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https://issues.apache.org/jira/browse/ZOOKEEPER-1675?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13616477#comment-13616477
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Alexander Shraer commented on ZOOKEEPER-1675:
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Hi,
Its pretty much the same as an empty write, and perhaps an easy way to
implement it without introducing a new operation.
Unlike writes, I don't think we want to log the syncs. Anyway, users can
currently do "write + read" instead of "sync + read" to get strong semantics.
Good point about the connection change. Yes, this is a problem. I'm not sure
how this is implemented exactly, but intuitively we need the client to update
its last-seen-zxid once the sync completes. This way if the connection change
happens after the sync completes the client will only connect to someone at
least as up-to-date as the sync. And if the connection switch happens before
the sync completes then the sync will fail, which is also fine.
Alex
> Make sync a quorum operation
> ----------------------------
>
> Key: ZOOKEEPER-1675
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1675
> Project: ZooKeeper
> Issue Type: Bug
> Affects Versions: 3.4.0, 3.5.0
> Reporter: Alexander Shraer
>
> sync + read is supposed to return at least the latest write that completes
> before the sync starts. This is true if the leader doesn't change, but when
> it does it may not work. The problem happens when the old leader L1 still
> thinks that it is the leader but some other leader L2 was already elected and
> committed some operations. Suppose that follower F is connected to L1 and
> invokes a sync. Even though L1 responds to the sync, the recent operations
> committed by L2 will not be flushed to F so a subsequent read on F will not
> see these operations.
> To prevent this we should broadcast the sync like updates.
> This problem is also mentioned in Section 4.4 of the ZooKeeper peper (but the
> proposed solution there is insufficient to solve the issue).
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