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From "Bryan Duxbury (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2443) [hbase] Keep lazy cache of regions in client rather than an 'authoritative' list
Date Mon, 17 Dec 2007 20:56:43 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2443?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12552529
] 

Bryan Duxbury commented on HADOOP-2443:
---------------------------------------

So, this issue indicates that clients keep a dense list of regions and locations. Is there
a reason why they should keep a dense list instead of a sparse one? In production clusters,
there could be hundreds or thousands of regions, and caching them all seems to be pretty inefficient.


Instead, couldn't we just cache the regions we've found through trying to use them? That is,
when you're going to do some operation on a key, you ask the master/META regionserver to resolve
where your key belongs, and just remember the location and bounds of that region. That way,
you first check the cache to see if you already know where a key lives, and if it is a hit,
you go out and talk to the region server. If the server hasn't got that region anymore, you
get an NSRE, invalidate just the single cache item you used to get there. Isn't this how Bigtable
does it?

> [hbase] Keep lazy cache of regions in client rather than an 'authoritative' list
> --------------------------------------------------------------------------------
>
>                 Key: HADOOP-2443
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2443
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: contrib/hbase
>            Reporter: stack
>
> Currently, when the client gets a NotServingRegionException -- usually because its in
middle of being split or there has been a regionserver crash and region is being moved elsewhere
-- the client does a complete refresh of its cache of region locations for a table.
> Chatting with Jim about a Paul Saab upload issue from Saturday night, when tables are
big comprised of regions that are splitting fast (because of bulk upload), its unlikely a
client will ever be able to obtain a stable list of all region locations.  Given that any
update or scan requires that the list of all regions be in place before it proceeds, this
can get in the way of the client succeeding when the cluster is under load.
> Chatting, we figure that it better the client holds a lazy region cache: on NSRE, figure
out where that region has gone only and update the client-side cache for that entry only rather
than throw out all we know of a table every time.
> Hopefully this will fix the issue PS was experiencing where during intense upload, he
was unable to get/scan/hql the same table.

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