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From "Liu, Raymond" <raymond....@intel.com>
Subject How to implement this check put and then update something logic?
Date Wed, 08 May 2013 07:22:04 GMT

	Say, I have four field for one record :id, status, targetid, and count.
	Status is on and off, target could reference other id, and count will record the number of
"on" status for all targetid from same id.

	The record could be add / delete, or updated to change the status.

	I could put count in another table, or put it in the same table, it doesn't matter. As long
as it can work.

	My question is how can I ensure its correctness of the "count" field when run with multiple
client update the table concurrently?

	The closet thing I can think of is checkAndPut, but I will need two steps to find out the
change of count, since checkAndPut etc can only test a single value and with EQUAL comparator,
thus I can only check upon null firstly, then on or off. Thus when thing change during this
two step, I need to retry from first step until it succeed. This could be bad when a lot of
concurrent op is on going.

	And then, I need to update count by checkAndIncrement, though if the above problem could
be solved, the order of -1 +1 might not be important for the final result, but in some intermediate
time, it might not reflect the real count of that time.

	I know this kind of transaction is not the target of HBASE, APP should take care of it, then
, what's the best practice on this? Any quick simple solution for my problem? Client RowLock
could solve this issue, But it seems to me that it is not safe and is not recommended and

	Btw. Is that possible or practice to implement something like PutAndGet which put in new
row and return the old row back to client been implemented? That would help a lot for my case.

Best Regards,
Raymond Liu

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