Embodiments of the disclosure provide a method, system, and computer program product for managing a windowing operation. The method can include determining a sentinel value that defines a start of a grouping window for a stream of tuples and a terminating sentinel value that defines the end of the grouping window based upon an attribute contained in the stream of tuples. The stream of tuples can be monitored for the sentinel value and the terminating sentinel value by a stream operator. The stream operator can initiate a windowing operation that defines the start of the grouping window in response to a presence of the sentinel value and terminate the windowing operation in response to a presence of the terminating sentinel value.