title: run nifi flow once and notify me when it is finish
body: <p>I use rest api in my program,I made a processor group for convent a mongodb collection to json file:
  <a href="https://i.stack.imgur.com/0zZ65.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0zZ65.png" alt="enter image description here"></a>
  I want to run the scheduling only one time,so I set the "Run schedule" to 10000 sec.Then I will stop the group when the data flow have ran one time,and I made a Notify processor and add a DistributedMapCacheService.But the DistributedMapCacheClientService of the Notify processor only comunicates with the DistributedMapCacheService in nifi itself,It never nofity my program.</p>
  
  <p>I try to use my own socket server,but I only get a message "nifi" but no more message.
  My question is:If I only want scheduling run once and stop it,how do I know when shall I stop it?Or is there some other way to achieve my purpose,like detect if the json file exists or use incremental data(If the scheduling run twice,the data will be repeated twice)?</p>