Architectural support for dynamic reconfiguration of distributed workflow applications

In a distributed environment, it is inevitable that long running applications require support for dynamic reconfiguration because, for example, machines may fail, services may be moved or withdrawn and user requirements may change. In such an environment it is essential that the structure of running applications can be modified to reflect such changes. A complication is that such long running applications are frequently composed out of existing applications. The resulting application can be very complex in structure, containing many temporal dependencies between constituent applications. This paper describes an approach that supports the dynamic reconfiguration of a class of large-scale distributed applications that represent business processes (commonly referred to as workflows). An application composition and execution environment has been designed and implemented as a transactional workflow system that enables sets of inter-related tasks (applications) to be carried out and supervised in a dependable manner. A task model that is expressive enough to represent temporal dependencies between constituent tasks has been developed. The workflow system maintains this structure and makes it available through transactional operations for performing changes to it. Use of transactions ensures that changes can be carried out atomically with respect to running applications. The workflow system is general purpose and open: it has been designed and implemented as a set of CORBA services to run on top of a given ORB