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CCA Event Specification Proposal

Steven Parker

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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();}

}

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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:1. Make wildcarding optional?2. Assume events not performance critical

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Outstanding concerns (BenGripestm)

• Typemap payload is inefficient– Must copy to avoid hijacking by component

listeners

• Solution:1. Fix typemap by adding immutable tags

(freeze/thaw)

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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:1. Fix typemap by allowing objects

2. Add convenience methods for common values in Event interface

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Outstanding concerns (BenGripestm)

• No scoping considered

• Solution:1. Please give us use cases