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• light weight (a library)
• provide simple interface
• decouple the publisher and subscriber so that they need not be aware of other
• powerful enough to provide filters for events of a type to wildcard (all events)
• deliver events asynchronously

In order for CloudStack be able to generate rich set of events and have clean mechanism to generate events, its important to have state machine associated with every resource that is managed by CloudStack. So another goal of this proposal is to associate with every resource (User VM, Volume, Nic, Network, public IP, snapshot, template etc) with a state machine and generate events as part of the state change. So the publishers of events in CloudStack will become that StateListeners associated with the resource. 

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