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From Michael Segel <michael_se...@hotmail.com>
Subject RE: multiple scanners on same table will cause problem? Scan results change among different tries.
Date Thu, 22 Apr 2010 13:50:08 GMT

Ok...

This is something that I think we'll need input from a major contributor...

It looks like there may be an issue with respect to row locking...

I guess the questions to ask are:

- How does HBase handle row level locking?
-Concurrent reads/fetches of the same row?

To be honest and fair, HBase is still an immature product when compared to databases and there
going to be some issues that need to be fleshed out.  (Lets see where we are in 20+ years
;-)

I wish I knew more about the internals of HBase, but there are only so many hours in the day
and my wife forces me to work so I can keep up with her spending. ;-) (And if any of you happen
to ever meet her, please don't bring this up, she'll kill me. :-D   )

Lets see what St.Ack or Andrew have to say. This might be a JIRA issue.

Thx

-Mike



> Date: Thu, 22 Apr 2010 20:17:12 +0800
> Subject: Re: multiple scanners on same table will cause problem? Scan results 	change
among different tries.
> From: steven.zhuang.1984@gmail.com
> To: hbase-user@hadoop.apache.org
> 
> hi, Michael,
> 
>                Sorry for not making the question clear, there are multiple
> scanners scanning a single table, there might be the case multiple scanners
> reading from a single region.
>        please see answers inline.
> 
> On Thu, Apr 22, 2010 at 8:08 PM, Michael Segel <michael_segel@hotmail.com>wrote:
> 
> >
> > I'm sorry, but are you trying to say that you have multiple scanners trying
> > to read from a single region and the result sets do not match?
> >
> >  Yes, the result sets do not match.
> 
> > I guess it would be an easy test, enter a bunch of rows in to a region and
> > have a unique integer for each row. (1,2,3,...)
> > Then run a bunch of unfiltered scans in parallel, and generate a sum from
> > the scan. If any of the sums do not match, then you have a potential issue
> > on concurency/row locking, and row isolation level.  How does HBase handle
> > row level locking and isolation levels?
> >
> > I have iterate on the rows/columnfamilies/cells, and printed the content of
> each cell, found that there are some cells missing in some scan result set.
> 
> > -Mike
> >
> > > Date: Thu, 22 Apr 2010 17:07:47 +0800
> > > Subject: multiple scanners on same table will cause problem? Scan results
> >     change among different tries.
> > > From: steven.zhuang.1984@gmail.com
> > > To: hbase-user@hadoop.apache.org
> > >
> > > hi, All,
> > >           Has anybody do scan on one table using multiple scanners at the
> > > same time and  found some inconsistent problem?
> > >           I am doing query on a table using dozens(20-120) of scanners in
> > > parallel(multiple threads), trying to take advantage of the multiple
> > cores.
> > > But I found the scan results doesn't consist among several goes. I have
> > > checked my code, seems there is no bug in it. So I guess the problem may
> > > come from the HBase itself.
> > >           My HBase version is 0.20.3.
> >
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