ConcurrentHashMap scenario

Question

Hi all,
Following is my scenario, let me know if ConcurrentHashMap is the soln.
I have a cache (map) with one process say P1 (having n number of threads) does inserting into map and another process say P2 (with m number of threads) does updation of the values (that were inserted).
Threads of P1 insert unique key / value pairs, so i thought, to do insertion faster concurrent hashmap will be better choice (with concurrency level = number of threads of P1).
The issue is, threads of P2 get the value from the map and update the value. If, for a given key the insertion by a P1 thread and get() by P2 thread happens simultaneously, P2 thread may get the value as null....if hashtable is used, I will not get this situation as get and put are synchronized...but I will not get the performance benefit of Conccurent hashmap (during insertion)..
This will be a large scale application with up to two million inserts tobe taken care by P1 threads(in total) and same number of updates by P2 threads.
Please let me know your comments on this...

Answer 1

harry123 wrote:
There is no guarantee but we expect most of the times this will be the case, for other cases when the P2 gets a value first (and the corresponding insert by P1 has not been done).....
P2 will put the value in a different cache, and its P1's responsibility to check the other cache and merge it the main cache.
Well this solution can be applied to the issue I presented but in that we are increasing the chances of P2 not finding the value when it needs to be updated...thereby increasing number of values that go into second cache..there by putting more work on P1This is what I thought: you don't have a guarantee in either case (hashmap or concurrenthashmap). Since you're not providing a happens-before guarantee with HashMap, I don't see it as a big deal that you're not getting such a guarantee from ConcurrentHashMap either. If it is important, you need to provide that guarantee somehow, or structure your code to deal with a lack thereof.
I'm not clear on what your end goal is; if you could elaborate, then maybe someone could provide suggestions.