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From: Jonathan Hsieh <jon@cloudera.com>
Date: Wed, 29 Aug 2012 10:46:20 -0700
Message-ID: 
 <CAAha9a2VcMedFmBNqMWBp=A9izBpEV3NiCbyfsmHCGb0CU=nTw@mail.gmail.com>
Subject: Re: A general question on maxVersion handling when we have Secondary
 index tables
To: dev@hbase.apache.org
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Let me rephrase to make sure I'm on the same page for the ram's question:

We do three inserts on row 1 at different times to the same column (which
is being indexed in a secondary table)  (Are we assuming only a 1-to-1
secondary->primary mapping?)

t1< t2 <t3
put ("row1", "cf:c", "val1", t1)
put ("row1", "cf:c", "val2", t2)
put ("row1", "cf:c", "val3", t3)

What happens is in the primary table we have:

row1 / cf:c = val1 @ t1
row1 / cf:c = val3 @ t2
row1 / cf:c = val3 @ t3

I'm assuming that these writes happen to a secondary table like this:
put ("val1", "r", "row1", t1)
put ("val2", "r", "row1", t2)
put ("val3", "r", "row1", t3)

an in the secondary table we have:

val1 / r = row1 @ t1
val2 / r = row1 @ t2
val3 / r = row1 @ t3

The core question is how and when can we efficiently and correctly get rid
of the now invalid val1, val2 rows in the index table.

Let's look at some of the strawmen:
1) periodic scan of secondary table that add delete markers for invalid
entries (removed on major compact)
2) lazily delete marker on reads that are invalid (we are @t4, attempt to
read via "val2" in 2ndary index, see primary value is invalid so do a
checkAndDelete val2 from 2ndary).  would get removed on major compact.
3) delete on update.  This means we need to know if we are modifying a
value and thus incurs a at least an extra read per write.

Ram, does this seem like the right question and potential options to
consider?

Jon.

On Wed, Aug 29, 2012 at 8:12 AM, Ramkrishna.S.Vasudevan <
ramkrishna.vasudevan@huawei.com> wrote:

> When we have many to one mapping between main and secondary index table may
> be we will end up in hitting many RS. If there is one to one mapping may be
> that is not a problem.
>
> Basically my intention of this discussion was mainly to discuss on the
> version maintenance on any type of secondary index particularly to remove
> the stale data in the index table that would have expired.
>
> Regards
> Ram
>
>
> > -----Original Message-----
> > From: Ted Yu [mailto:yuzhihong@gmail.com]
> > Sent: Wednesday, August 29, 2012 7:45 PM
> > To: dev@hbase.apache.org
> > Subject: Re: A general question on maxVersion handling when we have
> > Secondary index tables
> >
> > 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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