithya Ganeshan of NITK for her valuable comments and fruitful discussions about this paper, and Prof. Guttorm Sindre of NTNU for his support.

5

References

[14] Z. Maamar, M. Lahkim, D. Benslimane, P. Thiran, and S. Subramanian. Web Services Communities - Concepts & Operations -. In Proceedings of International Conference on Web Information Systems and Technologies (WEBIST), Barcelona, Spain, 2007.

[1] L. Ardissono, R. Furnari, A. Goy, G. Petrone, and M. Segnan. Fault Tolerant Web Service Orchestration by Means of Diagnosis. In Proceedings of European Workshop on Software Architectures (EWSA), Nantes, France, 2006. [2] L. Baresi, S. Guinea, and P. Plebani. Policies and Aspects for the Supervision of BPEL Processes. In Proceedings of International Conference on Advanced Information Systems Engineering (CAiSE), Trondheim, Norway, 2007. [3] B. Benatallah, Q. Z. Sheng, and M. Dumas. The Self-Serv Environment for Web Services Composition. IEEE Internet Computing, 7(1), January/February 2003. [4] J. Bentahar, Z. Maamar, D. Benslimane, and P. Thiran. An Argumentation Framework for Communities of Web Services. IEEE Intelligent Systems, 22(6), 2007. [5] J. Bentahar, Z. Maamar, D. Benslimane, and P. Thiran. Using Argumentative Agents to Manage Communities of Web Services. In Proceedings of International Workshop on Web and Mobile Information Systems (WAMIS), Niagara Falls, Canada, 2007. [6] J. Bentahar, Z. Maamar, D. Benslimane, W. Wan, P. Thiran, and S. Subramanian. Argumentative Agents for Communities of Web Services. Special Issue on Service Intelligence and Service Science in the Journal of Service Oriented Computing and Applications (SOCA), 2009 (In-press). [7] M. K. S. Chan, J. Bishop, J. Steyn, L. Baresi, and S. Guinea. A Fault Taxonomy for Web service Composition. In Proceedings of Workshop on Engineering Service-Oriented Applications: Analysis, Design and Composition (WESOA), Vienna, Austria, 2007. [8] S. Elnaffar, Z. Maamar, H. Yahyaoui, J. Bentahar, and P. Thiran. Reputation of Communities of Web Services - Preliminary Investigation. In Proceedings of International Conference on Advanced Information Networking and Applications - Workshops (AINAW), Okinawa, Japan, 2008. [9] O. Ezenwoye and S. M. Sadjadi. TRAP/BPEL: A Framework for Dynamic Adaptation of Composite Services. In Proceedings of International Conference on Web Information Systems and Technologies (WEBIST), Barcelona, Spain, 2007. [10] S. A. Gurguis and A. Zeid. Towards Autonomic Web Services: Achieving Self-Healing Using Web Services. ACM SIGSOFT Software Engineering Notes, 30(4), 2005. [11] J. O. Kephart and D. M. Chess. The Vision of Autonomic Computing. IEEE Computer, 36(1), 2003. [12] N. Laranjeiro and M. Vieira. Towards fault tolerance in web services compositions. In Proceedings of Workshop on Engineering Fault Tolerant Systems (EFTS), Dubrovnik, Croatia, 2007. [13] S. H. Lee and H. Choi. The Fast Bully Algorithm: For Electing a Coordinator Process in Distributed Systems. In Proceedings of International Conference on Information Networking, Wireless Communications Technologies and Network Applications-Part II (ICOIN), Cheju Island, Korea, 2002.

[15] Z. Maamar, Q. Z. Sheng, and D. Benslimane. Sustaining Web Services High-Availability Using Communities. In Proceedings of International Conference on Availability, Reliability and Security (ARES), Fukuoka, Japan, 2008. [16] Z. Maamar, S. Subramanian, P. Thiran, D. Benslimane, and J. Bentahar. An Approach to Engineer Community of Web services: Concepts, Architecture, Operation and Deployment. International Journal of E-Business Research (IJEBR), 5(4), 2009 (In-press). [17] H. Maruyama. New trends in e-Business: from B2B to web services. New Generation Computing, 20(1), 2002. [18] B. Medjahed and A. Bouguettaya. A Dynamic Foundational Architecture for Semantic Web Services. Distributed and Parallel Databases, Kluwer Academic Publishers, 17(2), March 2005. [19] H. G. Molina. Elections in a Distributed Computing System. IEEE Transactions on Compututers, 31(1), 1982. [20] M. Mrissa, P. Thiran, C. Ghedira, D. Benslimane, and Z. Maamar. Using context to enable semantic mediation in web service communities. In Proceedings of international workshop on Context enabled source and service selection, integration and adaptation (CSSSIA), Beijing, China, 2008. [21] M. P. Papazoglou and D. Georgakopoulos. Introduction. Communications of the ACM, 46(10), 2003. [22] N. Ruberg, G. Ruberg, and I. Manolescu. Towards costbased optimizations for data-intensive web service computations. In Proceedings of Brazilian Symposium on Databases (SBBD), Munich, Germany, 2004. [23] S. Subramanian, P. Thiran, Z. Maamar, and D. Benslimane. Engineering Communities of Web services. In Proceedings of International Conference on Information Integration and Web-based Applications and Services (iiWAS), Jakarta, Indonesia, 2007. [24] Y. Taher, D. Benslimane, M. C. Fauvet, and Z. Maamar. Towards an Approach for Web services Substitution. In Proceedings of International Database Engineering and Applications Symposium (IDEAS), Montreal, Canada, 2006. [25] Q. B. Vo, M. Huhns R. Kowalczyk, and Z. Maamar. Introduction. Special issue on Service-Oriented Computing: Agents, Semantics, and Engineering in the International Journal on Agent-Oriented Software Engineering, 2009 (Inpress). [26] A. E. Walsh, editor. UDDI, SOAP, and WSDL: The Web Services Specification Reference Book. Prentice Hall Professional Technical Reference, 2002.

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Highly-Available Web Service Community

vices to run the customized Fast Bully Algorithm. Finally, a ... to the user or application when master Web service is ... 2 Customization of Fast Bully Algorithm.

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