[infinispan-dev] [ISPN-116] Async cache store: aggregation of multiple changes on a single key

[Galder Zamarreno]

On 07/09/2009 11:55 AM, Manik Surtani wrote:
>
> On 8 Jul 2009, at 19:53, Jason T. Greene wrote:
>
>> Manik Surtani wrote:
>>> * Make the concurrent map volatile
>>> * When iterating, first create a new ConcurrentMap and replace the
>>> old one with the new one so all concurrent threads write to the new Map
>>> * Iterate over the old map
>>
>> That would lead to race conditions since a concurrent writing thread
>> could write to the "old" map, either by getting a recent yet incorrect
>> read off the volatile, or reading it right before it changes.
>
> True, since referencing the map and writing to it isn't atomic.
>
> We could guard access to the map with a read/write lock. Safe, if a
> little heavy-handed... map writers would acquire a RL (since we want
> concurrent access here) but the async flushing thread would need to wait
> for a WL to swap the map reference, releasing the lock after the map
> reference has been swapped.

Yeah, I don't think it could be volatile because it only applies to the 
variable itself, not the contents pointed at. The R/W lock approach 
looks like a valid one and better than using synchronized blocks. 
Another thing I had in my mind is whether we need the copy to be a 
ConcurrentMap, we could probably just use a Immutables.immutableMapCopy().

I looked at the two implementations (ConcurrentHashMap(Map) constructor 
vs Immutables.immutableMapCopy() which would Object.clone() on the map) 
but I'm not sure which one would be faster. Since clone() would rely on 
a native clone() impl, I'd imagine that to be faster.

> --
> Manik Surtani
> manik at jboss.org
> Lead, Infinispan
> Lead, JBoss Cache
> http://www.infinispan.org
> http://www.jbosscache.org
>
>
>
>
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-- 
Galder Zamarreño
Sr. Software Engineer
Infinispan, JBoss Cache