public InputOperatorImpl getInputOperator(SystemStream systemStream) {
    return this.inputOperators.get(systemStream);
  }
Get the {@link InputOperatorImpl} corresponding to the provided input {@code systemStream}.

@param systemStream  input {@link SystemStream}
@return  the {@link InputOperatorImpl} that starts processing the input message
samza-core/src/main/java/org/apache/samza/operators/impl/OperatorImplGraph.java