code: private String getEdgeId(String source, String destination, String edgeName) {
      return Joiner.on(FLOW_EDGE_LABEL_JOINER_CHAR).join(source, destination, edgeName);
    }
docs: Get an edge label from the edge properties
  @param source source data node id
  @param destination destination data node id
  @param edgeName simple name of the edge (e.g. file name without extension of the edge file)
  @return a string label identifying the edge