As the "openflow-spec-v1.3.1"said ,"When the controller wants to ensure message dependencies have been met or wants to receive notica-
tions for completed operations, it may use an OFPT_BARRIER_REQUEST message. "

The controller may use a barrier message to ensure that previous messages have been fully processed.
To do this, the controller will send an OFPT_BARRIER_REQUEST message. The network element will finish processing all previous messages including sending replies and/or error messages before executing any additional commands. When it completes processing all previous messages, it will send the OFPT_BARRIER_REPLY message.

Now i need new functions that the barrier message should be sent after flowmod being sent to ensure that the flowmod message has been processed successfully and the APP over the ODL could get the result about the barrier message through the RESTAPI.