1. A computer program product for processing streaming data in a data processing system, the computer program product comprising: a computer readable storage medium for execution by a processing circuit for performing a method comprising: receiving a stream of data; determining whether a new event of the stream of data has a key which is the same key as a previous event being currently maintained as a stored event; in response to determining the new event has the same key as the previous event, replacing the previous event with the new event and removing from an aggregation an effect of the previous event; including in the aggregation one or more values of the new event; and outputting the aggregation.