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From Devaraj Das <d...@hortonworks.com>
Subject Re: [Shadow Regions / Read Replicas ] External replication disqualified?
Date Wed, 04 Dec 2013 06:41:43 GMT
On Tue, Dec 3, 2013 at 6:49 PM, Jonathan Hsieh <jon@cloudera.com> wrote:

> The read replicas doc mentions something a little more intrusive in the "3
> options" section but doesn't seem to disqualify it.
>
>
I don't quite see what you are referring to actually... Can you please
copy-paste a relevant line from the design doc.


> Relatedly just as another strawman, for the "mostly read only" use case and
> "bulk load only" usecases, why not use normal replication against two
> clusters in the same HDFS / datacenter and add a "bulk load replication"
> feature?
>
>
We considered this and the issue is that the resource usage on the HDFS
would be doubled (for the store files) for the two replica case.


> We'd get latency in the seconds (closer to my expected definition of
> eventual consistency)
>
> Jon
>
> On Tue, Dec 3, 2013 at 6:47 PM, Jonathan Hsieh <jon@cloudera.com> wrote:
>
> >
> >
> > On Tue, Dec 3, 2013 at 2:04 PM, Enis Söztutar <enis.soz@gmail.com>
> wrote:
> >
> >> On Tue, Dec 3, 2013 at 11:51 AM, Jonathan Hsieh <jon@cloudera.com>
> >> wrote:>
> >>  >
> >> > On Tue, Dec 3, 2013 at 11:07 AM, Enis Söztutar <enis@apache.org>
> wrote:
> >> >
> >> > > Thanks Jon for bringing this to dev@.
> >> > >
> >> > >
> >> > > On Mon, Dec 2, 2013 at 10:01 PM, Jonathan Hsieh <jon@cloudera.com>
> >> > wrote:
> >> > >
> >> > > > Fundamentally, I'd prefer focusing on making HBase "HBasier"
> >> instead of
> >> > > > tackling a feature that other systems architecturally can do
> better
> >> > > > (inconsistent reads).   I consider consistent reads/writes being
> >> one of
> >> > > > HBase's defining features. That said, I think read replicas makes
> >> sense
> >> > > and
> >> > > > is a nice feature to have.
> >> > > >
> >> > >
> >> > > Our design proposal has a specific use case goal, and hopefully we
> can
> >> > > demonstrate the
> >> > > benefits of having this in HBase so that even more pieces can be
> >> built on
> >> > > top of this. Plus I imagine this will
> >> > > be a widely used feature for read-only tables or bulk loaded tables.
> >> We
> >> > are
> >> > > not
> >> > > proposing of reworking strong consistency semantics or major
> >> > architectural
> >> > > changes. I think by
> >> > > having the tables to be defined with replication count, and the
> >> proposed
> >> > > client API changes (Consistency definition)
> >> > > plugs well into the HBase model rather well.
> >> > >
> >> > >
> >> > I do agree think that without any recent updating mechanism, we are
> >> > limiting this usefulness of this feature to essentially *only* the
> >> > read-only or bulk load only tables.  Recency if there were any
> >> > edits/updates would be severely lagging (by default potentially an
> hour)
> >> > especially in cases where there are only a few edits to a primarily
> bulk
> >> > loaded table.  This limitation is not mentioned in the tradeoffs or
> >> > requirements (or a non-requirements section) definitely should be
> listed
> >> > there.
> >> >
> >>
> >> Obviously the amount of lag you would observe depends on whether you are
> >> using
> >> "Region snapshots", "WAL-Tailing" or "Async wal replication". I think
> >> there
> >> are still
> >> use cases where you can live with >1 hour old stale reads, so that
> "Region
> >> snapshots"
> >> is not *just* for read-only tables. I'll add these to the tradeoff's
> >> section.
> >>
> >
> > Thanks for adding it there -- I really think it is a big headline caveat
> > on my expectation of "eventual consistency".  Other systems out there
> that
> > give you eventually consistency on the millisecond level for most cases,
> > while this initial implementation would has eventual mean 10's of minutes
