title: How to sync data between elasticsearch clusters?
body: <p>I want elasticsearch data backup in different physical location. 
  I have tried to put all elasticsearch nodes into a same cluster at first, but when program query or update elasticsearch, large data will transfer on internet. It will cause a lot of money for network traffic and there is a network delay.</p>
  
  <p>Is there any easy way to sync data between two elasticsearch clusters? so that I can only sync the changed data on the internet.</p>
  
  <p>PS:
  I don't so care about data sync delay, less then 1 min is acceptable</p>