29.7. Illustrating External and Interval Events
Event Types
It is useful to categorize events as follows:
External event— Also known as a system event, is caused by something (for example, an actor) outside our system boundary. SSDs illustrate external events. Noteworthy external events precipitate the invocation of system operations to respond to them.
When a cashier presses the “enter item” button on a POS terminal, an external event has occurred.
Internal event— Caused by something inside our system boundary. In terms of software, an internal event arises when a method is invoked via a message or signal that was sent from another internal object. Messages in interaction diagrams suggest internal events.
When a Sale receives a makeLineItem
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