Page #110 (Loosely Coupled Events (LCEs))

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Summary

An event is a notification of a change in some interesting data. The publisher fires an event, and one or more subscribers receive the event.

Event system implementation can be broadly categorized as tightly coupled events (TCEs) or loosely coupled events (LCEs).

Under TCE, the publisher and the subscriber have a direct connection to each other. The lifetimes of the publisher and the subscriber are tightly coupled. A connection point is a commonly used technique for TCEs.

Another example of TCE implementation is MSMQ. When a message listener enables notification on a specific MSMQ queue, MSMQ fires an event when a message arrives in the queue.

Under LCE, the publisher and the subscriber do not have a direct connection to each other. COM+ events is a mechanism based on LCE. Under COM+ events, the publisher and the subscriber are decoupled by an event class.

COM+ events can have persistent subscribers or transient subscribers. Persistent subscribers can survive a system shutdown while the transient subscribers cannot.

COM+ events support queuing the method call, from the publisher to the event class as well as from the event class to the subscribers.

COM+ events also support filtering of data, both at the publisher s side as well as the subscriber s side.

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