1. A computer-implemented method for processing a stream of tuples, the method comprising: establishing an operator graph for a plurality of processing elements that each include a respective one or more operators configured to perform a respective one or more operations on tuples of streaming data, wherein the operator graph defines at least one execution path through the operators of the plurality of processing elements, and wherein a first processing element in the operator graph includes a barrier operator that joins output of a plurality of upstream operators included in the one or more of the plurality of processing elements; determining at least one timeout condition has occurred at the barrier operator; generating, in response to the determining at least one timeout condition has occurred at the barrier operator, substitute data tuple values for missing tuple output data from at least one of the plurality of upstream operators; generating a first barrier tuple output by joining the substitute data tuple values with received data tuple values from the plurality of upstream operators; storing a copy of the received data tuple values; receiving, at the barrier operator and after the storing of the copy of the received data tuple values, a set of one or more tuples; determining the set of one or more tuples comprises the missing tuple output data; and generating a second barrier tuple output by joining the missing tuple output data values with the copy of the received data tuple values.