Using Chemical Reactions to Model Service Composition

摘要

Internet is evolving from a network of computers and information into a network of services allowing applications to be built by selecting services and composing them in a loosely coupled manner. These Service Based Applications (SBA) are composed of a number of possibly independent services that are provided by many actors under different conditions (like price, time to deliver, and so on). Service provision conditions may change in time depending on provider policies or other environmental changes, so it is necessary to organize compositions of services on demand in response to dynamic requirements and circumstances. In this paper we propose to use a chemical computational model to address this problem by decoupling the process of finding services composing an SBA requested by a user, from their actual enactment. An SBA request is described in terms of an abstract workflow where only service functionalities of the single components and their execution order (i.e. the application control flow) are specified, along with parameters representing the conditions under which the user expects the application to be delivered. The proposed approach allows to model the process of instantiating the required functionalities with actual service implementations as an evolving and always running middleware mechanism that can take into account the current state of the context when the composition is required. Furthermore, the evolutionary nature of the chemical system provides a form of adaptation since once compositions of services are computed with the available services, new compositions can be computed as soon as new services become available or the conditions of existing ones change.