The use-case model(System Model)
The use-case model provides detailed information about the behaviors of the system or application that you are developing. It contains use-case diagrams and activity diagrams that describe how users interact with the system.
The use-case model identifies the requirements of the system in terms of the functionality that must exist to achieve the goals set out by the user or to solve a problem identified by the user. Uses cases describe the major behaviors that you identify in the requirements and describe the value that the results give the users; they do not describe how the system operates internally. Actors are the users of the system and represent the different roles that people and other systems play when they interact with the system.
Use-case diagrams depict the relationships between the uses cases and actors, and activity diagrams describe the flow of objects and control in each identified behavior.
The Analysis model
The analysis model describes the structure of the system or application that you are modeling. It consists of class diagrams and sequence diagrams that describe the logical implementation of the functional requirements that you identified in the use case model.
The analysis model identifies the main classes in the system and contains a set of use case realizations that describe how the system will be built. Class diagrams describes the static structure of the system by using stereotypes to model the functional parts of the system. Sequence diagrams realize the use cases by describing the flow of events in the use cases when they are executed. These use case realizations model how the parts of the system interact within the context of a specific use case.
You can think of the analysis model as the foundation of the design model since it describes the logical structure of the system, but not how it will be implemented.
The design model
The design model builds on the analysis model by describing, in greater detail, the structure of the system and how the system will be implemented. Classes that were identified in the analysis model are refined to include the implementation constructs.
The design model is based on the analysis and architectural requirements of the system. It represents the application components and determines their appropriate placement and use within the overall architecture.
In the design model, packages contain the design elements of the system, such as design classes, interfaces, and design subsystems, that evolve from the analysis classes. Each package can contain any number of subpackages that further partition the contained design elements. These architectural layers form the basis for a second-level organization of the elements that describe the specifications and implementation details of the system.
Within each package, sequence diagrams illustrate how the objects in the classes interact, state machine diagrams to model the dynamic behavior in classes, component diagrams to describe the software architecture of the system, and deployment diagrams to describe the physical architecture of the system.
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