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HBase, mail # user - writing and reading from a region at once


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Re: writing and reading from a region at once
ramkrishna vasudevan 2013-04-29, 16:26
Check this out
http://www.slideshare.net/cloudera/3-learning-h-base-internals-lars-hofhansl-salesforce-final

Regards
Ram
On Mon, Apr 29, 2013 at 7:59 PM, Aaron Zimmerman <
[EMAIL PROTECTED]> wrote:

> Thanks J-D.  I'm interested in learning how HBase handles MVCC - do you
> know of any resources explaining?  I have data streaming into hbase around
> 6 million records a day, and I'm scanning the tables pretty much constantly
> with mapreduce jobs that rollup and store the data elsewhere.  So I want to
> understand better exactly which rows are present in a given scan.
>
> If I create a scan that reads an entire region, I would expect a "read
> committed" level to lock that region during the read (which might take a
> few minutes).    So what happens to the rows that are inserted during the
> scan?
>
> AZ
>
>
> On Apr 25, 2013, at 4:34 PM, Jean-Daniel Cryans <[EMAIL PROTECTED]>
> wrote:
>
> > Inline.
> >
> > J-D
> >
> >
> > On Thu, Apr 25, 2013 at 1:09 PM, Aaron Zimmerman <
> > [EMAIL PROTECTED]> wrote:
> >
> >> Hi,
> >>
> >>  If a region is being written to, and a scanner takes a lease out on the
> >> region, what will happen to the writes?  Is there a concept of
> "Transaction
> >> Isolation Levels"?
> >>
> >
> > There's MVCC, so reads can happen while someone else is writing. What you
> > should expect from HBase is "read committed".
> >
> >
> >>
> >>  I don't see errors in Puts while the tables are being scanned?  But it
> >> seems that I'm losing writes somewhere, is it possible the writes could
> >> fail silently?
> >>
> >
> > Is it temporary while you're scanning or there's really data missing at
> > the end of the day? The former might happen on some older HBase versions
> > while the latter should never happen unless you lower the durability
> level
> > yourself and have machine failures.
> >
> > J-D
>
>