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