Event Notification with message oriented middleware Proposal

Problem

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 resources 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.

Proposal

This proposal is to introduce EventBus abstraction with 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

• 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. Publisher publishes the event and forgets.

EventBus

Notion of EventBus shall be introduced which shall have below interface.

public interface EventBus {
    /**
     * publish an event on to the event bus
     *
     * @param event event that needs to be published on the event bus
     */
    void publish(Event event) throws EventBusException;
    /**
     * subscribe to events that matches specified event topics
     *
     * @param topic defines category and type of the events being subscribed to
     * @param subscriber subscriber that intends to receive event notification
     * @return UUID returns the subscription ID
     */
    UUID subscribe(EventTopic topic, EventSubscriber subscriber) throws EventBusException;
    /**
     * unsubscribe to events of a category and a type
     *
     * @param subscriber subscriber that intends to unsubscribe from the event notification
     */
    void unsubscribe(UUID subscriberId, EventSubscriber subscriber) throws EventBusException;
}

public class Event {
    String eventCategory;
    String eventType;
    String eventSource;
    String resourceType;
    String resourceUUID;
    String description;
}

public interface EventSubscriber {
    /**
     * Callback method. EventBus calls this method on occurrence of subscribed event
     *
     * @param event details of the event
     */
    void onEvent(Event event);
}

public class EventTopic {
    String eventCategory;
    String eventType;
    String resourceType;
    String resourceUUID;
    String eventSource;
}