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From "Suraj Varma (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4999) Constraints - Enhance checkAndPut to do atomic arbitrary constraint checks
Date Tue, 13 Dec 2011 17:52:30 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13168539#comment-13168539
] 

Suraj Varma commented on HBASE-4999:
------------------------------------

I see this being used similar to a Filter which is specified client side and executes on the
server. Just like we have some out of the box filters (added over time as well), I see some
being provided out of the box. The ability to add client specified custom constraints would
be akin to custom server side filters.

I think specifying this completely on the server side via configuration (e.g. a constraint
like column_name > threshold-value) may work & be sufficient for some cases. I'm thinking
of cases where the column names are dynamic (i.e. have ids or attribute values embedded in
them) and cannot be specified purely server side via configuration. For these, we will need
client specified constraints. Similarly, if I have multiple constraints on different column
qualifiers that are a subset of all the qualifiers, the ability to specify this client side
is much more flexible.

So - again - this would work similar to how filters work, in my mind.

                
> Constraints - Enhance checkAndPut to do atomic arbitrary constraint checks
> --------------------------------------------------------------------------
>
>                 Key: HBASE-4999
>                 URL: https://issues.apache.org/jira/browse/HBASE-4999
>             Project: HBase
>          Issue Type: Improvement
>          Components: client, coprocessors
>    Affects Versions: 0.92.0
>            Reporter: Suraj Varma
>              Labels: CAS, checkAndPut, constraints
>             Fix For: 0.94.0
>
>
> Related work: HBASE-4605
> It would be great if checkAndPut (CAS) can be enhanced to not just use a value comparison
as a gating factor for the put, but rather have the capability of doing arbitrary constraint
checks on the column value (where the current comparinator approach is a subset of possible
constraints that can be checked). Commonly used constraints (like comparisons) can be provided
out of the box and we should have the ability to accept custom constraints set by the client
for the checkAndPut call. 
> One use-case would be the ability to implement something like the below in HBase.
> Pseudo sql: 
> update table-name
> set column-name = new-value
> where (column-value - new-value) > threshold-value
> ... where the mutation would go through only if the specified constraint in the where
clause is true.
> Current options include using a co-processor to do preCheckAndPut/postCheckAndPut constraint
checks - but this is not atomic. i.e. the row lock needs to be released by the co-processor
before the real checkAndPut call, thus not meeting the atomic requirement. 
> Everything above is still meant to be at row level (so, no cross-row constraint checking
is implied here).
> And ideal end result would be that an HBase client would be able to specify a set of
constraints on multiple column qualifiers as part of the checkAndPut call. The call goes through
if all the constraints are satisfied or doesn't if any of the constraints fail. And the above
checkAndPut should be atomically executed (just like current checkAndPut semantics).

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