1. CCA Event Specification Proposal
Steven Parker

2. Events
The essence of events are very simple. Abstract the notion of an occurrence in the system from what should be done when it occurs.
So, instead of having a function that is called when something happens, raise the event (hey, this happened), and let the handlers for that event to process that event.

3. Events: Asynchronous Processing
Events don’t have to be asynchronous
You can raise an event, and wait for it to be handled if you want
Of course, events are most useful when asynchronous processing is supported
You raise the event, and return right away.
The event handlers fire and complete as they can.

4. Event Contents Every event needs a body
The body contains the information needed to handle the event.
For example, changed text in the text box.
It is common, but not required to include a reference to the event sender.
Also, in some systems, there is a event header and properties
More common in distributed systems.

5. Events: Publish-Subscribe Model
One can add the notion of topics, which are named channels on which events are sent and received.
To send an event on a topic, you publish that event
To receive the events that are sent on a topic, you subscribe to that topic.

6. Event Topics It is often useful to organize topics into an hierarchy.
This organizes the namespace better
Also, this allows us to subscribe to topic groups
To get all events that are published to topics within that group.

7. Topic Examples
In these examples, we use the topic naming scheme espoused by the TIBCO middleware system
One could use a URI based scheme, but the idea is the same.
Example 1: A simple topic.
signals.processExit
We have a processExit topic in the signals topic group.

8. Topic Examples Example 2: Another simple topic
workflow.control.exceptions.processExit
While the topic has the same name, it is in a different group, therefore, it is different.
Example 3: Wildcard topic.
signals.*
This matches every topic in the signals group (including sub-groups).

9. Topic Wildcards
The use of wildcards allows us to subscribe to a whole set of topics at once.
Of course, the top-level wildcard topic ‘*’ allows us to receive every event in the system.
With some exceptions.
More complex matching can be supported
Allowing topics like *.signals.*.processExit
We disallow this in the first drafts of the spec.

10. CCA Event Specification
Centers on four classes, one interface
EventService: Manages the EventService
Including global resources, properties, etc.
TopicManager: Factory for Topic objects
Topics can not be made directly.
Topic: Represents a topic
Stores properties, allows for events to be raised, event handlers to added, removed.

11. CCA Event Specification
Event: Allows for the creation of an event
This includes the event body and a header
Event body and header are typemaps
EventHandler: Abstract Interface
Any actual event handler must implement this interface to receive events.

12. Example (C++ Fragment)
Topic echoTopic = TopicManager.CreateTopic(“services.echo”);
echoTopic.AddHandler(“handlerKey1”, new EchoEventHandler());
if (echoTopic != null) {
echoTopic.RaiseEvent(new Event(“Hello World”)); }
class EchoEventHandler : IEventHandler {
void HandleEvent(Object Sender, Event newEvent)
cout << newEvent.Body(); }

13. Outstanding concerns (BenGripestm)
Strings for event topics is less efficient than classes
Much easier for the user
Allows more flexible wildcarding
Avoids explosion of tiny classes
Conscious tradeoff
Potential solutions:
Make wildcarding optional?
Assume events not performance critical

14. Outstanding concerns (BenGripestm)
Typemap payload is inefficient
Must copy to avoid hijacking by component listeners
Solution:
Fix typemap by adding immutable tags (freeze/thaw)

15. Outstanding concerns (BenGripestm)
Event payload is cumbersome
Inconvenient to get common data from typemaps
Event payload has component strings instead of component pointers (typemap cannot hold objects)
Solutions:
Fix typemap by allowing objects
Add convenience methods for common values in Event interface

16. Outstanding concerns (BenGripestm)
No scoping considered
Solution:
Please give us use cases