A method of achieving fault tolerance in a distributed stream processing system organized as a directed acyclic graph includes the initial step of managing a stream process within the distributed stream processing system including one or more operators. The one or more operators of the stream process are communicatively associated with one or more downstream operators. The method includes the steps of maintaining one or more data copies of a processing state of the one or more operators until the one or more data copies can be safely discarded, notifying the one or more operators when it is safe to discard at least one of the at least one of the one or more data copies of the processing state; and using an identifier to denote the data copy of the processing state to be safely discarded.