Tuesday, October 26, 2010

gemfire cache-callback vs cache-listener concepts

Here I'm more interested in the concepts rather than the 2 implementations. If you want to study the nitty-gritty, then you better know the event objects. I feel events are simpler than listeners.

The CacheCallback interface is the superinterface of most cache event handlers. CacheCallback has a single method, close().

Top 3 essential operations on a region - get/put/subscribe. I think subscribe means cache to notify listeners using callback methods and event objects.

The order in which the listeners are added is important, because it is the order in which they are called. GemFire maintains an ordered list of the region's listeners. You can execute getCacheListeners on the AttributesFactory or AttributesMutator to fetch the current ordered list. The AttributesMutator.removeCacheListener method takes a specified listener off the list.

The cache listeners are *sequential* -- listener1 must finish its work before listener2 begins. GemFire guarantees the order, as long as the same thread calls each listener. You can put the actual work into Command objects to execute on other threads -- execution order is not guaranteed.

Cache listener notifications are invoked synchronously, so they will cause the cache *modification* operation to block if the callback method blocks. This is particularly likely if you perform cache operations from a callback. To avoid this, use the Executor interface discussed in Writing Callback Methods.

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