if recover worked the way you outline, we would have a problem indeed. fortunately, we specifically address this case.

the problem is in your first step. when b is elected leader, he will not proposal 10, he will propose 100000000000001. the zxid is made up of two parts, the high order bits are an epoch number and the low order bits are a counter. when every a new leader is elected, he will increment the epoch number and reset the counter.

when A restarts you have the opposite problem, you need to make sure that A forgets 10 because we have skipped it and committing it will mean that 10 is delivered out of order. we take advantage of the epoch number in that case as well to make sure that A forgets about 10.

there is some discussion about this in: http://hadoop.apache.org/zookeeper/docs/r3.1.1/zookeeperInternals.html#sc_atomicBroadcast

we have a presentation as well that i'll put up that may make it more clear.


rag...@yahoo.com wrote:
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?


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