Section: New Results

Specific studies: Web services orchestrations

Participants : Ajay Kattepur, Albert Benveniste, Claude Jard.

Web services orchestrations and choreographies refer to the composition of several Web services to perform a co-ordinated, typically more complex task. We decided to base our study on a simple and clean formalism for WS orchestrations, namely the Orc formalism proposed by Jayadev Misra and William Cook  [55] .

Main challenges related to Web services QoS (Quality of Service) include: 1/ To model and quantify the QoS of a service. 2/ To establish a relation between the QoS of queried Web services and that of the orchestration (contract composition); 3/ To monitor and detect the breaching of a QoS contract, possibly leading to a reconfiguration of the orchestration. Typically, the QoS of a service is modeled by a contract (or Service Level Agreement, SLA) between the provider and the consumer of a given service. To account for variability and uncertainty in QoS, we proposed in previous work soft probabilistic contracts specified as probabilistic distributions involving the different QoS parameters; we studied contract composition for such contracts; we developed probabilistic QoS contract monitoring; and we studied the monotonicity of orchestrations; an orchestration is monotonic if, when a called service improves its performance, then so does the overall orchestration.

Last year, in the framework of the Associated Team FOSSA with the University of Texas at Austin (John Thywissen (PhD), Jayadev Misra and William Cook), we extended our approach to general QoS parameters, i.e., beyond response time. We now encompass composite parameters, which are thus only partially, not totally, ordered. We developed a general algebra to capture how QoS parameters are transformed while traversing the orchestration and we extended our study of monotonicity. Finally, we have developed corresponding contract composition procedures. This year, John Thywissen (from UT Austin) and Ajay Kattepur have prototyped a toolbox for Orc to support QoS-management. A journal paper is submitted.

A key task in extending Orc for QoS was to extend the Orc engine so that causalities between the different site calls are made explicit at run time while execution progresses. This benefits from our previous work on Orc semantics, but a new set of rules has been proposed to generate causalities in an efficient way, by covering new features of the language. This is joint work of Claude Jard, Ajay Kattepur and John Thywissen from Austin. An implementation on Orc is under development and a publication is in preparation.

Besides this main line of work, the additional topic of Negotiation Strategies for Probabilistic Contracts in Web Services Orchestrations has been addressed by Ajay Kattepur as part of his thesis, see [31] . Service Level Agreements (SLAs) have been proposed in the context of web services to maintain acceptable quality of service (QoS) performance. This is specially crucial for composite service orchestrations that can invoke many atomic services to render functionality. A consequence of SLA management entails efficient negotiation proto- cols among orchestrations and invoked services. In composite services where data and QoS (modeled in a probabilistic setting) interact, it is difficult to pick an individual atomic service to negotiate with. A superior improvement in one negotiated domain (eg. latency) might mean deterioration in another domain (eg. cost). In this work, we propose an integer programming formulation based on first order stochastic dom- inance as a strategy for re-negotiation over multiple services. A consequence of this is better end-to-end performance of the orchestration compared to random strategies for re-negotiation. We also demonstrate this optimal strategy can be applied to negotiation protocols specified in languages such as Orc. Such strategies are necessary for composite services where QoS contributions from individual atomic services vary significantly.