Architectural Patterns
Viable software architectures are built according to some overall structuring principle. We describe these principles with architectural patterns.
An architectural pattern expresses a fundamental structural organization schema for software systems. It provides a set of predefined subsystems, specifies their responsibilities, and includes rules and guidelines for organizing the relationships between them.
Architectural patterns are templates for concrete software architectures. They specify the system-wide structural properties of an application and have an impact on the architecture of its subsystems. The selection of an architectural pattern is therefore a fundamental design decision when developing a software system.
The Model-View-Controller pattern is one of the best-know examples of an architectural pattern. It provides a structure for interactive software systems.
Design Patterns
The subsystems of a software architecture, as well as the relationships between them, usually consist of several smaller architectural units. We describe these using design patterns.
A design pattern provides a scheme for refining the subsystems or components of a software system, or the relationships between them. It describes a commonly recurring structure of communication components that solves a general design problem within a particular context.
Design patterns are medium-scale patterns. They are smaller in scale than architectural patterns but tend to be independent of a particular programming language or programming paradigm, The application of a design pattern has no effect on the fundamental structure of a software system, but may have a strong influence on the architecture of subsystem.
Idioms
Idioms deal with the implementation of particular design issues.
An idiom is a low-level pattern specific to programming language. An idiom describes how to implement particular aspects of components or the relationships between them using the features of the given language.
Idioms represent the lowest-level patterns. The address aspects of both design and implements.
Most idioms are language-specific, the capture existing programming experience. Often the same idiom looks different for different languages, and sometimes an idiom that useful for one programming language does not make sense in another. For example, the C++ community uses reference-counting idioms to manage dynamically allocated resources; Small talk provides a garbage collection mechanism, so has no need for such idioms.
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