How to increase flexibility has always been an issue relating tothe applicability of any workflow system. We address this issue in onecase where an agent needs to alter the control flow prescribed in theoriginal definition to some previous executions. This phenomenon istermed ad-hoc recovery. Ad-hoc recovery is usually caused byunpredictable reasons, such as unexpected output Of some individualtasks, events or exceptions due to the changing environment. Due to itsirregularity in nature, ad-hoc recovery in general cannot be dealt withby approaches to handling failure recoveries. On the other hand, sinceit allows normal control flows to be diverged from at run time withoutmodifying workflow schema, it enjoys higher flexibility than the fixedschema systems while pays a lower cost than schema evolution. In thispaper, we introduce the concept of ad-hoc recovery, and characterize itsmain features. We also discuss some implementation issues
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