An apparatus to facilitate design of a stream processing flow that satisfies an objective, wherein the flow includes at least three processing groups, wherein a first processing group includes a data source and an operator, a second processing group includes a data source and an operator and a third processing group includes a join operator at its input and another operator, wherein data inside each group is organized by channels and each channel is a sequence of data, wherein an operator producing a data channel does not generate new data for the channel until old data of the channel is received by all other operators in the same group, and wherein data that flows from the first and second groups to the third group is done asynchronously and is stored in a queue if not ready for processing by an operator of the third group.