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https://issues.apache.org/jira/browse/ZOOKEEPER-1277?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13149973#comment-13149973
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Flavio Junqueira commented on ZOOKEEPER-1277:
---------------------------------------------
The scenario I have in mind to say this is incorrect is more or less the
following:
# Leader L is currently in epoch 3 and it moves to epoch 4 in the way this
patch proposes by simply adding 2 to hzxid. The leader proposes a transaction
with zxid <4,1>, which is acknowledged by some follower F, but not a quorum;
# Concurrently, a new leader L' arises and selects 4 as its epoch (it hasn't
talked to L or F);
# L' proposes a transaction with zxid <4,1>, which is different from the
transaction L proposed with the same zxid and this transaction is acknowledged
by a quorum;
# L eventually gives up on leadership after noticing that it is not supported
by a quorum;
# L' crashes;
# A new leader arises and its highest zxid is <4,1>. It doesn't have to
synchronize with any of the followers because they all have highest zxid <4,1>.
We have servers that have different transaction values for the same zxid, which
constitutes an inconsistent state.
> servers stop serving when lower 32bits of zxid roll over
> --------------------------------------------------------
>
> Key: ZOOKEEPER-1277
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1277
> Project: ZooKeeper
> Issue Type: Bug
> Components: server
> Affects Versions: 3.3.3
> Reporter: Patrick Hunt
> Assignee: Patrick Hunt
> Priority: Blocker
> Fix For: 3.3.4
>
> Attachments: ZOOKEEPER-1277_br33.patch
>
>
> When the lower 32bits of a zxid "roll over" (zxid is a 64 bit number, however
> the upper 32 are considered the epoch number) the epoch number (upper 32
> bits) are incremented and the lower 32 start at 0 again.
> This should work fine, however in the current 3.3 branch the followers see
> this as a NEWLEADER message, which it's not, and effectively stop serving
> clients. Attached clients seem to eventually time out given that heartbeats
> (or any operation) are no longer processed. The follower doesn't recover from
> this.
> I've tested this out on 3.3 branch and confirmed this problem, however I
> haven't tried it on 3.4/3.5. It may not happen on the newer branches due to
> ZOOKEEPER-335, however there is certainly an issue with updating the
> "acceptedEpoch" files contained in the datadir. (I'll enter a separate jira
> for that)
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