5.3 系统顺序图

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