What are Business Events in Oracle Apps?

Business event is, as the term means, is simply an event that occured in business. Business is all about people taking decisions in their respective functions on the information available at that time and communicating further down the supply chain as quickly and accurately as they can: Upon order booking warehouse ships the order or based on invoice apporval payables department pays either the vendor or an employee. All these are business events in the flow of work from one department to another so that common objective is achieved at the end of the day.
Business events in workflow are no different from the above scenario. As the human actions that flow from one department to another, workflow also passes the message from source to destination so that information can be processed in the supply chain. Also business events can be raised and read accross applications and systems.
From technical point of view, just like a department which ”performs” the action that should be communicated, there is a source that “raises” the event. This event when raised is “queued” into an “agent”. Then there is an “agent listener” this picks up the queue from this agent processes it or “dequeues” it. While dequeueing, the “agent listener” processes the rule function that is attached as a ”subscription” using the information that contained in the queue message (“parameters”).
Take an example. Say a new employee joined your company. The day she reports to work, you generate an employee record for her. Say,once she is assinged an employee number, you have to communicate to payroll and systems departments (among others). This information either can be an email to both the departments or inter-office memo for each department. This memo will be picked by mail department and delivered. Once this mail is received, payroll and systems divisions use this information like employee number to complete their own work.
So if you extend this example to business events in wokflow, HR Department raises this event (creation of employee). This event will have a message to be delivered (parameters). This message will be picked up by mail department and delivered (Agent). Once delviered, respective departments receive this information (listened to the queue). This message will be delivered to an appropriate person in the department (subscriber) who will process this information (executing code or kicking a workflow off).
Nailing this down to simple diagram in technical terms, this is how it looks.

There are can be two kinds of subscriptions: Deferred or Immediate. Deferred subscriptions are those that are queued and to be processed by listener. The above example is of that kind. HR deferred the work to be processed by Payroll. On the other hand there are chances that the HR and Payroll handled by the same person. In that case there is nothing to be deferred. This is synchronous process and both the functions are performed in the same department (no need of mail delivery) or by the same person.
And subscribers can process the information in two different ways: either kick off a workflow or execute pl/sql package or java class. If the rule function controls this. Also based on the rule function (workflow vs pl/sql of Java function) appropriate queue is used. If it is pl/sql WF_DEFERRED is used and if it is Java then WF_JAVA_DEFERRED is used.