hadoop-hive-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Joydeep Sen Sarma (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-1293) Concurreny Model for Hive
Date Tue, 17 Aug 2010 07:58:24 GMT

    [ https://issues.apache.org/jira/browse/HIVE-1293?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12899320#action_12899320

Joydeep Sen Sarma commented on HIVE-1293:

a little bummed that locks need to be held for entire query execution. that could mean a writer
blocking readers for hours.

hive's query plans seem to be of two distinct stages:
1. read a bunch of stuff, compute intermediate/final data
2. move final data into output locations

ie. - a single query never reads what it writes (into a final output location). even if #1
and #2 are mingled today - they can easily be put in order.

in that sense - we only need to get shared locks for all read entities involved in #1 to begin
with. once phase #1 is done, we can drop all the read locks and get the exclusive locks for
all the write entities in #2, perform #2 and quit. that way exclusive locks are held for a
very short duration. i think this scheme is similarly deadlock free (now there are two independent
lock acquire/release phases - and each of them can lock stuff in lex. order).

> 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.

This message is automatically generated by JIRA.
You can reply to this email to add a comment to the issue online.

View raw message