The method can include receiving the first stream of tuples to be processed by the stream operator hosted by one or more computer processors, the stream operator having at least one input port and a condition input port. The method can also include receiving, at runtime, a customized condition at the condition input port of the stream operator. The method can also include monitoring a first group of one or more tuples from the first stream of tuples at a first window of the stream operator. The method can also include determining whether the first group of one or more tuples at the first window fulfills the customized condition. The method can also include deleting, in response to the first group of one or more tuples fulfilling the customized condition, the first group of one or more tuples.