Workkow Evolution ?

A basic step towards exibility in workkow systems is the consistent and eeective management of workkow evolution, i.e. of changing existing workkows while they are operational. One of the most challenging issue is the handling of running instances when their schemata are modiied: simple solutions can be devised, but they often imply loos-ing all the work done or failing in capturing the advantages ooered by workkow modiications; this is unacceptable for many applications. In this paper we address the problem of workkow evolution, from both a static and a dynamic point of view. We deene a complete, minimal, and consistent set of modiication primitives that allow modiications of workkow schemata and we introduce a taxonomy of policies to manage evolution of running instances when the corresponding workkow schema is modiied. Formal criteria are introduced, based on a simple workkow conceptual model, in order to determine which running instances can be transparently migrated to the new version. A case study, relating the assembling of a desktop computer, will exemplify the introduced concepts.