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HBase >> mail # dev >> Re: [Shadow Regions / Read Replicas ] External replication disqualified?


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Re: [Shadow Regions / Read Replicas ] External replication disqualified?
The read replicas doc mentions something a little more intrusive in the "3
options" section but doesn't seem to disqualify it.

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'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 <[EMAIL PROTECTED]> wrote:

>
>
> On Tue, Dec 3, 2013 at 2:04 PM, Enis Söztutar <[EMAIL PROTECTED]> wrote:
>
>> On Tue, Dec 3, 2013 at 11:51 AM, Jonathan Hsieh <[EMAIL PROTECTED]>
>> wrote:>
>>  >
>> > On Tue, Dec 3, 2013 at 11:07 AM, Enis Söztutar <[EMAIL PROTECTED]> wrote:
>> >
>> > > Thanks Jon for bringing this to dev@.
>> > >
>> > >
>> > > On Mon, Dec 2, 2013 at 10:01 PM, Jonathan Hsieh <[EMAIL PROTECTED]>
>> > 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

// Jonathan Hsieh (shay)
// Software Engineer, Cloudera
// [EMAIL PROTECTED]