Preconditions, Postconditions, and Provisional Execution in Software Processes

Preconditions and postconditions are widely used in execution models for software processes. The conditions that control software processes, however, can be complex and di cult to evaluate in the context of ongoing development activities. If evaluations and tasks are conducted in parallel, evaluations may be blocked for lack of access to data that are held by tasks. Conversely, task initiations and terminations may be blocked because their controlling conditions cannot be evaluated. Costly delays or even deadlocks may result. Potential con icts between condition evaluations and process tasks can be reduced by a number of strategies, such as limiting the extent of conditions, or serializing evaluations and tasks. However, each of these strategies has its drawbacks (such as failure to capture full process semantics, or reduced process concurrency). This paper introduces a new strategy for condition-based process execution, specifically, provisional execution of process tasks. When it is not possible to completely evaluate all preconditions or postconditions because of con icts with ongoing activities, a manager may allow some tasks to be executed provisionally. That is, a task may be initiated before all its preconditions are satis ed, or terminated before all its postconditions are satis ed. The blocked conditions are not ignored; rather, their evaluation is deferred but still required. These conditions must be satis ed or the task is subject to abort. Provisional execution allows increased exibility in both the execution of process tasks and the evaluation of controlling conditions. It can lead to increased concurrency, reduced concurrency con icts, and generally greater adaptability in process execution. 2