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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.
Opinions vary as to the scope of software architecture:
There is no sharp distinction between software architecture versus design and requirements engineering (see Related fields below). They are all part of a “chain of intentionality” from high-level intentions to low-level details.
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.
Software architecture is an “intellectually graspable” abstraction of a complex system. This abstraction provides a number of benefits:
The term software architecture was first used in the late 1960s , but became prevalent only in the beginning of the 1990s. The field of computer science had encountered problems associated with complexity since its formation. Earlier problems of complexity were solved by developers by choosing the right data structures, developing algorithms, and by applying the concept of separation of concerns. Although the term “software architecture” is relatively new to the industry, the fundamental principles of the field have been applied sporadically by software engineering pioneers since the mid-1980s. Early attempts to capture and explain software architecture of a system were imprecise and disorganized, often characterized by a set of box-and-line diagrams. 
Software architecture as a concept has its origins in the research of Edsger Dijkstra in 1968 and David Parnas in the early 1970s. These scientists emphasized that the structure of a software system matters and getting the structure right is critical. During the 1990s there was a concerted effort to define and codify fundamental aspects of the discipline, with research work concentrating on architectural styles (patterns), architecture description languages, architecture documentation, and formal methods.
Research institutions have played a prominent role in furthering software architecture as a discipline. Mary Shaw and David Garlan of Carnegie Mellon wrote a book titled Software Architecture: Perspectives on an Emerging Discipline in 1996, which promoted software architecture concepts such as components, connectors, and styles. The University of California, Irvine's Institute for Software Research's efforts in software architecture research is directed primarily in architectural styles, architecture description languages, and dynamic architectures.
IEEE 1471-2000, Recommended Practice for Architecture Description of Software-Intensive Systems, was the first formal standard in the area of software architecture. It was adopted in 2007 by ISO as ISO/IEC 42010:2007. In November 2011, IEEE 1471-2000 was superseded by ISO/IEC/IEEE 42010:2011, Systems and software engineering — Architecture description (jointly published by IEEE and ISO).
While in IEEE 1471, software architecture was about the architecture of “software-intensive systems”, defined as “any system where software contributes essential influences to the design, construction, deployment, and evolution of the system as a whole”, the 2011 edition goes a step further by including the ISO/IEC 15288 and ISO/IEC 12207 definitions of a system, which embrace not only hardware and software, but also “humans, processes, procedures, facilities, materials and naturally occurring entities”. This reflects the relationship between software architecture, Enterprise Architecture and Solution Architecture.
There are many activities that a software architect performs. A software architect typically works with project managers, discusses architecturally significant requirements with stakeholders, designs a software architecture, evaluates a design, communicates with designers and stakeholders, documents the architectural design and more. There are four core activities in software architecture design. These core architecture activities are performed iteratively and at different stages of the initial software development life-cycle, as well as over the evolution of a system.
Architectural Analysis is the process of understanding the environment in which a proposed system or systems will operate and determining the requirements for the system. The input or requirements to the analysis activity can come from any number of stakeholders and include items such as:
Architectural Synthesis or design is the process of creating an architecture. Given the requirements determined by the analysis, the current state of the design and the results of any evaluation activities, the design is created and improved. See  for a discussion of various techniques for improving a current design.
Architecture Evaluation is the process of determining how well the current design or a portion of it satisfies the requirements derived during analysis. An evaluation can occur whenever an architect is considering a design decision, it can occur after some portion of the design has been completed, it can occur after the final design has been completed or it can occur after the system has been constructed. Some of the available software architecture evaluation techniques include Architecture Tradeoff Analysis Method (ATAM) and TARA. Frameworks for comparing the techniques are discussed in frameworks such as SARA Report  and Architecture reviews: practice and experience.
Architecture Evolution is the process of maintaining and adapting an existing software architecture to meet requirement and environmental changes. As software architecture provides a fundamental structure of a software system, its evolution and maintenance would necessarily impact its fundamental structure. As such, architecture evolution is concerned with adding new functionality as well as maintaining existing functionality and system behaviour.
Architecture requires critical supporting activities. These supporting activities take place throughout the core software architecture process. They include knowledge management and communication, design reasoning and decision making, and documentation.
Software architecture supporting activities are carried out during core software architecture activities. These supporting activities assist a software architect to carry out analysis, synthesis, evaluation and evolution. For instance, an architect has to gather knowledge, make decisions and document during the analysis phase.
Software architecture description involves the principles and practices of modeling and representing architectures, using mechanisms such as: architecture description languages, architecture viewpoints, and architecture frameworks.
An architecture description language (ADL) is any means of expression used to describe a software architecture (ISO/IEC/IEEE 42010). Many special-purpose ADLs have been developed since the 1990s, including AADL (SAE standard), Wright (developed by Carnegie Mellon), Acme (developed by Carnegie Mellon), xADL (developed by UCI), Darwin (developed by Imperial College London), DAOP-ADL (developed by University of Málaga), SBC-ADL (developed by National Sun Yat-Sen University), and ByADL (University of L'Aquila, Italy).
