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From "Van Klaveren, Brian N." <b...@slac.stanford.edu>
Subject Re: Question about the two-phrase commit
Date Wed, 07 Jan 2015 00:26:30 GMT


From http://zookeeper.apache.org/doc/r3.4.6/zookeeperInternals.html:

There is a corner case in which a follower that has proposals, U, not seen by a leader arrives.
Proposals are seen in order, so the proposals of U will have a zxids higher than zxids seen
by the leader. The follower must have arrived after the leader election, otherwise the follower
would have been elected leader given that it has seen a higher zxid. Since committed proposals
must be seen by a quorum of servers, and a quorum of servers that elected the leader did not
see U, the proposals of you have not been committed, so they can be discarded. When the follower
connects to the leader, the leader will tell the follower to discard U.


-Brian


On Jan 6, 2015, at 4:19 PM, Ibrahim <i.s.el-sanosi@newcastle.ac.uk<mailto:i.s.el-sanosi@newcastle.ac.uk>>
wrote:

Thank you Edward for replaying.

In my opinion, your first case can’t happen, because the election mechanism will elect the
node that has the latest transaction  (zxid) seen. In this case, it will be only your second
case is correct, unless the node that has latest transaction crashes forever.

Ibrahim
From: Edward Carter [via zookeeper-user] [mailto:ml-node+s578899n7580741h95@n2.nabble.com]
Sent: Tuesday, January 06, 2015 11:53 م
To: Ibrahim El-sanosi (PGR)
Subject: Re: Question about the two-phrase commit

If an operation is logged to disk on a minority of servers, then what happens on restart depends
on which server wins the leader election.  Since the new operation is on a minority, depending
on network conditions, a server that did *not* log the operation might win the leader election.
 In this case, the operation will be discarded.  If a server that *did* log the operation
wins the leader election (as would be the case in the absence of network or latency issues),
then the operation will be propagated to the remaining servers in the ensemble.

Edward

On Jan 6, 2015, at 3:42 PM, Ibrahim <[hidden email]</user/SendEmail.jtp?type=node&node=7580741&i=0>>
wrote:

Hi Alexander,

So, you meant that if there is operation loged to the disk but it hasn't got
enough majority, then if the zookeeper servers restart, the new leader will
re-broadcast the loged operation again to the followers to commit that
operation.

Does above what you meant by (NEWLEADER message acts as the accept and
UPTODATE message acts as commit).

Am I right?

Thank you

Ibrahim



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