> > or even handfuls of minutes behind (with the snapshots flush mechanism)!
> >
> > There are a handful of other things in the phase one part of the
> > implementation section that limit the usefulness of the feature to a
> > certain kind of constrained hbase user.  I'll start another thread for
> > those.
> >
> >
> >>
> >> We are proposing to implement "Region snapshots" first and "Async wal
> >> replication" second.
> >> As argued, I think wal-tailing only makes sense with WALpr so, that work
> >> is
> >> left until after we have WAL
> >> per region.
> >>
> >>
> > This is our main disagreement -- I'm not convinced that wal tailing only
> > making sense for the wal per region hlog implementation.  Instead of
> > bouncing around hypotheticals, it sounds like I'll be doing more
> > experiments to prove it to myself and to convince you. :)
> >
> >
> >>
> >> >
> >> > With the current design it might be best to have a flag on the table
> >> which
> >> > marks it read-only or bulk-load only so that it only gets used by
> users
> >> > when the table is in that mode?  (and maybe an "escape hatch" for
> power
> >> > users).
> >> >
> >>
> >> I think we have a read-only flag already. We might not have bulk-load
> only
> >> flag though. Makes sense to add it
> >> if we want to restrict allowing bulk loads but preventing writes.
> >>
> >> Great.
> >
> >>
> >> >
> >> > [snip]
> >> > >
> >> > > - I think the two goals are both worthy on their own each with their
> >> own
> >> > > > optimal points.  We should in the design makes sure that we can
> >> support
> >> > > > both goals.
> >> > > >
> >> > >
> >> > > I think our proposal is consistent with your doc, and we have
> >> considered
> >> > > secondary region promotion
> >> > > in the future section. It would be good if you can review and
> comment
> >> on
> >> > > whether you see any points
> >> > > missing.
> >> > >
> >> > >
> >> > > I definitely will. At the moment, I think the hybrid for the
> >> wals/hlogs I
> >> > suggested in the other thread seems to be an optimal solution
> >> considering
> >> > locality.  Though feasible is obviously more complex than just one
> >> approach
> >> > alone.
> >> >
> >> >
> >> > > > - I want to making sure the proposed design have a path for
> optimal
> >> > > > fast-consistent read-recovery.
> >> > > >
> >> > >
> >> > > We think that it is, but it is a secondary goal for the initial
> work.
> >> I
> >> > > don't see any reason why secondary
> >> > > promotion cannot be build on top of this, once the branch is in a
> >> better
> >> > > state.
> >> > >
> >> >
> >> > Based on the detail in the design doc and this statement it sounds
> like
> >> you
> >> > have a prototype branch already?  Is this the case?
> >> >
> >>
> >> Indeed. I think that is mentioned in the jira description. We have some
> >> parts of the
> >> changes for region, region server, HRI, and master. Client changes are
> on
> >> the way.
> >> I think we can post that in a github branch for now to share the code
> >> early
> >> and solicit
> >> early reviews.
> >>
> >> I think that would be great.  Back when we did snapshots, we had active
> > development against a prototype and spent a bit of time breaking it down
> > into manageable more polished pieces that had slightly lenient reviews.
> >  This exercise really helped us with our interfaces.  We committed code
> to
> > the dev branch which limited merge pains and diff for modifications made
> by
> > different contributors.  In the end when we had something we were happy
> > with on the dev branch we merged with trunk and fixed bugs/diffs that
> > cropped up in the mean time.  I'd suggest a similar process for this.
> >
> >
> > --
> > // Jonathan Hsieh (shay)
> > // Software Engineer, Cloudera
> > // jon@cloudera.com
> >
> >
>
>
>
> --
> // Jonathan Hsieh (shay)
> // Software Engineer, Cloudera
> // jon@cloudera.com
>

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