The online whiteboard of Kristofer Palmvik
Next in my series on Design Patterns and their implementation in PL/SQL – also see previous articles on Dependency Injection of Values, Dependency Injection of Interfaces and the Template Pattern – is a discussion of the Observer Pattern. We will see a fairly elegant implementation of this pattern that provides for a very loosely (runtime) coupled architecture where packages can advise procedures of custom events.
What is extremely fancy and powerful – and not nearly used enough – is the architectural pattern that AQ allows us to introduce. Queues are a key concept for achieving decoupling. Decoupling itself is like the holy grail of architects – because it allows agility and reuse. Through a Queue, a publisher (or discoverer) of events can make them available to consumers that may or may not exist and that may or may not be available at the time of publishing the event. The publisher of the event does its duty by putting the event on the queue and is then free to go. The queue infrastructure will take it from there.