[jira] [Commented] (HBASE-3766) Manage Connections Through Reference Counts

["Jean-Daniel Cryans (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-3766?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13018572#comment-13018572 ] 

Jean-Daniel Cryans commented on HBASE-3766:
-------------------------------------------

First, thanks for starting this discussion.

About the patch, I see you increment in HTable's constructor, but if the user is reusing a conf object then the counter becomes wrong right?

Also the counter itself isn't accessed in a synchronize way (as multiple threads using different HTables would need) so this could pose a problem.

> Manage Connections Through Reference Counts
> -------------------------------------------
>
>                 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
>
>         Attachments: HBASE-3766.patch
>
>   Original Estimate: 1h
>  Remaining Estimate: 1h
>
> 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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