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https://issues.apache.org/jira/browse/HBASE-7815?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13576176#comment-13576176
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Lars Hofhansl commented on HBASE-7815:
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Seems like an API fix that we should have in 0.94 as well.
> Too subtle behavior for HConnection#getRegionLocation reload parameter and
> performance risk
> -------------------------------------------------------------------------------------------
>
> Key: HBASE-7815
> URL: https://issues.apache.org/jira/browse/HBASE-7815
> Project: HBase
> Issue Type: Bug
> Components: Client, regionserver
> Affects Versions: 0.96.0
> Reporter: nkeywal
> Priority: Minor
>
> HConnection#getRegionLocation(table, row, reload=true) and
> HConnection#getRegionLocation(table, row, reload=false) are not equivalent
> when the cache is empty: the first will check the table status while the
> second will not.
> As a consequence, the client won't have the same exception if the table is
> disabled. With reload==true, we will have a DoNotRetryIOException, with a
> message saying that the table is disabled. With reload==false we will have a
> NotServingException. It's quite difficult to guess, as it's not mentioned in
> the javadoc.
> Second effect is that the client is going to ZooKeeper to check this table
> state. In ServerCallable, if the first try is not successful, we will then go
> all the time to ZK to check this status. So if a region server stops, all its
> clients will connect to ZK, possibly multiple time if the recovery takes some
> time. With a few hundreds clients, it's not very nice...
> I'm not sure of the solution. A possible improvement in ServerCallable would
> be to do a reload only at the first retry instead of all of them, but:
> - it's not without side effects, even if it's limited
> - the real cost is the first try, as it may creates a ZK connection.
> Another thing to do would be to limit the reload to the case it makes sense.
> In locateRegionInMeta there is a test on the exception:(e instanceof
> RegionOfflineException || e instanceof NoServerForRegionException).
> May be this logic could be put in ServerCallable as well, but we need to
> cover all cases.
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