Managed function execution for processing data streams in real time may be. A function that describes one or more operations to be performed with respect to one or more data streams may be received via programmatic interface for a managed stream processing system. Stream processing nodes capable of applying the function may be determined and execution of the one or more operations may be initiated at the stream processing nodes as data records of the data stream are received. Results of the application of the processing function may be provided to one or more destinations specified for the function. Performance metrics may also be collected for the execution of the function and provided to a client that submitted the function.