系统顺序图 System Sequence Diagram
把待建系统看成一个黑盒子，研究参与者与系统边界的交互
SSD
System Sequence Diagram
What is SSD
A SSD is a picture that shows, for one particular scenario of a use case, the events that external actors generate, inter-system events , and their order
All systems are treated as a black box
the emphasis of the diagram is events that cross the system boundary from actors to systems
System Event
external input events
actor generates events to a system
system operation
to handle the system event , for example
when a cashier enters an item's ID, the cashier is requesting the POS system to record that item's sale (the enterItem event). That event initiates an operation upon the system