1. A computer-implemented method for processing a data stream, comprising: executing, by operation of one or more computer processors, a first operation on a first tuple of the data stream by a first processing element to produce a first result tuple; executing a second operation on the first tuple of the data stream by a second processing element, wherein the second operation is executed in parallel with the first operation; determining one or more attributes of the first tuple used in executing the second operation on the second processing element; and comparing the determined one or more attributes of the first tuple with attributes of the first result tuple produced from executing the first operation by the first processing element, to determine whether the second operation executed by the second processing element is valid.