IEEE TRANSACTIONS ON SYSTEMS, MAN, AND CYBERNETICS—PART C:APPLICATIONS AND REVIEWS, VOL. 38, NO. 3,MAY 2008 319 The Pervasive Workﬂow: A Decentralized Workﬂow System Supporting Long-Running Transactions Frederic Montagut, Student Member, IEEE, Reﬁk Molva, Member, IEEE, and Silvan Tecumseh Golega Abstract—Workﬂow technologies are becoming pervasive in that they enable the execution of business processes in distributed and ubiquitous computing environments. As long-running trans- actions, the execution of workﬂows in environments without dedi- cated infrastructures raises transactional requirements due to the dynamicity of resources available to run a workﬂow instance and the integration of relaxed atomicity constraints at both design and instantiation time. In this paper, we propose an adaptive transac- tional protocol for the pervasive workﬂow model developed in a previous work to support the execution of business processes in the pervasive setting. The execution of this protocol takes place in two phases. First, candidate business partners are assigned to tasks using an algorithm wherein the selection process is based on both functional and transactional requirements. The workﬂow execution further proceeds through a hierarchical coordination protocol managed by the workﬂow initiator and controlled based on a decision table computed as an outcome of the business part- ner assignment procedure. The resulting workﬂow execution is compliant with the deﬁned consistency requirements, and the co- ordination decisions depend on the transactional characteristics offered by the partners assigned to each task. An implementation of our theoretical results relying on ontology Web Language for Series and Business Process Execution Language technologies is further detailed as a proof of concept. Index Terms—Decentralized workﬂows, transaction-aware com- position, transactional consistency. I. INTRODUCTION W ORKFLOW technologies are becoming pervasive in that they enable the execution of long-running business processes and transactions in distributed and ubiquitous envi- ronments [1]–[3]. The adequate execution support for pervasive workﬂows has to cope with the lack of dedicated infrastructure for management and control tasks in order to provide business users with means to leverage the resources available in their surrounding environment. To that effect, a ﬁrst step has been achieved by the design of a fully decentralized workﬂow ar- chitecture based on the service oriented computing paradigm (SOC) [4]. Featuring a dynamic assignment of tasks to work- ﬂow partners, this architecture allows users to initiate work- Manuscript received November 27, 2006; revised February 25, 2007, June 8, 2007, and December 18, 2007. This work was supported in part by European Union (EU) Information on Science and Technology (IST) Directorate General as a part of FP6 IST projects MOSQUITO, in part by the R4eGov, and in part by the Systems, Applications and Products (SAP) Research Laboratories, France S.A.S. This paper was recommended by Guest Editor H. Patrick. F. Montagut is with the Systems, Applications and Products (SAP) Research Laboratories, France, 06250 Mougins, France (e-mail: frederic.montagut@sap.com). R. Molva is with the Institut Eurecom, 06904 Sophia-Antipolis, France (e-mail: reﬁk.molva@eurecom.fr). S. T. Golega is with the Hasso-Plattner-Institut, 900460-14440 Potsdam, Germany (e-mail: silvangolega@gmail.com). Digital Object Identiﬁer 10.1109/TSMCC.2008.919184 ﬂows in any environment where surrounding users’ resources can be advertised by various means including a service dis- covery mechanism. Yet, this architecture does not provide any guarantee on the consistency of the outcome reached by the process execution. Considering the lack of reliability akin to distributed environments, data and transaction consistency is a main issue. Transactional requirements raised by the execution of processes on top of the pervasive workﬂow infrastructure are twofold: on the one hand, the workﬂow execution is dynamic in that the workﬂow partners offering different characteristics can be assigned to tasks depending on the resources available at run-time, and on the other hand, atomicity of the workﬂow execution can be relaxed as intermediate results produced by the workﬂow may be kept despite the failure of one partner. Existing transactional protocols [5], [6] are not adapted to solve this paradigm as they do not offer enough ﬂexibility to cope, for instance, with the run-time assignment of computational tasks. In this paper, we propose an adaptive transactional proto- col for the pervasive workﬂow management system developed in [4]. The execution of this protocol takes place in two phases. First, business partners are assigned to tasks using an algorithm wherein the selection process is based on functional and trans- actional requirements. These transactional requirements are de- ﬁned at the workﬂow design stage using the acceptable ter- mination states (ATS) model. The workﬂow execution further proceeds through a hierarchical coordination protocol managed by the workﬂow initiator and controlled using a decision table computed as an outcome of the business partner assignment procedure. The resulting workﬂow execution is compliant with the deﬁned consistency requirements and the coordination de- cisions depend on the characteristics of the partners assigned to each task. Besides, it should be noted that the practical solutions that are presented in this paper do not only answer speciﬁc re- quirements introduced by the pervasive workﬂow model but are sufﬁciently generic to be applied to other workﬂow architectures supporting long-running transactions. The remainder of the paper is organized as follows. Section II introduces preliminary deﬁnitions and the methodology under- pinning our approach. We present an example of pervasive workﬂow execution in section III for the purpose of illustrat- ing our results throughout the paper. Section IV introduces a detailed description of the transactional model used to repre- sent the characteristics offered by business partners. Section V describes how transactional requirements expressed by means of the ATS model are derived from the inherent properties of termination states. Sections VI and Sections VII present the transaction-aware partner assignment procedure and the asso- ciated coordination protocol, respectively. An implementation 1094-6977/$25.00 © 2008 IEEE