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From "raghul@yahoo.com" <rag...@yahoo.com>
Subject Divergence in ZK transaction logs in some corner cases?
Date Sat, 28 Mar 2009 01:49:27 GMT

ZK gurus,

I think the ZK transaction logs can diverge from one another in some corner cases. I have
one such corner case listed below, could you please confirm if my understanding is correct?

Imagine a 5 srever ensemble (A,B,C,D,E). All the servers are @ zxid 9. A is the leader and
it starts a new PROPOSAL (@zxid 10). A writes the proposal to the log, so A moves to zxid
10. Others haven't received the PROPOSAL yet and A crashes. Now the following happens:

1. B is elected as the newleader. B bumps up its in-mem zxid to 10. Since other nodes are
at the same zxid, it sends a SNAP so that the others can rebuild their data tree. In-memory
zxid of all other nodes moves to 10.  

2.  A comes back now, it accepts B as the leader as soon as the leader (B) and N/2 other nodes
vouch for B as the leader. So A joins the ensemble. Every zookeeper node is at zxid 10.

3. A new request is submitted to B. B runs PROPOSAL and COMMIT phases and the cluster moves
up to zxid 11. But the transaction log of A is different from that of everyone else now. So
the transaction logs have diverged.

Could you confirm if this can happen? Or am I reading the code wrong?

Thanks
Raghu


      

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