Using the Lock Service
Use the lock service when you have a business process or resource within a business process that should never have more than one instance running at a time.
The Lock service will need to be included twice in the business process. The first instance to lock the resource and the second instance to unlock the resource. Generally the lock duration is set to 2-3 times what the estimated time of execution will be so that the lock does not time-out before the business process reaches the unlock step.