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Event notification framework provides a means for the Management Server components to publish and subscribe to &PRODUCT; events. Event notification is achieved by implementing the concept of event bus abstraction in the Management Server. An event bus is introduced in the Management Server that allows the &PRODUCT;components and extension plug-ins to subscribe to the events by using the Advanced Message Queuing Protocol (AMQP) client. In &PRODUCT;, a default implementation of event bus is provided as a plug-in that uses the RabbitMQ AMQP client. The AMQP client pushes the published events to a compatible AMQP server. Therefore all the &PRODUCT; events are published to an exchange in the AMQP server. A new event for state change, resource state change, is introduced as part of Event notification framework. Every resource, such as user VM, volume, NIC, network, public IP, snapshot, and template, is associated with a state machine and generates events as part of the state change. That implies that a change in the state of a resource results in a state change event, and the event is published in the corresponding state machine on the event bus. All the &PRODUCT; events (alerts, action events, usage events) and the additional category of resource state change events, are published on to the events bus. The following are some of the use cases: Usage or Billing Engines: A third-party cloud usage solution can implement a plug-in that can connects to &PRODUCT; to subscribe to &PRODUCT; events and generate usage data. The usage data is consumed by their usage software. AMQP plug-in can place all the events on the a message queue, then a AMQP message broker can provide topic-based notification to the subscribers. Publish and Subscribe notification service can be implemented as a pluggable service in &PRODUCT; that can provide rich set of APIs for event notification, such as topics-based subscription and notification. Additionally, the pluggable service can deal with multi-tenancy, authentication, and authorization issues. As a &PRODUCT; administrator, perform the following one-time configuration to enable event notification framework. At run time no changes can control the behaviour. Open 'componentContext.xml. Define a bean named eventNotificationBus as follows: name : Specify a name for the bean. server : The name or the IP address of the RabbitMQ AMQP server. port : The port on which RabbitMQ server is running. username : The username associated with the account to access the RabbitMQ server. password : The password associated with the username of the account to access the RabbitMQ server. exchange : The exchange name on the RabbitMQ server where &PRODUCT; events are published. A sample bean is given below: <bean id="eventNotificationBus" class="org.apache.cloudstack.mom.rabbitmq.RabbitMQEventBus"> <property name="name" value="eventNotificationBus"/> <property name="server" value="127.0.0.1"/> <property name="port" value="5672"/> <property name="username" value="guest"/> <property name="password" value="guest"/> <property name="exchange" value="cloudstack-events"/> </bean> The eventNotificationBus bean represents the org.apache.cloudstack.mom.rabbitmq.RabbitMQEventBus class. Restart the Management Server.