Hierarchical web services compositions: visibility, compensation and monitoring

Biswas, Debmalya (2005) Hierarchical web services compositions: visibility, compensation and monitoring. Masters thesis, Memorial University of Newfoundland.

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Abstract

Industry and researchers acknowledge Web services as being at the heart of next generation distributed systems. The most promising feature of the Web services platform is its ability to form new services by combining the capabilities of already existing services, i.e., its composability. The existing services may themselves be composed of other services, leading to a hierarchical composition. In this work, we focus on the visibility, compensation and monitoring aspects for hierarchical compositions. -- Most works on mechanisms to provide extended functionalities like transactions, monitoring, security, etc. for Web services compositions consider single-level compositions with an implicit assumption that they can be straightforwardly extended to hierarchical compositions. As such, they fail to appreciate an important and unique aspect of hierarchical compositions, the visibility aspect. For example, a service provider may not be aware of any providers in the hierarchy other than its parent and children. On the other hand, a service provider may be aware of all other providers in the hierarchy. Towards this end, we introduce the notion of Spheres of Visibility (SoV). Basically, SoV provides an abstraction to capture the upward/downward visibility aspects in a hierarchical composition. We expect other compositional aspects like transactions, monitoring, security, etc. to build on this abstraction. -- We discuss in detail what "compensation" means in a Web services context, analyze proposed models and show how compensation can be implemented efficiently in hierarchical compositions. We identify two aspects, Cost of Compensation (CoC) and End User Involvement, missing from most of the proposed models. Current transaction models also constrain the act of compensation to the original service provider. Given the ability to bind with a service provider at run-time (dynamic binding), we believe that this is an unnecessary restriction and outline a mechanism for provider independent compensation. We also introduce the notion of side-effects in a hierarchical composition to determine if provider independent compensation is possible for a given scenario. Finally, we outline a compensation mechanism for hierarchical compositions incorporating the above aspects while conforming to the visibility restrictions modeled as So V. -- With respect to monitoring, we focus on capturing the state of a hierarchical Web services composition at any given point of time (snapshot). Such information is useful not only for reporting the current status to the end-user but also for answering specific queries related to the execution. Analogous to distributed systems, capturing the state of a hierarchical Web services composition is difficult because of the absence of a global observer, inherent nondeterminism, unexpected communication delays, etc. In addition, for Web services compositions, the "components" of the distributed system may not be known in advance (due to dynamic binding). We discuss in detail how some of the snapshot algorithms proposed in literature can be adapted for hierarchical Web services compositions. Snapshots usually reflect a state of the system which "might have occurred". We outline algorithms to acquire a state that "actually occurred", from such snapshots. Finally, we discuss how the acquired snapshots help us in answering execution status related queries.

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