In one implementation, a system for processing a data stream can comprise a station engine, an execution engine, and a synchronize engine. A station engine can provide a stream operator to receive application logic, punctuate the data stream, and determine a number of input channels for parallel processing. The execution engine can perform a behavior of the application logic during a process operation. The synchronize engine can hold data of the data stream associated with a window until each input channel has reached a data boundary based on a boundary parameter.