A controller for controlling the processing of a data stream by a data stream processing system having a plurality of data stream management systems, DSMSs. Each DSMS is arranged to execute a respective continuous query to generate an output data stream having continuous query execution results. The controller controls the execution of the continuous query on a windowed portion of a data stream when different DSMSs receive different parts of the data for the windowed portion. The controller selects, for processing of the windowed portion of the data stream, a single DSMS that is to execute the respective continuous query. The controller generates a control signal to cause only the selected DSMS to execute the continuous query using data in the data stream received thereby, so that the continuous query is executed on data in the windowed portion of the data stream only by the selected DSMS.