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https://issues.apache.org/jira/browse/HBASE-24603?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17142469#comment-17142469
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Bharath Vissapragada commented on HBASE-24603:
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Some tests have regressed with the sync() call never returning, which is weird
since there shouldn't be many transactions in the test context for the
followers to catchup. I'm trying to dig into the ZK code to understand it
better to see if its a ZK bug.
> Zookeeper sync() call is async
> ------------------------------
>
> Key: HBASE-24603
> URL: https://issues.apache.org/jira/browse/HBASE-24603
> Project: HBase
> Issue Type: Improvement
> Components: master, regionserver
> Affects Versions: 3.0.0-alpha-1, 2.3.0, 1.7.0
> Reporter: Bharath Vissapragada
> Assignee: Bharath Vissapragada
> Priority: Critical
> Fix For: 3.0.0-alpha-1, 2.3.1, 1.7.0, 2.4.0
>
>
> Here is the method that does a sync() of lagging followers with leader in the
> quorum. We rely on this to see a consistent snapshot of ZK data from multiple
> clients. However the problem is that the underlying sync() call is actually
> asynchronous since we are passing a 'null' call back. See the ZK API
> [doc|https://zookeeper.apache.org/doc/r3.5.7/apidocs/zookeeper-server/index.html]
> for details. The end-result is that sync() doesn't guarantee that it has
> happened by the time it returns.
> {noformat}
> /**
> * Forces a synchronization of this ZooKeeper client connection.
> * <p>
> * Executing this method before running other methods will ensure that the
> * subsequent operations are up-to-date and consistent as of the time that
> * the sync is complete.
> * <p>
> * This is used for compareAndSwap type operations where we need to read the
> * data of an existing node and delete or transition that node, utilizing
> the
> * previously read version and data. We want to ensure that the version
> read
> * is up-to-date from when we begin the operation.
> */
> public void sync(String path) throws KeeperException {
> this.recoverableZooKeeper.sync(path, null, null);
> }
> {noformat}
> We rely on this heavily (at least in the older branches that do ZK based
> region assignment). In branch-1 we saw weird "BadVersionException" exceptions
> in RITs because of the inconsistent view of the ZK snapshot. It could
> manifest differently in other branches. Either way, this is something we need
> to fix.
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