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From "HBase Review Board (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2946) Increment multiple columns in a row at once
Date Mon, 25 Oct 2010 21:44:20 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2946?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12924737#action_12924737

HBase Review Board commented on HBASE-2946:

Message from: "Ryan Rawson" <ryanobjc@gmail.com>

bq.  On 2010-10-24 20:05:55, Ryan Rawson wrote:
bq.  > trunk/src/main/java/org/apache/hadoop/hbase/regionserver/MemStore.java, line 495
bq.  > <http://review.cloudera.org/r/1088/diff/2/?file=15909#file15909line495>
bq.  >
bq.  >     yeah you cant compare against memstoreTS because if you have this in here you
wont be able to ever increment values that were inserted into the future. you'd just leave
them there and continually see it in the 'get' part and then in this code bit leave it in
place and create a new KV that is masked by the future KV.
bq.  >     
bq.  >     It won't be possible for that insert to be part of an uncommitted change because
of the rowlock however. So no atomic-rules will have been broken.
bq.  Jonathan Gray wrote:
bq.      i'm not sure i follow.  am i doing it wrong?  this should be exactly the same as
what's there.
bq.      also, we are breaking the atomic-rules across the Increment.  Each column is not
broken but across them it is (for reads not writes).
bq.      It seems like we could use RWCC though for increments.  I think it's fine that if
you're using increment you can never write data into it w/ another API (or really, with manual

there is no easy way to include RWCC right now, because you have to be careful about 'committing'
the new values before removing the old ones.  This is a bit tricky because normally the RWCC
'commit' happens at the HRegion level, and right now we are inside memstore/store.  Hopefully
people won't need atomic reads of multi incremented values.  If one is doing counters this
would be the case.  Javadoc should definitely yell about this.

bq.  On 2010-10-24 20:05:55, Ryan Rawson wrote:
bq.  > trunk/src/main/java/org/apache/hadoop/hbase/regionserver/MemStore.java, line 377
bq.  > <http://review.cloudera.org/r/1088/diff/2/?file=15909#file15909line377>
bq.  >
bq.  >     I'm not sure how this code is optional for your new 'upsert', here are some
use cases that I found painful:
bq.  >     
bq.  >     - when the KV is in snapshot we can create new KVs in memstore with the same
TS.  This means you have a dup and before we had this new 'dup' code it ruined counts badly.
bq.  >     - the Math.max() bit it to ensure the new KV isnt being created in the past
a bit and accidently duplicating  a timestamp inside the snapshot.
bq.  Jonathan Gray wrote:
bq.      what do you mean by optional?  there shouldn't be any real difference.  this code
is basically the exact same code that was there but now pulled into a method that can be reused.

The original updateColumnValue code was structured like this:

- look in snapshot
- loop over kvset

Create new KV, insert it

- loop over kvset

But you only included the 2nd half.  Your new 'upsert' only includes half the implementation
of updateColumnValue. 

This comes up in this scenario:

- increment at TS=100
- snapshot, now we have kvset={} and a KV, TS=100 in snapshot
- increment at TS=100

Without the code i have above we'll end up with 2 KVs, both of TS=100, one in kvset, one in
snapshot, which then becomes a HFile. 

When future reads happen, if they don't see the 'correct' KV, you will end up with lost updates.

In theory the 'read memstore first' and 'order duplicate TS KVs' patches should resolve this.
 Lost updates reallllllly suck though.  I'd probably recommend the conservative course, because
I already went thru this and it sucks.  If you decide that you are willing to risk it, I might
recommend this particular test case:

- increment some data at TS=X
- snapshot at TS=X
- increment some data at TS=X, now you have 2 * KV, TS=X
- flush to hfile
- increment at TS=X, ensure you dont lose any updates
- snapshot/flush with TS=X in kvset->snapshot->hfile
- compact the 2 hfiles both with TS=X in them
- do read and make sure the value is what you expect.

By using the manual environment edge you can control the currentTimeMillis() returned inside
HRegion and down.

This covers the worst case scenario I think, since normally one would get increments and you
shouldn't get TS=X * 2 in a single HFile. 

- Ryan

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> Increment multiple columns in a row at once
> -------------------------------------------
>                 Key: HBASE-2946
>                 URL: https://issues.apache.org/jira/browse/HBASE-2946
>             Project: HBase
>          Issue Type: New Feature
>          Components: client, regionserver
>            Reporter: Jonathan Gray
>            Assignee: Jonathan Gray
> Currently there is no way to do multiple increments to a single row in one RPC.  This
jira is about adding an HTable and HRegionInterface method to increment multiple columns within
a single row at once.

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