Design pattern (computer science)

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In software engineering, a design pattern is a general reusable solution to a commonly occurring problem in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that can be used in many different situations. Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved. Algorithms are not thought of as design patterns, since they solve computational problems rather than design problems.

Not all software patterns are design patterns. Design patterns deal specifically with problems at the level of software design. Other kinds of patterns, such as architectural patterns, describe problems and solutions that have alternative scopes.


[edit] History

Patterns originated as an architectural concept by Christopher Alexander (1977/79). In 1987, Kent Beck and Ward Cunningham began experimenting with the idea of applying patterns to programming and presented their results at the OOPSLA conference that year.[1][2] In the following years, Beck, Cunningham and others followed up on this work.

Design patterns gained popularity in computer science after the book Design Patterns: Elements of Reusable Object-Oriented Software was published in 1994 (Gamma et al.). That same year, the first Pattern Languages of Programming Conference was held and the following year, the Portland Pattern Repository was set up for documentation of design patterns. The scope of the term remains a matter of dispute. Notable books in the design pattern genre include:

Although the practical application of design patterns is a phenomenon, formalization of the concept of a design pattern languished for several years.[3]

[edit] Practice

Design patterns can speed up the development process by providing tested, proven development paradigms. Effective software design requires considering issues that may not become visible until later in the implementation. Reusing design patterns helps to prevent subtle issues that can cause major problems, and it also improves code readability for coders and architects who are familiar with the patterns.

In order to achieve flexibility, design patterns usually introduce additional levels of indirection, which in some cases may complicate the resulting designs and hurt application performance.

By definition, a pattern must be programmed anew into each application that uses it. Since some authors see this as a step backward from software reuse as provided by components, researchers have worked to turn patterns into components. Meyer and Arnout claim a two-thirds success rate in componentizing the best-known patterns.[4]

Often, people only understand how to apply certain software design techniques to certain problems. These techniques are difficult to apply to a broader range of problems. Design patterns provide general solutions, documented in a format that doesn't require specifics tied to a particular problem.

[edit] Structure

Design patterns are composed of several sections (see Documentation below). Of particular interest are the Structure, Participants, and Collaboration sections. These sections describe a design motif: a prototypical micro-architecture that developers copy and adapt to their particular designs to solve the recurrent problem described by the design pattern. A micro-architecture is a set of program constituents (e.g., classes, methods...) and their relationships. Developers use the design pattern by introducing in their designs this prototypical micro-architecture, which means that micro-architectures in their designs will have structure and organization similar to the chosen design motif..

In addition, patterns allow developers to communicate using well-known, well understood names for software interactions. Common design patterns can be improved over time, making them more robust than ad-hoc designs.

[edit] Domain specific patterns

Efforts have also been made to codify design patterns in particular domains, including use of existing design patterns as well as domain specific design patterns. Examples include User Interface design patterns,[5] Information Visualization [6], Secure Usability[7] and web design.[8]

The Pattern Languages of Programming Conference (annual, 1994—) proceedings includes many examples of domain specific patterns.

[edit] Classification

Design patterns were originally grouped into the categories Creational patterns, Structural patterns, and Behavioral patterns, and described them using the concepts of delegation, aggregation, and consultation. For further background on object-oriented design, see coupling and cohesion. For further background on object-oriented programming, see inheritance, interface, and polymorphism. Another classification has also introduced the notion of architectural design pattern which may be applied at the architecture level of the software such as the Model-View-Controller pattern.

