Abstract
The need for exibility in workows and the problem of unanticipated exceptions are well recognized, and in fact a number of solutions have already been proposed in areas such as computer-supported collaborative work and software process modeling. On the other hand, workow systems frequently maintain forms and other information in databases, and there is also strong evidence for the need to consider exceptions to the constraints imposed by the schema on the data in databases. This paper presents a framework for treating both kinds of exceptions uniformly by applying ideas from programming language exception handling (but where the responsible agents of workows are allowed to handle exceptions on-line) to a situation where most aspects of workows have been rei ed as persistent ob jects in special classes and with special attributes. As a result, only a small number of new constructs need to be introduced, and power is achieved through orthogonality. Unlike most previous work, the framework pays particular attention to the consequences of permitting deviations from the norm to persist, in part by de ning exceptions as violations of constraints and then developing a taxonomy of constraints. Also as a departure from prior e orts, the paper takes the view that the evolution of the schema (workow or data) is a di erent problem from that of individual exceptions, but that a limited form of incremental evolution { providing exception handlers ahead of time { is a useful abstraction mechanism to help organize the details of complex workows.