改进Sagas事务模型实现工作流中的任务回滚

raditional workflow management systems lack the abilities of handling transactions, thus cannot recover quickly enough when it fails, affecting the applying of workflow. Sagas transaction model, a long lived transaction which is composed of ordered transactions set. If a transaction is abnormal, the compensate transactions will be implemented in the opposite direction to remove the results and quite. This can cause a waste of resource. An improved Sagas is proposed, if a transaction is abnormal, do part of the compensate transactions to get back to a rollback point. And apply the improved model to workflow to settle the problem of return a task and cancel a task, so when there is a failure the users can do something to remedy.