WORKFLOW EVOLUTION

A basic step towards flexibility in workflow systems is the consistent and effective management of workflow evolution, i.e. of changing exis ting workflows while they are operational. One of the most challenging issue is the handling of running instances when their schemata are mo dified: simple solutions can be devised, but they often imply losing a ll the work done or failing in capturing the advantages offered by wor kflow modifications; this is unacceptable for many applications. In th is paper we address the problem of workflow evolution, from both a sta tic and a dynamic point of view. We define a complete, minimal, and co nsistent set of modification primitives that allow modifications of wo rkflow schemata and we introduce a taxonomy of policies to manage evol ution of running instances when the corresponding workflow schema is m odified. Formal criteria are introduced, based on a simple workflow co nceptual model, in order to determine which running instances can be t ransparently migrated to the new version. A case study, relating the a ssembling of a desktop computer, will exemplify the introduced concept s.