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https://issues.apache.org/jira/browse/HIVE-1293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12898010#action_12898010
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John Sichi commented on HIVE-1293:
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After seeing some other issues, had a chat with Namit about semantics; here's
what we worked out.
* Normally, locks should only be held for duration of statement execution.
* However, LOCK TABLE should take a global lock (not tied to any particular
session or statement).
* UNLOCK TABLE should remove both kinds of lock (statement-level and global).
Likewise, SHOW LOCKS shows all.
* For fetching results, we'll need a parameter to control whether a dirty read
is possible. Normally, this is not an issue since we're fetching from saved
temp results, but when using select * from t to fetch directly from the
original table, this behavior makes a difference. To prevent dirty reads,
we'll need the statement-level lock to span the duration of the fetch.
To avoid leaks, we need to make sure that once we create a ZooKeeper client, we
always close it.
> Concurreny Model for Hive
> -------------------------
>
> Key: HIVE-1293
> URL: https://issues.apache.org/jira/browse/HIVE-1293
> Project: Hadoop Hive
> Issue Type: New Feature
> Components: Query Processor
> Reporter: Namit Jain
> Assignee: Namit Jain
> Fix For: 0.7.0
>
> Attachments: hive.1293.1.patch, hive.1293.2.patch, hive.1293.3.patch,
> hive.1293.4.patch, hive.1293.5.patch, hive_leases.txt
>
>
> Concurrency model for Hive:
> Currently, hive does not provide a good concurrency model. The only
> guanrantee provided in case of concurrent readers and writers is that
> reader will not see partial data from the old version (before the write) and
> partial data from the new version (after the write).
> This has come across as a big problem, specially for background processes
> performing maintenance operations.
> The following possible solutions come to mind.
> 1. Locks: Acquire read/write locks - they can be acquired at the beginning of
> the query or the write locks can be delayed till move
> task (when the directory is actually moved). Care needs to be taken for
> deadlocks.
> 2. Versioning: The writer can create a new version if the current version is
> being read. Note that, it is not equivalent to snapshots,
> the old version can only be accessed by the current readers, and will be
> deleted when all of them have finished.
> Comments.
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