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Introduce a event bus
There are multiple parties like event-driven workflows systems, monitoring system, usage & billing system would be interested in getting notification of a events in the cloud in near instantaneous manner. In CloudStack event could be state change of virtual or psychical resources, an action performed by user (action events), policy based events (alerts). Currently CloudStack has no notification mechanism to let the interested parties know of the occurrence of an event. Polling by invoking listEvents and listAlerts is the only way to know occurrence of events. Also there are no externsion hooks one can implement to get the event notification. Though there is StateListener interface which has both pre and post state transition event call back methods, only user VM resource has state listener implemented. Most of the resorces volume, network, nic, etc has states but does not have state machine and StateListener. So for a component interested getting notification need to get tightly coupled into orchestration workflow. For e.g, code to generate usage date (usage events) is spread across the orchestration components.
This proposal is to introduce an in-process event bus in the management server and by which achieve event notifications. Scope of the event bus is only with in the management server, so only CloudStack components and extension plug-in's can be subscribers to the events. Goal of this proposal is to have bare minimum mechanism in the CloudStack code, using which more powerful notification mechanism (like SMS, email etc) can be built as CloudStack extensions.
Ideal requirements for such event bus is to be
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.
For e.g. UserVMStateListener which is listener for the state changes of user VM's would create event object and post on to the event bus as below.
public boolean postStateTransitionEvent(State oldState, Event event, State newState, VirtualMachine vo, boolean status, Object opaque) {
_eventBus.post(new UserVmStateChangeEventVO(....));
}
A subscriber interested in the user VM state change would register to the event and get the events as below
_eventBus.register(this)
@Subscribe
public void userVMStateChangetHandler(UserVmStateChangeEventVO event)
Following are some use cases demonstrated in the diagram below
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