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From Eric Burin des Roziers <eric_...@yahoo.com>
Subject Re: put to WAL and scan/get operation concurrency
Date Fri, 06 May 2011 08:45:37 GMT
Thanks Stack,  I hadn't read the percolator paper (doing it now).  I think I am not describing
my question properly.  Basically, based on the hbase-trx implementation, when the transaction
commits, there is a time window where a Get() might read partial rows since it implements
the snapshot isolation by writing records to a different location (than the actual HTable)
before the commit().  In the percolator paper, cell versions are used as snapshot isolation
and uses an as-of timestamp when doing a Get().

Another unrelated question: when a region server fails, does the client (while doing a get/scan)
get notified (exception)?  Basically, I want to ensure that an operation (such as a rollup/aggregate)
does not compute the wrong amounts due to missing data.

Thanks again for your help,
-Eric


________________________________
From: Stack <stack@duboce.net>
To: user@hbase.apache.org; Eric Burin des Roziers <eric_bdr@yahoo.com>
Sent: Thursday, May 5, 2011 10:05 PM
Subject: Re: put to WAL and scan/get operation concurrency

On Thu, May 5, 2011 at 11:43 AM, Eric Burin des Roziers
<eric_bdr@yahoo.com> wrote:
> Hi Jean-Daniel,
>
> Yes, I need to have a multi-row transactional aware HBase for the types of processing
I need to do.  I need to avoid having partial rows available and I am in the process of selecting
a way to implement such a transaction isolation.  I currently have 2 choices: (1) use the
HBase-trx or (2) implement my own leveraging the verioning that HBase provides.  In light
of this I wanted to understand the inner workings of HBase a little more.


You have read the megastore and percolator papers?  They discuss x-row
transactions.


> For example, I want to understand if scans read data from the MemStore even if it has
not yet been flushed to the HFiles yet.

It does.

> HBase replicates the data 3 times (depending on your configs).  Does it do that as well
for the MemStore.

The data in memstore is first put in the WAL which is replicated three times.



> Say the client wants to inserts 10 lines which happen to fall across 2 regions.  If
region 2 fails, then another client will still be able to read the rows inserted in region
1, but not region 2.  Since HBase replicates data to other servers, region 2 lines could
be available on other servers, right?
>

Would suggest you read the bigtable paper.  It'll answer most of your
questions more eloquently than I can (To answer your question, only
one region serves a specific piece of data.  It depends on your
transaction implementation as to whether the half written data is
readable by the client).


> The second aspect that I would like to understand is the implementation of the HBase-trx.
 It seems that I can still have a failure point when the transactional WAL (THLog) flushed
the data to the main Wal.  using the above example, I can get into a situation where I will
only be able to read a subset of the initial 10 lines initially inserted.  Is that right?
>

I think, pardon me if I'm reading this wrong, you have begun on a
wrong foot so your question doesn't add up right.

St.Ack
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