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From "James Kennedy (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1528) HClient for multiple tables
Date Wed, 27 Jun 2007 20:50:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12508646
] 

James Kennedy commented on HADOOP-1528:
---------------------------------------

If by "HBase instance" we mean a Configuration object pointing to a specific HMaster, then
one HConnection per HBase instance is how i've implemented it.  The static methods in HConnection
ensure that HConnections are initialized as needed based on given Configuration object.  After
that, any requests for an HConnection are keyed by Configuration. Repeated requests always
get the same HConnection for a given Configuration.

Imagine using a single HConnection for many transactions across many (sometimes the same)
tables.  You don't want to "close" an HTable because another transaction on the same table
may execute moments afterwards... the caching has been centralized to avoid repeated meta
lookups. 

Only when an HTable times out accessing an HRegion and needs to findRegion(),  does it call
a HConnection.closeTable() which essentially clears the cache for that table.  This is followed
by  a synchronized HConnection.getTableServers() method which will update the cache with the
new regions.





> HClient for multiple tables
> ---------------------------
>
>                 Key: HADOOP-1528
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1528
>             Project: Hadoop
>          Issue Type: Task
>          Components: contrib/hbase
>    Affects Versions: 0.14.0
>            Reporter: James Kennedy
>
> I have an app that needs to access multiple HBase tables concurrently.  The current HClient
can only have one table open at a time even though it caches region servers of multiple tables
as they are looked up.
> This means that my application layer must open multiple HClients, one per table, perhaps
caching those HClients in a pool to reuse them (and their cached table data) as appropriate.
> or
> Shall I write an HClient patch that makes the HClient  multi-table thread-safe?
> Jim's suggestion is to implement an HClient singleton (call it HClientManager?) that
does the actual caching/resync of root/meta regions.  Individual HClients will still be one
table, one update row at a time but will rely on the singleton for the cached table info.
 We want HClients to be created and disposed as fast as possible with a minimum of meta lookups.
> Jim, what about non-root/meta regions, shouldn't they be cached and refreshed via the
singleton also?  It may still be possible that a region split/resync will occur during on
HClient session so does the HClientManager need to be able to notify the corresponding HClients
in that event?

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