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https://issues.apache.org/jira/browse/HIVE-584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12754280#action_12754280
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Ning Zhang commented on HIVE-584:
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Just had an offline conversation with Zheng, trying to understand the concepts:
Life time:
- Session is the longest living object. It corresponds to the life time from
user connecting to Hive using Hive CLI or Hive Server till closing it.
- The DB connection life time is the time that Hive connects to the
metastore. Currently its life time is always the same as that of the session.
But going forward, it could be different since a session could be connecting to
different metastores (by providing a different HiveConf). This is useful if
there are multiple instances of Hadoop clusters, each of which maintain a
different Hive DB.
> Clean up global and ThreadLocal variables in Hive
> -------------------------------------------------
>
> Key: HIVE-584
> URL: https://issues.apache.org/jira/browse/HIVE-584
> Project: Hadoop Hive
> Issue Type: Improvement
> Affects Versions: 0.3.0, 0.3.1
> Reporter: Zheng Shao
> Fix For: 0.5.0
>
> Attachments: hive-584-2009-9-10.patch, hive-584-2009-9-11.patch
>
>
> Currently in Hive code there are several global and ThreadLocal variables
> that need to be cleaned.
> Specifically, the following classes are involved:
> 1. HiveConf: contains hive configurations (and a classloader)
> 2. Hive class: contains a static member Hive db. Hive class contains a member
> HiveConf conf, as well as a ThreadLocal storage of IMetaStoreClient.
> 3. SessionState: contains a static ThreadLocal storage of SessionState.
> SessionState class contains a Hive db, a HiveConf conf, a history logger, and
> a bunch of standard input/output streams
> 4. CliSessionState: SessionState plus some command options and the command
> file name.
> 5. All classes that try to get Hive db or HiveConf from global static Hive
> db, or SessionState.
> There are several problems with the current design. To name a few:
> 1. SessionState instances are ThreadLocal, but SessionState contains Hive db
> which also contains ThreadLocal storage. Not sure a db can be shared across
> different threads or not? What is the global static Hive db?
> 2. We pass HiveConf and Hive db in two ways to classes like Task: Sometimes
> through initialize(), sometimes through SessionState. This complicates the
> code a lot. It's hard to know which HiveConf and which db we should use.
> We need to think about a better way to do it.
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