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From Jonathan Hsieh <...@cloudera.com>
Subject Re: A general question on maxVersion handling when we have Secondary index tables
Date Wed, 29 Aug 2012 16:11:14 GMT
Ted,

Ram's summarizes the concern succinctly -- to answer the specific question
it isn't for versions -- it is for the case where a secondary index can
point to many many primary rows.  (let's say we have a rowkey userid and we
want to have a 2ndary index based on the state portion of there address
 --- we'll end up pointing to many many primary rows).

Jon.



On Wed, Aug 29, 2012 at 7:15 AM, Ted Yu <yuzhihong@gmail.com> wrote:

> Thanks for the detailed response, Jon.
>
> bq. it would mean that a query based on secondary index would
> potentially have to hit every region server that has a region in the
> primary table.
>
> Can you elaborate on the above a little bit ?
> Is this because secondary index would point us to more than one region in
> the data table because several versions are saved for the same row ?
>
> My thinking was to ease management of simultaneous (data and index) region
> split through region colocation.
>
> Cheers
>
> On Wed, Aug 29, 2012 at 6:47 AM, Jonathan Hsieh <jon@cloudera.com> wrote:
>
> > I'm more of a fan of having secondary indexes added as an external
> feature
> > (coproc or new client library on top of our current client library) and
> > focusing on only adding apis necessary to make 2ndary indexes possible
> and
> > correct on/in HBase.  There are many different use patterns and
> > requirements and one style of secondary index will not be good for
> > everything.  Do we only care about this working well for highly
> selectivity
> > keys?  What are possible indexes (col name, value, value prefix,
> everything
> > our filters support?)  Do we care more about writes or reads, ACID
> > correctness or speed, etc?  Also, there are several questions about how
> we
> > handle other features in conjunction with 2ndary indexes: replication,
> bulk
> > load, snapshots, to name a few.
> >
> > Maybe it makes sense to spend some time defining what we want to index
> > secondarily and what a user api to this external api would be.  Then we
> > could have the different implementations under-the-covers, and allow for
> > users to swap implementations for the tradeoffs that fit their use cases.
> >  It wouldn't be free to change but hopefully "easy" from a user point of
> > view.
> >
> > Personally, I've tend to favor more of a percolator-style implementation
> --
> > it is a client library and built on top of hbase. This approach seems to
> be
> > more "HBase-style" with it's emphasis consistency and atomicity, and
> seems
> > to require only a few mondifications to HBase core. Sure it likely slower
> > than my read of Jesse's proposal, but it seems always always consistent
> and
> > thus predictable in cases where there are failures on deletes and
> updates.
> > We'd need  HBase API primitives like checkAndMutate call (check with
> > multiple delete/put on the same row), and possibly an atomic multitable
> > bulkload.  I'm not sure that it is replication compatible, and there are
> > probably questions we'll need to answer once snapshots solidifies.
> >
> > Ted's idea of colocating regions (like the index table's
> > regions) definitely feels like a primitive (pluggable, likely-per-table
> > region assignment plans) that we could add to HBase core. This
> requirement
> > though for 2ndary indexes seems to imply an approach similar to
> cassandra's
> > approach -- having a local index of each region on region server and
> > colocating them.  Is this right?  If so, this is essentially a filtering
> > optimization --  it would mean that a query based on secondary index
> would
> > potentially have to hit every region server that has a region in the
> > primary table.  This is great approach if the index lookup has high
> > cardinality but if the secondary index is highly selective, you'd have to
> > march through a bunch or RS's before getting an answer.
> >
> > Jon.
> >
> > On Tue, Aug 28, 2012 at 9:18 PM, Ramkrishna.S.Vasudevan <
> > ramkrishna.vasudevan@huawei.com> wrote:
> >
> > > Hi
> > >
> > > Yes I was talking about the dead entry in the index table rather than
> the
> > > actual data table.
> > >
> > > Regards
> > > Ram
> > >
> > > > -----Original Message-----
> > > > From: Wei Tan [mailto:wtan@us.ibm.com]
> > > > Sent: Tuesday, August 28, 2012 9:22 PM
