COM & .NET Component Services

Chapter 9. COM+ Event Service

In a component-oriented program, an object provides services to clients by letting clients invoke methods on the object’s interfaces. But what if a client (or more than one client) wants to be notified about an event that occurs on the object side? Traditionally, the client implements a callback interface called a sink interface . To notify the client of an occurring event, the object calls a method on the sink interface. Each method on a sink interface corresponds to a type of event fired by the object.

This model raises a few questions: How does the object access the sink interfaces? How do clients find out which sink interfaces the object fires events on? How do the clients unsubscribe from event notification?

As you will see shortly, the COM+ events service is an exciting new service that evolved to address the classic problems of event notification and reception. COM+ events are also known as Loosely Coupled Events (LCE), because they provide an effective way of decoupling components. They put the logic for publishing and subscribing to events outside the scope of the component. Besides significantly improving on the classic COM model for handling events, LCE takes full advantage of such COM+ services as transactions, queuing, and security. Managing event publishing and subscription can be done both declaratively via the Component Services Explorer and programmatically using the COM+ Catalog.

To fully appreciate the elegance of the COM+ events ...