Decentralized establishment of consistent, multi-lateral collaborations

Multi-lateral collaborations are based on the interaction of several parties. In particular, each party contributes different tasks to the execution of the collaboration. The coordination of these different tasks, that is, the handling of the dependencies between the different tasks, is known as a workflow. When this coordination ensures a successful interaction between the different parties the workflow is called consistent, guaranteeing deadlock-freeness. Currently, a multi-lateral collaboration is set up by people meeting and discussing the collaboration, specifying the workflow (also called the global workflow) and checking its consistency. Afterwards the global workflow is split into parts (also called local workflows) performed by the individual parties. Following this top-down approach, the combination of the local workflow guarantees consistency of the global workflow. Applying a bottom up approach, that is, deriving global properties from local ones is known to be hard in distributed systems. Thus, the issue is to provide an approach which can determine global consistency based on local consistency decisions. Recent technologies such as Service Oriented Architecture (SOA) support decentralized and loosely coupled applications. In particular, parties make the provided functionality available as a service, which is maintained and controlled completely independent of a centralized coordinator. Further, the loose coupling supports the late binding of services, that is, a service requestor may decide at run-time which service provider to use for that particular collaboration. As a consequence of these decentralized decisions and the lack of a centralized coordinator, the top-down approach is not applicable to SOA, but requires a bottom up approach. Deciding consistency of a global workflow in a decentralized way requires additional local information which is provided by the method proposed in this thesis. In particular, information on parameter constraints and execution sequences between local workflows has to be exchanged and propagated through the collaboration to gather sufficient information. It is shown that this propagated information suffices to determine global workflow consistency in a decentralized way. Further, Web Services are used as a concrete technology supporting the SOA paradigm and the theoretical results are applied to this technology as a proof of concept to illustrate the applicability of the approach presented. The approach can be applied to synchronous or asynchronous communication models. While there already exist approaches for asynchronous communication models, there are none available for the synchronous case. Therefore, a formal model for synchronous communication is introduced which is called annotated Finite State Automata. This model extends standard Finite State Automata by annotating states with logical expressions to differentiate between mandatory and optional automata transitions. An optional transition can be illustrated by a party providing the option to receive one of two messages, where the interacting party may use one of the options. Optional transitions represent standard automata semantics. However, a mandatory transition can be illustrated by a party sending either one of two messages, where the receiving party is required to support both options, because supporting only a single option results in a deadlock if the sender selects the other option. This formal model is introduced and discussed in detail for bilateral and multi-lateral collaborations. In particular, the propagation of parameter constraints and execution sequences are defined based on this model and the construction of the corresponding global workflow is introduced. For the asynchronous communication model, Workflow Nets are used as a formal model based on an existing approach to constructing the corresponding global workflow. However, since the computational complexity of Workflow Nets prevents satisfactory application of the propagation definitions, the execution sequences of the Workflow Nets involved are represented as annotated Finite State Automata and thus allow handling of the synchronous and asynchronous communication model based on a single formal model, that is, annotated Finite State Automata. Based on the common formal model, it can be shown that the propagation of parameter constraints and execution sequence constraints result in a fixed point, where further propagation will not change the local workflow any further. Based on this fixed point, it can be shown that if all parties decide locally that the workflow is consistent then the global workflow is also consistent. Therefore, the final consistency decision is based on the consensus of the parties' local consistency decisions. The approach is implemented within the Web Services framework. In particular, a partial mapping of the Web Service process specification language (Business Process Execution Language for Web Services (BPEL)) to annotated Finite State Automata is provided. Based on this mapping an extension of standard service discovery, considering process descriptions in terms of BPEL, is implemented. Further, the relevant operations for annotated Finite State Automata are implemented. Finally, in order to be able to apply the approach presented, a decentralized collaboration establishment approach is introduced.