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Coherence provides Maptrigger functionality.
What is the problem you are trying to solve by decorating the object? Is it to suppress persistence in certain cases? Or to control how its done? Something else?
This is a tricky area, especially if you switch to write-behind, when as you suggest, it is not possible to update the backing map from the CacheStore. Also, updates may be coalesced, losing intermediate values.
Client application sets flags on rows of a table it has updated. Basically the data we store in the cache are tables, where some of them are massive in size. This way the algorithm that persists the data knows which rows of the table require SQL update. After persisting the value I need to reset those flags to initial state.
Unfortunately this behavior needs do be copied from a legacy system that we need to mimic at some level. In a future release we plan to avoid decorating the object and to implement CQRS style of communication, but that's not doable now.
My first idea to cope with this issue was to have a separate cache we could write to objects that are decorated and such cache would server a purpose of a write queue. A CacheStore would be associated with this cache. And the read cache would be updated with the same value but after putting to write cache and after resetting the flags.
I have been looking at MapTrigger but that doesn't solve my issue due to the place of MapTrigger in the chain of operations - before CacheStore.
Edited by: 984164 on 29-Jan-2013 05:50
I would recommend to have another CacheService that keeps the decoration related data for your objects and access this Cache in your actual object CacheStore to read this information.
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It is possible to subclass and customize ReadWriteBackingMap or LocalCache used as internal map inside of it. This way you cloud override put method and make required transformation, you cloud also modify content of backing map directly. This technique is rarely used and messy, but it alows you to implement very non-trivial BM behavior.
With backing listener you should also be aware of global cache lock issue http://blog.ragozin.info/2012/12/coherence-101-beware-of-cache-listeners.html.
You could try asynchronously call out from your cachestore (e.g. by ThreadPoolExecutor) to update your cache entry after it has been persisted. You might want to use optimistic locking at the cache entry level, or even table row within the cache entry so as not to lose updates that came in during the persistence/status update process. Of course the status update itself would cause another call to your CacheStore, but with rows no longer marked for persistence so you could ignore those.
Done right, this would allow asynch persistence and should work even with coalesced updates. The asynch step in updating status is necessary as you would not be able to re-entrantly call the cache service from within the CacheStore.
Thank you Alexey. That sounds like a plan. I guess I could override the ReadWriteBackingMap and modify putInternal method accordingly.
I initially assumed that this approach would not cover write behind scenario, but I have just noticed WriteQueue reference.