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From "Joydeep Sen Sarma (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-1293) Concurreny Model for Hive
Date Tue, 17 Aug 2010 08:52:19 GMT

    [ https://issues.apache.org/jira/browse/HIVE-1293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12899347#action_12899347
] 

Joydeep Sen Sarma commented on HIVE-1293:
-----------------------------------------

also - i am missing something here:

+      for (WriteEntity output : plan.getOutputs()) {
+        lockObjects.addAll(getLockObjects(output.getTable(), output.getPartition(), HiveLockMode.EXCLUSIVE));
+      }

getLockObjects():

+    if (p != null) {
...
+      locks.add(new LockObject(new HiveLockObject(p.getTable()), mode));
+    }

doesn't this end up locking the table in exclusive mode if a partition is being written to?
(whereas the design talks about locking the table in shared mode only?)

> 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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