Modify, put and get : operations under transaction

Question

Hello,
I use Coherence 3.0 with CONCUR_PESSIMISTIC.
When threads t1 and simultaneously t2 get the object obj1, they obtain it whitout waiting any lock. It's OK.
However, I've created a "modify" operation which is :
- get a transaction
- get the object obj1
- "modify it in my app" (for example obj = obj+1)
- put the new value
- commit transaction
This works but, in the following scenario, I get incoherent result, since get operations don't block.
Threads t1 and t2 modify the object obj1 at the same time. So, for example obj1 == 0 at the beginning. Then :
- t1 get a transaction
- t2 get a transaction
- t1 get the old obj1 value
- t2 get the old obj1 value
- t1 put the new value (obj1 == 1) in the cache;
- t2 put the new value (obj1 == 1) in the cache;
So at the end, obj1 == 1 in the cache, instead of 2!
obj1 = 0 + 1 (thread 1)
obj1 = 1 + 1 (thread 2, with old value == 1, thanks to thread 1)
I hope I'm clear enough...
So, my question is : what should I do?
Note that I can't say anything on the get/write operations ratio.
Should I use "put" operation to get the old value in modify operation (instead of get)? Won't it be heavy (in load term)?
Note that when I use TRANSACTION_GET_COMMITTED, the get of the "modify" operation doesn't block, but when I use TRANSACTION_REPEATABLE_GET, it is the get operation (the basic, not embedded in the modify operation) which is blocking.
I'd like : get basic operations don't be blocking and coherent "modify" operation. Is it possible?
Isn't it too heavy (in load) to block all the "modify" operation? What about versioning, to detect a change (between a get (old value) and put (new value))? Have code example?
Thanks for your advice,
Vincent
Message was edited by: Vincent

Answer 1

Hi Vincent,
It sounds like you can simply use locking in this scenario (cache locking semantics are similar to that of a Java synchronized keyword - if thread does use lock() it will block is there is contention, and if it doesnt use lock() it will not block at all):
read:
<code>
value = cache.get(key); // does not block
</code>
write:
<code>
try {
cache.lock(key, timeout); // will block if there is contention
value = cache.get(key);
cache.put(key, value + 1);
} finally {
cache.unlock(key);
</code>
Regards,
Dimitri