Workflow Specification Static specification of task coordination: Tasks and dependencies among them are defined before the execution of the workflow starts Can establish preconditions for execution of each task:tasks are executed only when their preconditions are satisfied. Defined preconditions through dependencies: Execution states of other tasks. "task t;cannot start until task t,has ended" Output values of other tasks. "task t can start if task t;returns a value greater than 25" External variables,that are modified by external events. task t must be started within 24 hours of the completion of task t" Database System Concepts,5th Ed. 25.14 ©Silberschat乜，Korth and SudarshanDatabase System Concepts, 5 25.14 ©Silberschatz, Korth and Sudarshan th Ed. Workflow Specification Static specification of task coordination: Tasks and dependencies among them are defined before the execution of the workflow starts. Can establish preconditions for execution of each task: tasks are executed only when their preconditions are satisfied. Defined preconditions through dependencies:  Execution states of other tasks. “task ti cannot start until task tj has ended”  Output values of other tasks. “task t i can start if task t j returns a value greater than 25”  External variables, that are modified by external events. “task ti must be started within 24 hours of the completion of task tj