Mailing-List: contact issues-help@hive.apache.org; run by ezmlm
Precedence: bulk
Reply-To: dev@hive.apache.org
Date: Mon, 25 Apr 2016 20:11:12 +0000 (UTC)
From: "Sushanth Sowmyan (JIRA)" <jira@apache.org>
To: issues@hive.apache.org
Message-ID: <JIRA.12953082.1458789311000.27460.1461615072894@Atlassian.JIRA>
In-Reply-To: <JIRA.12953082.1458789311000@Atlassian.JIRA>
References: <JIRA.12953082.1458789311000@Atlassian.JIRA>
 <JIRA.12953082.1458789311537@arcas>
Subject: [jira] [Commented] (HIVE-13348) Add Event Nullification support for
 Replication
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit


    [ https://issues.apache.org/jira/browse/HIVE-13348?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15256953#comment-15256953 ] 

Sushanth Sowmyan commented on HIVE-13348:
-----------------------------------------

Sorry, to clarify, the idea is not to nullify the events in the main eventlog itself - we will still maintain those, and they are under the purview of the metastore currently - the idea is that when a program calls HCatClient.getReplicationTasks which exposes an Iterator<ReplicationTask>, where currently, there is a 1:1 map from Event to ReplicationTask, and we should ideally have a many-one.

Thus, this filtering would be downstream of the actual collection of events, it would be in-stream for the processing of replication events.

Or are you suggesting that even for replication, we should allow the capability to send along noop-replication-tasks as marker tasks for those events which were nullified, so we can have an audit on the destination? That could be done too, and would be performant as well.

> Add Event Nullification support for Replication
> -----------------------------------------------
>
>                 Key: HIVE-13348
>                 URL: https://issues.apache.org/jira/browse/HIVE-13348
>             Project: Hive
>          Issue Type: Sub-task
>          Components: Import/Export
>            Reporter: Sushanth Sowmyan
>              Labels: gsoc2016
>
> Replication, as implemented by HIVE-7973 works as follows:
> a) For every singly modification to the hive metastore, an event gets triggered that logs a notification object.
> b) Replication tools such as falcon can consume these notification objects as a HCatReplicationTaskIterator from HCatClient.getReplicationTasks(lastEventId, maxEvents, dbName, tableName).
> c) For each event,  we generate statements and distcp requirements for falcon to export, distcp and import to do the replication (along with requisite changes to export and import that would allow state management).
> The big thing missing from this picture is that while it works, it is pretty dumb about how it works in that it will exhaustively process every single event generated, and will try to do the export-distcp-import cycle for all modifications, irrespective of whether or not that will actually get used at import time.
> We need to build some sort of filtering logic which can process a batch of events to identify events that will result in effective no-ops, and to nullify those events from the stream before passing them on. The goal is to minimize the number of events that the tools like Falcon would actually have to process.
> Examples of cases where event nullification would take place:
> a) CREATE-DROP cases: If an object is being created in event#34 that will eventually get dropped in event#47, then there is no point in replicating this along. We simply null out both these events, and also, any other event that references this object between event#34 and event#47.
> b) APPEND-APPEND : Some objects are replicated wholesale, which means every APPEND that occurs would cause a full export of the object in question. At this point, the prior APPENDS would all be supplanted by the last APPEND. Thus, we could nullify all the prior such events. 
> Additional such cases can be inferred by analysis of the Export-Import relay protocol definition at https://issues.apache.org/jira/secure/attachment/12725999/EXIMReplicationReplayProtocol.pdf or by reasoning out various event processing orders possible.
> Replication, as implemented by HIVE-7973 is merely a first step for functional support. This work is needed for replication to be efficient at all, and thus, usable.


--
This message was sent by Atlassian JIRA
(v6.3.4#6332)