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From "Enis Soztutar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10070) HBase read high-availability using eventually consistent region replicas
Date Tue, 03 Dec 2013 02:59:36 GMT

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Enis Soztutar commented on HBASE-10070:
---------------------------------------

bq. Enis Soztutar in your doc does "group-based assignment" mean assigning multiple regions
on a single transaction?
I was trying to refer to not having co-location constraints for secondary replicas whose primaries
are hosted by the same RS. For example, if R1(replica=0), and R2(replica=0) are hosted on
RS1, R1(replica=1) and R2(replica=1) can be hosted by RS2 and RS3 respectively. This can definitely
use the hdfs block affinity work though. 

bq. However, I don't think we get into a situation where all RS's must read all other RS's
logs – we only need to have the shadows RS's to read the primary RS's log.
I am assuming a random distribution of secondary regions per above. In this case, for replication=2,
a region server will have half of it's regions in primary and the other in secondary mode.
For all the regions in the secondary mode, it has to tail the logs of the rs where the primary
is hosted. However, since there is no co-location guarantee, the primaries are also randomly
distributed. For n secondary regions, and m region servers, you will have to tail the logs
of most of the RSs if n > m with a high probability (I do not have the smarts to calculate
the exact probability) 

> HBase read high-availability using eventually 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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