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From Mudit Verma <mudit.f2004...@gmail.com>
Subject Watch: One time event -> Loss of event
Date Thu, 12 Jun 2014 15:40:22 GMT
Hi All, 

In my last couple of projects, I have heavily used zookeeper watches. However, ZK watch is
a one time event.  

I strongly feel that zookeeper should have a way to set a watch permanently (unless removed
by the user through an API or the znode is deleted). This would guarantee that no data change
event is missed by the client. Otherwise, we can miss data change event that occurs between
receiving the call back from previous watch and resetting the new watch again.  This problem
is also stated in documentation. 

I am sure even others might have felt this need sometime or the other.  This missing capability
makes ZK watcher functionality best effort and not full proof.  Consequently, clients who
can not afford to miss any event, can not use ZooKeeper reliably. 

I think we should have these APIs unless there is some theoretical or practical problem with

1. API to set a watch permanently.  
2. Removing a watch when not needed. 

Looking forward for a discussion.


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