> > > > To: dev@hbase.apache.org
> > > > Cc: Sandeep Tata
> > > > Subject: Re: A general question on maxVersion handling when we have
> > > > Secondary index tables
> > > >
> > > > Thanks for sharing a pointer to your implementation.
> > > > My two cents:
> > > > timestamp is a way to do MVCC and setting every KV with the same TS
> > > > will
> > > > get concurrency control very tricky and error prone, if not
> impossible
> > > > I think Ram is talking about the dead entry in the index table rather
> > > > than
> > > > data table. Deleting old index entries upfront when there is a new
> put
> > > > might be a choice.
> > > >
> > > >
> > > > Best Regards,
> > > > Wei
> > > >
> > > > Wei Tan
> > > > Research Staff Member
> > > > IBM T. J. Watson Research Center
> > > > 19 Skyline Dr, Hawthorne, NY  10532
> > > > wtan@us.ibm.com; 914-784-6752
> > > >
> > > >
> > > >
> > > > From:   Jesse Yates <jesse.k.yates@gmail.com>
> > > > To:     dev@hbase.apache.org,
> > > > Date:   08/28/2012 04:00 AM
> > > > Subject:        Re: A general question on maxVersion handling when we
> > > > have
> > > > Secondary index tables
> > > >
> > > >
> > > >
> > > > Ram,
> > > >
> > > > If I understand correctly, I think you can design your index such
> that
> > > > you
> > > > don't actually use the timestamp (e.g. everything gets put with a TS
> =
> > > > 10
> > > > -
> > > > or some other non-special, relatively small number that's not 0 as
> I'd
> > > > worry about that in HBase ;) Then when you set maxVersions to 1,
> > > > everything
> > > > should be good.
> > > >
> > > > You get a couple of wasted bytes from the TS, but with the prefixTrie
> > > > stuff
> > > > that should be pretty minimal overhead. If you do need to keep track
> of
> > > > the
> > > > timestamp you should be able to munge that back up into the column
> > > > qualifier (and just know that that last 64 bits is the timestamp).
> > > > Again a
> > > > little more CPU cost, but its really not that big of an overhead. It
> > > > seems
> > > > like you don't really care about the TS though, in which case this
> > > > should
> > > > be pretty simple.
> > > >
> > > > Out of curiosity, what are people using for their secondary indexing
> > > > solutions? I know there are a bunch out there, but don't know what
> > > > people
> > > > have adopted, what they like/dislike, design tradeoffs made and why.
> > > >
> > > > Disclaimer: I recently proposed a secondary indexing solution myself
> > > > (shameless self-plug:
> > > > http://jyates.github.com/2012/07/09/consistent-enough-secondary-
> > > > indexes.html
> > > > )
> > > > and its something I'm working on for Salesforce - open sourced at
> some
> > > > point, promise!
> > > >
> > > > -Jesse
> > > > -------------------
> > > > Jesse Yates
> > > > @jesse_yates
> > > > jyates.github.com
> > > >
> > > >
> > > > On Tue, Aug 28, 2012 at 12:24 AM, Ramkrishna.S.Vasudevan <
> > > > ramkrishna.vasudevan@huawei.com> wrote:
> > > >
> > > > > Hi All
> > > > >
> > > > >
> > > > >
> > > > > When we try to build any type of secondary indices for a given
> table
> > > > how
> > > > > can
> > > > > one handle maxVersions in the secondary index tables.
> > > > >
> > > > >
> > > > >
> > > > > For eg,
> > > > >
> > > > > I have inserted
> > > > >
> > > > >  Row1  -  Val1  => t
> > > > >
> > > > > Row1 - Val2 => t+1
> > > > >
> > > > > Row1 - Val3. => t+2
> > > > >
> > > > >
> > > > >
> > > > > Ideally if my max versions is only one then Val3 should be my
> result
> > > > If
> > > > I
> > > > > query on main table for row1.
> > > > >
> > > > >
> > > > >
> > > > > Now in my index I will be having all the above 3 entries.  Now how
> > > > can
> > > > we
> > > > > remove the older entries from the index table that does not fit
> into
> > > > > maxVersions.
> > > > >
> > > > >
> > > > >
> > > > > Currently while scanning and the code that avoids the max Versions
> > > > does
> > > > not
> > > > > give any hooks to know the entries skipped thro versions.
> > > > >
> > > > > So any suggestions on this, I am still seeing the code for any
> other
> > > > > options
> > > > > but suggestions welcome.
> > > > >
> > > > >
> > > > >
> > > > > Regards
> > > > >
> > > > > Ram
> > > > >
> > > > >
> > >
> > >
> > >
> >
> >
> > --
> > // Jonathan Hsieh (shay)
> > // Software Engineer, Cloudera
> > // jon@cloudera.com
> >
>



-- 
// Jonathan Hsieh (shay)
// Software Engineer, Cloudera
// jon@cloudera.com

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