Team Distribcom

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New Results
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Section: New Results

Specific studies: Web services orchestrations

Participants : Sidney Rosario, Ajay Kattepur, Albert Benveniste, Claude Jard, Anne Bouillard.

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  [46] .

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 consumer of a given service. To account for variability. In 2007, we proposed soft probabilistic contracts specified as probabilistic distributions involving the different QoS parameters. We studied contract composition for such contracts, see 2007 Activity Report. Since then, the following directions have been developed.

First, we observed that the current practice regarding SLA seems to ignore the fact that orchestrations may not be monotonic , meaning that it is possible that if a called service improves its performance, then the overall orchestration performance nevertheless decreases. This artifact does not occur in SLA for networks; it can, however, occur for Web services because the latter involve complex interactions between control, data, and time (through the use of timers). In [22] we have established conditions (necessary and sufficient ones) ensuring monotonicity of orchestrations.

In a contract-based paradigm, one important duty of the orchestration is QoS contract monitoring. We have developed statistical techniques for soft probabilistic QoS contract monitoring. This work is part of the invited paper [52] of the ICWS07 special issue of IEEE Transactions on Services Computing and was presented at IM 09, special track on new results [26] . This monitoring service has been added to the TOrQuE ( T ool for Or chestration simulation and Qu ality of service E valuation ) tool developed since 2007 by Sidney Rosario regarding soft probabilistic QoS contracts for Web services orchestrations.

Last, we have further extended our approach by soft probabilistic contracts to general QoS parameter, i.e., beyond response time. In particular, we now encompass composite parameters, which are thus only partially, not totally, ordered. We have developed a general algebra to capture how QoS parameters are transformed while traversing the orchestration and we have extended our study of monotonicity. Finally, we have developed corresponding contract composition procedures, which are iterative and must involve negotiation, unlike the simple case that we studied previously. This was presented at ICWS 09 [27] and this paper was selected for the special issue of International Journal of Web Services Research  [15] .


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