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 To: user@hbase.apache.org; Eric Burin des Roziers 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 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