I've been reading about how the GemFire distributed data store/management/cache system performs notifications. While this reading, i had this question.
Gemfire seems to be using MBeans to create notifications during events. How different/suitable is using MBeans to create notifications instead of implementing a Listener based aproach ? (not just in GemFire but, generally)
Note: I am very new to the topic of MBean. Just with the understanding that it's main purpose is to expose resources to be managed.
CONTEXT
...topic of MBean... it's main purpose is to expose resources to be managed.
That is correct. (GemFire) Resources exposed as MBeans can both be queried and altered, depending on what the MBean exposes for the resource (e.g. Region, DiskStore, Gateway, AEQ, etc), using JMX.
GemFire's JMX interface can then be consumed by applications and tools that use the JMX API. GemFire's Gfsh (command-line shell and management tool) along with Pulse (web monitoring tool) are both examples of JMX clients and the kinds of applications you could write that use JMX.
You can also use the standard JDK tools like jconsole or jvisualvm to connect to a GemFire Manager (managing node in the cluster that federates the view of all the members in the cluster as well as the ability to control any single member from the Manager). See GemFire's section in the User Guide on Management for more details.
Contrasting that with GemFire Callbacks, callbacks (e.g. CacheListener) can be used by peer/client cache applications to register interests in certain types of events, like Region entry creation/updates, etc. Other callbacks like CacheLoaders can used to read-through to an external data source (e.g. RDBMS) on a Cache miss. Likewise, the CacheWriter can be used to 'write-through' to an external data source on a Cache (Region) create/update, or perhaps asynchronously with a AEQ/AsyncEventListener performing a 'write-behind' to the external data source.
There are many other callbacks and ways in which these callbacks can be used, but nearly all are used programmatically in an GemFire client/peer Cache application to "receive" notifications of some type.
For more details, see the GemFire User Guide on Events and Event Handling.
ANSWER
Now, when it comes to "sending" notifications, GemFire does a fair amount of distribution on your application's behalf. JMX is primarily used to send notifications about management changes... a Region was add, the eviction policy changed, a Function was deployed, etc. In contrast, GemFire sends distribution events when data changes to other members in the cluster that are interested in the event. "Interested" members typically includes other nodes in the cluster that host the same Region and have the same key/values, which need to be updated, and in certain cases atomically (in a TX) for consistency sakes.
Now, if you want to send notifications from your application, then you are better off using Spring and Spring Data GemFire to configure and access GemFire. Spring provides exceptional support for application messaging.
Of course, other options are available including JMS, which Spring provides integration support.
All and all, the events/notifications that are sent and the distribution mechanism used highly depends on the event/notification type. As well, the manner in which to be notified (JMX Notification vs. GemFire Callback) is also dependent on the type of message and purpose.
Sorry for the lengthy explanation; it is loaded/broad question and complex subject that can vary greatly depending on the use case.
Hope this helps (a little ;-)