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https://issues.apache.org/jira/browse/HBASE-10070?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14005198#comment-14005198
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Enis Soztutar commented on HBASE-10070:
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bq. 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. .... Might be good to update the lead in to the timeline
consistency part of the doc.
Agreed. Yes, the name was inspired by the PNUTS model. I like the concise form.
I'll add it to the doc.
bq. we can define pluggable strategy on how to execute RPCs
The Consistency enum is mostly concerned about semantics, while execution layer
(RPC) is concerned about latency + performance. Even within a given Consistency
model, you may want different execution strategies I think (like for TIMELINE
consistency, parallel and parallel with delay, or go to first replica, then
second, then third, etc). In the committed code in branch, the consistency
model implies hard coded execution model.
bq. What happens if I ask for TIMELINE and cluster is not deployed with read
replicas? Ignored
Good question. The execution strategy for TIMELINE replicas is to do the
primary RPC first, then if no response after some delay (10ms by default) do
the RPCs for secondaries. If the region has only 1 replica, there won't be any
RPC, so it will just wait for the primary RPC to response back.
> 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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