Name Description In Design Patterns (book) In Code Complete[9]
Creational patterns
Abstract factory Provide an interface for creating families of related or dependent objects without specifying their concrete classes. Yes Yes
Factory method Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. Yes Yes
Builder Separate the construction of a complex object from its representation so that the same construction process can create different representations. Yes No
Lazy initialization Tactic of delaying the creation of an object, the calculation of a value, or some other expensive process until the first time it is needed. No No
Object pool Avoid expensive acquisition and release of resources by recycling objects that are no longer in use No No
Prototype Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype. Yes No
Singleton Ensure a class has only one instance, and provide a global point of access to it. Yes Yes
Multiton Ensure a class has only named instances, and provide global point of access to them. No No
Resource acquisition is initialization Ensure that resources are properly released by tying them to the lifespan of suitable objects. No No
Structural patterns
Adapter Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces. Yes Yes
Bridge Decouple an abstraction from its implementation so that the two can vary independently. Yes Yes
Composite Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly. Yes Yes
Decorator Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality. Yes Yes
Facade Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use. Yes Yes
Flyweight Use sharing to support large numbers of fine-grained objects efficiently. Yes No
Proxy Provide a surrogate or placeholder for another object to control access to it. Yes No
Behavioral patterns
Chain of responsibility Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. Yes No
Command Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. Yes No
Interpreter Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language. Yes No
Iterator Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation. Yes Yes
Mediator Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently. Yes No
Memento Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later. Yes No
Null Object designed to act as a default value of an object. No No
Observer Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Yes Yes
State Allow an object to alter its behavior when its internal state changes. The object will appear to change its class. Yes No
Strategy Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it. Yes Yes
Specification Recombinable business logic in a boolean fashion No No
Template method Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure. Yes Yes
Visitor Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. Yes No
Concurrency patterns
Active Object The Active Object design pattern decouples method execution from method invocation that reside in their own thread of control. The goal is to introduce concurrency, by using asynchronous method invocation and a scheduler for handling requests. No No
Event-Based Asynchronous The event-based asynchronous design pattern addresses problems with the Asychronous Pattern that occur in multithreaded programs.[10] No No
Balking The Balking pattern is a software design pattern that only executes an action on an object when the object is in a particular state. No No
Double checked locking Double-checked locking is a software design pattern also known as "double-checked locking optimization". The pattern is designed to reduce the overhead of acquiring a lock by first testing the locking criterion (the 'lock hint') in an unsafe manner; only if that succeeds does the actual lock proceed.

The pattern, when implemented in some language/hardware combinations, can be unsafe. It can therefore sometimes be considered to be an anti-pattern.

No No
Guarded In concurrent programming, guarded suspension is a software design pattern for managing operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed. No No
Monitor object A monitor is an approach to synchronize two or more computer tasks that use a shared resource, usually a hardware device or a set of variables. No No
Read write lock A read/write lock pattern or simply RWL is a software design pattern that allows concurrent read access to an object but requires exclusive access for write operations. No No
Scheduler The scheduler pattern is a software design pattern. It is a concurrency pattern used to explicitly control when threads may execute single-threaded code. No No
Thread pool In the thread pool pattern in programming, a number of threads are created to perform a number of tasks, which are usually organized in a queue. Typically, there are many more tasks than threads. No No
Thread-specific storage Thread-local storage (TLS) is a computer programming method that uses static or global memory local to a thread. No No
Reactor The reactor design pattern is a concurrent programming pattern for handling service requests delivered concurrently to a service handler by one or more inputs. The service handler then demultiplexes the incoming requests and dispatches them synchronously to the associated request handlers. No No

[edit] Documentation

The documentation for a design pattern describes the context in which the pattern is used, the forces within the context that the pattern seeks to resolve, and the suggested solution.[11] There is no single, standard format for documenting design patterns. Rather, a variety of different formats have been used by different pattern authors. However, according to Martin Fowler certain pattern forms have become more well-known than others, and consequently become common starting points for new pattern writing efforts.[12] One example of a commonly used documentation format is the one used by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides (collectively known as the "Gang of Four", or GoF for short) in their book Design Patterns. It contains the following sections:

  • Pattern Name and Classification: A descriptive and unique name that helps in identifying and referring to the pattern.
  • Intent: A description of the goal behind the pattern and the reason for using it.
  • Also Known As: Other names for the pattern.
  • Motivation (Forces): A scenario consisting of a problem and a context in which this pattern can be used.
  • Applicability: Situations in which this pattern is usable; the context for the pattern.
  • Structure: A graphical representation of the pattern. Class diagrams and Interaction diagrams may be used for this purpose.
  • Participants: A listing of the classes and objects used in the pattern and their roles in the design.
  • Collaboration: A description of how classes and objects used in the pattern interact with each other.
  • Consequences: A description of the results, side effects, and trade offs caused by using the pattern.
  • Implementation: A description of an implementation of the pattern; the solution part of the pattern.
  • Sample Code: An illustration of how the pattern can be used in a programming language
  • Known Uses: Examples of real usages of the pattern.
  • Related Patterns: Other patterns that have some relationship with the pattern; discussion of the differences between the pattern and similar patterns.

[edit] Criticism

In the field of computer science, there exist some criticisms regarding the concept of design patterns.

