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https://issues.apache.org/jira/browse/SOLR-14425?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17383717#comment-17383717
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Mark Robert Miller commented on SOLR-14425:
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It's unlikely Solr tests would ever pick up any issue around using sync() or 
not given that we run tests with a single ZK instance and sync is for the case 
when client1 writes something to zk, communicates to client2 it should not find 
that in ZK, and client2 does a read on a different ZK instance that does not 
yet see the update on it's read. It's a workaround for zk not providing 
linearizable reads (as it does writes), which is done for much better read 
performance. But it's not likely to matter one way or another with a single zk 
instance as your zk cluster.

> Fix ZK sync usage to be synchronous (blocking)
> ----------------------------------------------
>
>                 Key: SOLR-14425
>                 URL: https://issues.apache.org/jira/browse/SOLR-14425
>             Project: Solr
>          Issue Type: Bug
>          Components: SolrCloud
>            Reporter: David Smiley
>            Priority: Minor
>          Time Spent: 0.5h
>  Remaining Estimate: 0h
>
> As of this writing, we only use one call to ZK's "sync" method.  It's related 
> to collection aliases -- I added this.  I discovered I misunderstood the 
> semantics of the API; it syncs in the background and thus returns 
> immediately.  Looking at ZK's sync CLI command and Curator both made me 
> realize my folly.  I'm considering this only a "minor" issue because I'm not 
> sure I've seen a bug from this; or maybe I did in spooky test failures over a 
> year ago -- I'm not sure.  And we don't use this pervasively (yet).
> It occurred to me that if Solr embraced the Curator framework abstraction 
> over ZooKeeper, I would not have fallen into that trap.  I'll file a separate 
> issue for that.



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