Modeling Systems with Scenario Sequence Diagrams

Decomposition of the System

Requirements are defined at each level and derive from the level above

• A-Spec
• B-Spec
• C-Spec

Architectural Components (Actors) exist at each level and derive from the level above. Actors execute behaviors.

• System is comprised of sub-system Actors
• A Sub-System is comprised of element Actors
• An Element is comprised of its component Actors

Behavior

The functions of the system (what the system must do) are the system Use Cases. Each necessary function (Use Case) is a requirement at that level of the system.

A function is realized through the actions (behaviors) of that layers actors interacting with each other. Those necessary actions are the requirements for that actor. Thus behavior is derived from the requirements of the layer above and allocate behavior (requirements) on the actors of the system layer below. Implementation (realization) of the behavior (Use Case) is left to the discretion of the designers of that layer.

Structured Decomposition

Use Cases decompose, Actors decompose.

Use Cases ==Behavior==requirements.

Actors==Architecture (and may be physical and or logical abstractions)

Communication