Software architecture descriptions are commonly organized into views, which are analogous to the different types of blueprints made in building architecture. Each view addresses a set of system concerns, following the conventions of its viewpoint, where a viewpoint is a specification that describes the notations, modeling and analysis techniques to use in a view that express the architecture in question from the perspective of a given set of stakeholders and their concerns (ISO/IEC/IEEE 42010). The viewpoint specifies not only the concerns framed (i.e., to be addressed) but the presentation, model kinds used, conventions used and any consistency (correspondence) rules to keep a view consistent with other views.
An architecture framework captures the "conventions, principles and practices for the description of architectures established within a specific domain of application and/or community of stakeholders" (ISO/IEC/IEEE 42010). A framework is usually implemented in terms of one or more viewpoints or ADLs.
An architectural pattern is a general, reusable solution to a commonly occurring problem in software architecture within a given context. Architectural patterns are often documented as software design patterns.
Following traditional building architecture, a 'software architectural style' is a specific method of construction, characterized by the features that make it notable" (Architectural style). "An architectural style defines: a family of systems in terms of a pattern of structural organization; a vocabulary of components and connectors, with constraints on how they can be combined." "Architectural styles are reusable 'packages' of design decisions and constraints that are applied to an architecture to induce chosen desirable qualities."
There are many recognized architectural patterns and styles, among them:
Some treat architectural patterns and architectural styles as the same, some treat styles as specializations of patterns. What they have in common is both patterns and styles are idioms for architects to use, they "provide a common language" or "vocabulary" with which to describe classes of systems.
There are also concerns that software architecture leads to too much Big Design Up Front, especially among proponents of Agile software development. A number of methods have been developed to balance the trade-offs of up-front design and agility. IEEE Software devoted a special issue to the interaction between agility and architecture.
Software architecture erosion (or "decay") refers to the gap observed between the planned and actual architecture of a software system as realized in its implementation. Software architecture erosion occurs when implementation decisions either do not fully achieve the architecture-as-planned or otherwise violate constraints or principles of that architecture. The gap between planned and actual architectures is sometimes understood in terms of the notion of technical debt.
As an example, consider a strictly layered system, where each layer can only use services provided by the layer immediately below it. Any source code component that does not observe this constraint represents an architecture violation. If not corrected, such violations can transform the architecture into a monolithic block, with adverse effects on understandability, maintainability, and evolvability.
Various approaches have been proposed to address erosion. "These approaches, which include tools, techniques and processes, are primarily classified into three generic categories that attempt to minimise, prevent and repair architecture erosion. Within these broad categories, each approach is further broken down reflecting the high-level strategies adopted to tackle erosion. These are: process-oriented architecture conformance, architecture evolution management, architecture design enforcement, architecture to implementation linkage, self-adaptation and architecture restoration techniques consisting of recovery, discovery and reconciliation."
There are two major techniques to detect architectural violations: reflexion models and domain-specific languages. Reflexion model (RM) techniques compare a high-level model provided by the system's architects with the source code implementation. Examples of commercial RM-based tools include the Bauhaus Suite (developed by Axivion), SAVE (developed by Fraunhofer IESE) and Structure-101 (developed by Headway Software). There are also domain-specific languages with focus on specifying and checking architectural constraints, including .QL (developed by Semmle Limited) and DCL (from Federal University of Minas Gerais).
Software architecture recovery (or reconstruction, or reverse engineering) includes the methods, techniques and processes to uncover a software system's architecture from available information, including its implementation and documentation. Architecture recovery is often necessary to make informed decisions in the face of obsolete or out-of-date documentation and architecture erosion: implementation and maintenance decisions diverging from the envisioned architecture.
Architecture is design but not all design is architectural. In practice, the architect is the one who draws the line between software architecture (architectural design) and detailed design (non-architectural design). There aren't rules or guidelines that fit all cases, although there have been attempts to formalize the distinction. According to the Intension/Locality Hypothesis, the distinction between architectural and detailed design is defined by the Locality Criterion, according to which a statement about software design is non-local (architectural) if and only if a program that satisfies it can be expanded into a program that does not. For example, the client–server style is architectural (strategic) because a program that is built on this principle can be expanded into a program that is not client–server—for example, by adding peer-to-peer nodes.
Requirements engineering and software architecture can be seen as complementary approaches: while software architecture targets the 'solution space' or the 'how', requirements engineering addresses the 'problem space' or the 'what'. Requirements engineering entails the elicitation, negotiation, specification, validation, documentation and management of requirements. Both requirements engineering and software architecture revolve around stakeholder concerns, needs and wishes.
There is considerable overlap between requirements engineering and software architecture, as evidenced for example by a study into five industrial software architecture methods that concludes that "the inputs (goals, constrains, etc.) are usually ill-defined, and only get discovered or better understood as the architecture starts to emerge" and that while "most architectural concerns are expressed as requirements on the system, they can also include mandated design decisions". In short, the choice of required behavior given a particular problem impacts the architecture of the solution that addresses that problem, while at the same time the architectural design may impact the problem and introduce new requirements. Approaches such as the Twin Peaks model  aim to exploit the synergistic relation between requirements and architecture.
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