System sequence diagram of UML and pattern application

The last time I wrote something about the domain model, I 'd like to write some knowledge about the system sequence diagram today...
Ready? Go!
Overview: The system sequence diagram is a product that is quickly and simply created to describe the input and output events related to the system discussed. They are the input of operation contract and Object design.
Sequence Chart example:

 

System sequence diagram (SSD): Used in a specific scenario of a use case, events generated by external participants, their sequence, and events within the system. All systems are regarded as black boxes, which emphasize events that span system boundaries from participants to the system.
Principles: SSD should be drawn for the primary success scenarios of each use case and frequently occurring or complex alternative scenarios
Why do I draw an SSD?
Basically, the software system must respond to the following three events:
1) external events from participants
2) time events or exceptions
Before you design how a software application works in detail, it is best to use its behavior as a "black box" to investigate and define it. System behaviors describe what the system does without explaining how to do it. A part of this description is the system sequence diagram.
Relationship between SSD and Use Cases
SSD shows the system events in a scenario in the use case. Therefore, it is generated from the test case,

Should use case text be displayed in SSD?
This is usually not the case. If you name an SSD properly, you can specify the corresponding use case. For example, processing a sales scenario
How do I name system events and operations?
System events should be expressed at the abstract level of the intent rather than the input device level of the physical.
The name of a system event starts with a verb (add..., input..., end..., generate ...), This can increase the clarity, because it can emphasize that these events are commands or requests.
How to model SSD involving other external systems?
SSD can also be used to describe the collaboration between systems.
Iterative and evolutionary SSD
SSD in up: SSD is part of the use case model, which visualizes the hidden interaction in the use case scenario. Although the creators of up realized and understood the purpose of these graphs, SSD was not directly mentioned in the initial up description.
Up stage
Initial Stage: SSD is usually not introduced at this stage unless you want to make a rough estimation of the technology involved. This estimation is based on the identification of system operations, for example, feature or cocomo2 (see www.ifpug.org)
Refinement phase: Most SSDs are created in the Refinement phase, which helps to identify the details of system events so as to clarify the operations that must be handled and understood by the system, and facilitate the compilation of system operation contracts, in addition, it may be helpful to support estimation.

The above is the system sequence diagram mentioned in "UML and Pattern Design". I hope to share more UML knowledge in this book ..