[edit] Workarounds for missing language features

Users of dynamic programming languages have discussed many design patterns as workarounds for the limitations of languages such as C++ and Java. For instance, the Visitor pattern need not be implemented in a language that supports multimethods. The purpose of Visitor is to add new operations to existing classes without modifying those classes. In C++, a class is declared as a syntactic structure with a specific and closed set of methods. In a language with multimethods, such as Common Lisp, methods for a class are outside of the class structure, and one may add new methods without modifying it. Similarly, the Decorator pattern amounts to implementing dynamic delegation, as found in Common Lisp, Objective C, Self and JavaScript.

Peter Norvig, in Design Patterns in Dynamic Programming, discusses the triviality of implementing various patterns in dynamic languages. [13] Norvig and others have described language features that encapsulate or replace various patterns that a C++ user must implement for themselves.

[edit] Does not differ significantly from other abstractions

Some authors[who?] allege that design patterns don't differ significantly from other forms of abstraction[citation needed], and that the use of new terminology (borrowed from the architecture community) to describe existing phenomena in the field of programming is unnecessary. The Model-View-Controller paradigm is cited as an example of a "pattern" which predates the concept of "design patterns" by several years.[14] It is further argued by some[who?] that the primary contribution of the Design Patterns community (and the Gang of Four book) was the use of Alexander's pattern language as a form of documentation; a practice which is often ignored in the literature.[citation needed]

[edit] See also

[edit] References

  1. ^ Smith, Reid (October 1987). "Panel on design methodology" in OOPSLA '87. OOPSLA '87 Addendum to the Proceedings. doi:10.1145/62138.62151. , "Ward cautioned against requiring too much programming at, what he termed, 'the high level of wizards.' He pointed out that a written 'pattern language' can significantly improve the selection and application of abstractions. He proposed a 'radical shift in the burden of design and implementation' basing the new methodology on an adaptation of Christopher Alexander's work in pattern languages and that programming-oriented pattern languages developed at Tektronix has significantly aided their software development efforts."
  2. ^ Beck, Kent; Ward Cunningham (September 1987). "Using Pattern Languages for Object-Oriented Program" in OOPSLA '87. OOPSLA '87 workshop on Specification and Design for Object-Oriented Programming'. Retrieved on 2006-05-26. 
  3. ^ Baroni, Aline Lúcia; Yann-Gaël Guéhéneuc and Hervé Albin-Amiot (June 2003). "Design Patterns Formalization". written at Nantes (PDF). École Nationale Supérieure des Techniques Industrielles et des Mines de Nantes. Retrieved on 2007-12-29. 
  4. ^ Meyer, Bertrand; Karine Arnout (July 2006). "Componentization: The Visitor Example". IEEE Computer (IEEE) 39 (7): 23–30. 
  5. ^ Laakso, Sari A. (2003-09-16). "Collection of User Interface Design Patterns". University of Helsinki, Dept. of Computer Science. Retrieved on 2008-01-31. 
  6. ^ Heer, J.; M. Agrawala (2006). "Software Design Patterns for Information Visualization". IEEE Transactions on Visualization and Computer Graphics 12 (5): 853. doi:10.1109/TVCG.2006.178. 
  7. ^ Simson L. Garfinkel (2005). Design Principles and Patterns for Computer Systems That Are Simultaneously Secure and Usable. 
  8. ^ "Yahoo! Design Pattern Library". Retrieved on 2008-01-31. 
  9. ^ McConnell, Steve (June 2004). "Design in Construction". Code Complete (2nd ed.). Microsoft Press. pp. 104. ISBN 978-0735619678. "Table 5.1 Popular Design Patterns" 
  10. ^ Christian Nagel, Bill Evjen, Jay Glynn, Karli Watson, and Morgan Skinner (2008). "Event-based Asynchronous Pattern". Professional C# 2008. Wiley. pp. 570–571. ISBN 0470191376. 
  11. ^ Gabriel, Dick. "A Pattern Definition". Retrieved on 2007-03-06. 
  12. ^ Fowler, Martin (2006-08-01). "Writing Software Patterns". Retrieved on 2007-03-06. 
  13. ^ Norvig, Peter (1998-03-17). "Design Patterns in Dynamic Programming". Retrieved on 2007-12-29. 
  14. ^ Reenskaug, Trygve. "MVC XEROX PARC 1978-79". Retrieved on 2008-06-09. 

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