How activiti handles the exception state? If error occurs in any of the task how the flow will move back to a step from where the task can be re executed again?
There are a couple of answers to this question. If the task is a java or script task and is running synchronously, then resumption of a failed instance (usually caused by a technical exception) with be from the last persisted state (remember, a BPM engine is really just a state machine).
Now, you can (obviously) add exception handlers (especially in 5.15 which introduces event handlers) for a specific task or for the process instance as a whole. These exception handler allow you to control transaction rollback (depends on the transaction manager you are using).
Finally, you can bubble a bpmnEvent up to the process model and handle exceptions/compensation using an attached/boundary event handler in the BPMN model.
Hope this helps. Greg Harley.