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https://issues.apache.org/jira/browse/SOLR-14425?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17100795#comment-17100795
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David Smiley commented on SOLR-14425:
-------------------------------------

ZK sync enqueues a sync into a communication channel and returns immediately.  
The normal use is that you _then_ do a read of something, which will occur 
_after_ the sync is done.  So as a caller of ZK sync, you don't really need to 
add a callback unless there is no immediate read following after -- perhaps if 
you need to return to a CLI or another process that will then do something.

I do understand the cause of the spooky test failures -- my comment on that 
failure see 
https://issues.apache.org/jira/browse/SOLR-12386?focusedCommentId=17089813&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-17089813
I've been mulling over what to do about this, and to educate myself further on 
ZK / Curator.

> 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
>      Security Level: Public(Default Security Level. Issues are Public) 
>          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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