Software architecture is the high level structure of a software system, the discipline of creating such structures, and the documentation of these structures. It is the set of structures needed to reason about the software system, and comprises the software elements, the relations between them, and the properties of both elements and relations. The architecture of a software system is a metaphor, analogous to the architecture of a building
Software architecture choices include specific structural options from possibilities in the design of software. For example, the systems that controlled the space shuttle launch vehicle have the requirement of being very fast, and very reliable, in principle. Therefore an appropriate real-time computing language would be chosen. Similarly, multiple redundant independently produced copies of a program running on independent hardware and cross-checking results would be a software system architecture choice to satisfy the need for reliability. Software architecture is about making fundamental structural choices which are costly to change once implemented, i.e., which are used to 'house' the more changeable elements of the program, e.g., an operating system.
Documenting software architecture facilitates communication between stakeholders, captures early decisions about the high-level design, and allows reuse of design components between projects.
Software architecture exhibits the following:
Multitude of stakeholders: software systems have to cater to a variety of stakeholders such as business managers, owners, users and operators. These stakeholders all have their own concerns with respect to the system. Balancing these concerns and demonstrating how they are addressed is part of designing the system.This implies that architecture involves dealing with a broad variety of concerns and stakeholders, and has a multidisciplinary nature.
Separation of concerns: the established way for architects to reduce complexity is by separating the concerns that drive the design. Architecture documentation shows that all stakeholder concerns are addressed by modeling and describing the architecture from separate points of view associated with the various stakeholder concerns. These separate descriptions are called architectural views (see for example the 4+1 Architectural View Model).
Quality-driven: classic software design approaches (e.g. Jackson Structured Programming) were driven by required functionality and the flow of data through the system, but the current insight is that the architecture of a software system is more closely related to its quality attributes such as fault-tolerance, backward compatibility, extensibility, reliability, maintainability, availability, security, usability, and other such –ilities. Stakeholder concerns often translate into requirements on these quality attributes, which are variously called non-functional requirements, extra-functional requirements, behavioral requirements, or quality attribute requirements.
Recurring styles: like building architecture, the software architecture discipline has developed standard ways to address recurring concerns. These “standard ways” are called by various names at various levels of abstraction. Common terms for recurring solutions are architectural style, strategy or tactic, reference architecture and architectural pattern.
Conceptual integrity: a term introduced by Fred Brooks in The Mythical Man-Month to denote the idea that the architecture of a software system represents an overall vision of what it should do and how it should do it. This vision should be separated from its implementation. The architect assumes the role of “keeper of the vision”, making sure that additions to the system are in line with the architecture, hence preserving conceptual integrity.