Why Draw an SSD?
An interesting and useful question in software design is this: What events are coming in to our system? Why? Because we have to design the software to handle these events (from the mouse, keyboard, another system, …) and execute a response.
Basically, a software system reacts to three things:
1) external events from actors (humans or computers),
2) timer events, and
3) faults or exceptions (which are often from external sources).
What is the Relationship Between SSDs and Use Cases?
An SSD shows system events for one scenario of a use case, therefore it is generated from inspection of a use case
What is the Logical Architecture? And Layers?
The logical architecture is the large-scale organization of the software classes into packages (or namespaces), subsystems, and layers. It's called the logical architecture because there's no decision about how these elements are deployed across different operating system processes or across physical computers in a network (these latter decisions are part of the deployment architecture).
A layer is a very coarse-grained grouping of classes, packages, or subsystems that has cohesive responsibility for a major aspect of the system. Also, layers are organized such that "higher" layers (such as the UI layer) call upon services of "lower" layers, but not normally vice versa.
Typically layers in an OO system include:
User Interface.
Application Logic and Domain Objects— software objects representing domain concepts (for example, a software class Sale) that fulfill application requirements, such as calculating a sale total.
Technical Services— general purpose objects and subsystems that provide supporting technical services, such as interfacing with a database or error logging. These services are usually application-independent and reusable across several systems.
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