1. A method for tracing dataflow in a distributed computing application, comprising: incrementally advancing a dataflow in a dataflow path of one or more dataflow paths according to two or more directives encoded in two or more data messages of the distributed computing application where the dataflow is being traced, the dataflow path comprising one or more operators comprising at least one merge operator operative to: (i) combine the two or more data messages; and (ii) combine the two or more directives; performing the two or more directives; forming an output data message from at least two of the two or more messages; forming an output directive from at least two of the two or more directives; and encoding the output directive in the output data message, wherein the incrementally advancing of the dataflow comprises providing one or more directives of the two or more directives by inferring a relationship between one or more input ports and one or more output ports of an operator of the one or more operators according to inference rules associated with evaluating externally observable behavior of the operator, and wherein one or more of the incrementally advancing of the dataflow and the performing of the two or more directives are implemented as instruction code performed on a processor device.