Manage Connections Through Reference Counts
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Key: HBASE-3766
URL: https://issues.apache.org/jira/browse/HBASE-3766
Project: HBase
Issue Type: Improvement
Components: client
Affects Versions: 0.90.2
Reporter: Karthick Sankarachary
Assignee: Karthick Sankarachary
Priority: Minor
Fix For: 0.90.3
As of now, the onus is on the developer to explicitly close connections once
they're done using them. Furthermore, since connections are managed based on
the identity of the {{Configuration}} object, one is forced to clone the
configuration object in order to be able to clean it up safely (for a case in
point, see {{HTablePool's}} constructor). As a matter of fact, this issue has
been well-documented in the HConnectionManager class:
{quote}
But sharing connections makes clean up of {{HConnection}} instances a little
awkward. Currently, clients cleanup by calling
{{#deleteConnection(Configuration, boolean)}}. This will shutdown the
zookeeper connection the {{HConnection}} was using and clean up all
{{HConnection}} resources as well as stopping proxies to servers out on the
cluster. Not running the cleanup will not end the world; it'll just stall the
closeup some and spew some zookeeper connection failed messages into the log.
Running the cleanup on a {{HConnection}} that is subsequently used by another
will cause breakage so be careful running cleanup. To create a {{HConnection}}
that is not shared by others, you can create a new {{Configuration}} instance,
pass this new instance to {{#getConnection(Configuration)}}, and then when
done, close it up by doing something like the following:
{code}
Configuration newConfig = new Configuration(originalConf);
HConnection connection = HConnectionManager.getConnection(newConfig);
// Use the connection to your hearts' delight and then when done...
HConnectionManager.deleteConnection(newConfig, true);
{code}
{quote}
Here, we propose a reference-count based mechanism for managing connections
that will allow {{HTables'}} to clean up after themselves. In particular, we
extend the {{HConnectionInterface}} interface so as to facilitate reference
counting, where, typically, a reference indicates that it is being used by a
{{HTable}}, although there could be other sources.
To elaborate, when a HTable is constructed, it increments the reference count
on the connection given to it. Similarly, when it is closed, that reference
count is decremented. In the event there are no more references to that
connection, {{HTable#close}} takes it upon itself to delete the connection,
thereby sparing the developer from doing so.
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