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From "Andrew Purtell (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10070) HBase read high-availability using timeline-consistent region replicas
Date Wed, 21 May 2014 18:41:45 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10070?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14005073#comment-14005073
] 

Andrew Purtell commented on HBASE-10070:
----------------------------------------

bq. Can I ask what definition of timeline consistency are we using here? 

I believe the Yahoo PNUTS paper is where many have first heard the term 'timeline consistency'.
Daniel Abadi summarizes it at http://dbmsmusings.blogspot.com/2010/04/problems-with-cap-and-yahoos-little.html
as "where replicas may not be consistent with each other but updates are guaranteed to be
applied in the same order at all replicas", which I think is very concise.

If we claim this about HBASE-10070, is that accurate? (I think yes.) 

If so, the documentation committed on HBASE-10513 doesn't summarize the term as cleanly in
my opinion. Might be good to update the lead in to the timeline consistency part of the doc.

> HBase read high-availability using timeline-consistent region replicas
> ----------------------------------------------------------------------
>
>                 Key: HBASE-10070
>                 URL: https://issues.apache.org/jira/browse/HBASE-10070
>             Project: HBase
>          Issue Type: New Feature
>            Reporter: Enis Soztutar
>            Assignee: Enis Soztutar
>         Attachments: HighAvailabilityDesignforreadsApachedoc.pdf
>
>
> In the present HBase architecture, it is hard, probably impossible, to satisfy constraints
like 99th percentile of the reads will be served under 10 ms. One of the major factors that
affects this is the MTTR for regions. There are three phases in the MTTR process - detection,
assignment, and recovery. Of these, the detection is usually the longest and is presently
in the order of 20-30 seconds. During this time, the clients would not be able to read the
region data.
> However, some clients will be better served if regions will be available for reads during
recovery for doing eventually consistent reads. This will help with satisfying low latency
guarantees for some class of applications which can work with stale reads.
> For improving read availability, we propose a replicated read-only region serving design,
also referred as secondary regions, or region shadows. Extending current model of a region
being opened for reads and writes in a single region server, the region will be also opened
for reading in region servers. The region server which hosts the region for reads and writes
(as in current case) will be declared as PRIMARY, while 0 or more region servers might be
hosting the region as SECONDARY. There may be more than one secondary (replica count >
2).
> Will attach a design doc shortly which contains most of the details and some thoughts
about development approaches. Reviews are more than welcome. 
> We also have a proof of concept patch, which includes the master and regions server side
of changes. Client side changes will be coming soon as well. 



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