What is Software Architecture ? What are subsystems and interfaces?

Software architecture defines a set of decisions about the organization. It includes:
- how to select structural elements.
- how to select their interfaces.
- how to select the behavior.
- how to select the composition of these structural and behavioral elements into larger subsystems.
- architectural style that guides this organization.
A software architecture is a description of the sub-systems and components of a software system and the relationships between them.
Software architecture is layered structure of software components and the manner in which these components interact.
A software architecture is modeled using package diagram of UML. A package is a model element that can contain other elements.
A subsystem is a combination of package and class. The advantages of defining subsystem are:
- Development of smaller units is possible.
- Re-usability increases.
- Handling complexity is managed properly.
- Maintainability eases.
- Supports portability.

An interface is a set of operations.
- Allows the separation of the declaration of behavior from the realization of the behavior.
- Serves as a contract to help in the independent development of the components by the development team, and ensures that the components can work together.
- There are two styles of communication subsystems use: client-server and peer-to-peer communication.