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Date: Wed, 16 Dec 2015 22:18:46 +0000 (UTC)
From: "Eugene Koifman (JIRA)" <jira@apache.org>
To: issues@hive.apache.org
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Subject: [jira] [Commented] (HIVE-11388) there should only be 1 Initiator
 for compactions per Hive installation
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    [ https://issues.apache.org/jira/browse/HIVE-11388?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15061006#comment-15061006 ] 

Eugene Koifman commented on HIVE-11388:
---------------------------------------

here is one general purpose mechanism:
create table MUTEX_TABLE(keyname varchar(512) PRIMARY KEY)

Then any process that requires a mutex needs to insert a row into this table (as long as everyone agrees on the key) and then do a "Select for update" on this row.  If the process dies, "select for update" lock is automatically released.

For example, if 2 Initiator instances want to schedule a compaction, each could
1. select * from MUTEX_TABLE where keyname="initiator" for update.
If the "initiator" row is already there, only 1 will succeed.  The other one, once it unblocks, will already see "this" compaction scheduled.
2. if select in 1 misses, then Initiator can insert "initiator" row and then goto 1.  Because of PK only 1 will succeed.

Since the keyname is arbitrary, it can be "db/table/partition" to coordinate Workers if necessary.

A little primitive, but workable and avoids ZooKeeper and allows all parts of Compaction/HouseKeeping to run on multiple MS nodes.

> there should only be 1 Initiator for compactions per Hive installation
> ----------------------------------------------------------------------
>
>                 Key: HIVE-11388
>                 URL: https://issues.apache.org/jira/browse/HIVE-11388
>             Project: Hive
>          Issue Type: Bug
>          Components: Transactions
>    Affects Versions: 1.0.0
>            Reporter: Eugene Koifman
>            Assignee: Eugene Koifman
>            Priority: Critical
>
> org.apache.hadoop.hive.ql.txn.compactor.Initiator is a thread that runs inside the metastore service to manage compactions of ACID tables.  There should be exactly 1 instance of this thread (even with multiple Thrift services).
> This is documented in https://cwiki.apache.org/confluence/display/Hive/Hive+Transactions#HiveTransactions-Configuration but not enforced.
> Should add enforcement, since more than 1 Initiator could cause concurrent attempts to compact the same table/partition - which will not work.


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