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From Thomas Vinod Johnson <Thomas.John...@Sun.COM>
Subject Re: What happens when a server loses all its state?
Date Wed, 17 Dec 2008 00:02:27 GMT
Mahadev Konar wrote:
> Hi Thomas,
>> More generally, is it a safe assumption to make that the ZooKeeper
>> service will maintain all its guarantees if a minority of servers lose
>> persistent state (due to bad disks, etc) and restart at some point in
>> the future?
> Yes that is true. 
Great - thanks Mahadev.

Not to drag this on more than necessary, please bear with me for one 
more example of 'amnesia' that comes to mind. I have a set of ZooKeeper 
servers A, B, C.
- C is currently not running, A is the leader, B is the follower.
- A proposes zxid1 to A and B, both acknowledge.
- A asks A to commit (which it persists), but before the same commit 
request reaches B, all servers go down (say a power failure).
- Later, B and C come up (A is slow to reboot), but B has lost all state 
due to disk failure.
- C becomes the new leader and perhaps continues with some more new 

Likely I'm misunderstanding the protocol, but have I effectively lost 
zxid1 at this point? What would happen when A comes back up?


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