Design patterns in software engineering.

In software engineering, a design pattern is a general solution to a common problem in software design. A design pattern isn't 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.

History

Patterns originated as an architectural concept by Christopher Alexander. 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. 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 Reusable Object-Oriented Software was published in 1994. That same year, the first Pattern Languages of Programs conference was held and the following year, the Portland Pattern Repository was set up for documentation of design patterns. The scope of the term remained a matter of dispute into the next decade.

Uses

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 improves code readability for coders and architects familiar with the patterns.

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.

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.

Classification

Design patterns can be classified based on multiple criteria, the most common of which is the basic underlying problem they solve. According to this criterion, design patterns can be classified into various classes, some of which are:

* Fundamental patterns * Creational patterns * Structural patterns * Behavioral patterns * Concurrency patterns * Event handling patterns

Documentation

The documentation for a design pattern should contain enough information about the problem that the pattern addresses, the context in which it is used, and the suggested solution. Nonetheless, authors use their own layouts to document design patterns, and these layouts usually resemble the essential parts. The authors usually include additional sections to provide more information, and organize the essential parts in different sections, possibly with different names.

A commonly used format is the one used by the Gang of Four which contains the following sections:

* Pattern Name and Classification: Every pattern should have a descriptive and unique name that helps in identifying and referring to it. Additionally, the pattern should be classified according to a classification such as the one described earlier. This classification helps in identifying the use of the pattern.

* Intent: This section should describe the goal behind the pattern and the reason for using it. It resembles the problem part of the pattern.

* Also Known As: A pattern could have more than one name. These names should be documented in this section.

* Motivation: This section provides a scenario consisting of a problem and a context in which this pattern can be used. By relating the problem and the context, this section shows when this pattern is used.

* Applicability: This section includes situations in which this pattern is usable. It represents the context part of the pattern.

* Structure: A graphical representation of the pattern. Class diagrams and Interaction diagrams can be used for this purpose.

* Participants: A listing of the classes and objects used in this pattern and their roles in the design.

* Collaboration: Describes how classes and objects used in the pattern interact with each other.

* Consequences: This section describes the results, side effects, and trade offs caused by using this pattern.

* Implementation: This section describes the implementation of the pattern, and represents the solution part of the pattern. It provides the techniques used in implementing this pattern, and suggests ways for this implementation.

* Sample Code: An illustration of how this pattern can be used in a programming language

* Known Uses: This section includes examples of real usages of this pattern.

* Related Patterns: This section includes other patterns that have some relation with this pattern, so that they can be used along with this pattern, or instead of this pattern. It also includes the differences this pattern has with similar patterns.

Critique

Some feel that the need for patterns results from using computer languages or techniques with insufficient abstraction ability. Under ideal factoring, a concept should not be copied, but merely referenced. But if something is referenced instead of copied, then there is no "pattern" to label and catalog. It is also said that design patterns encourage navigational database like structures instead of the allegedly cleaner relational approach where such structures are viewpoints instead of hardwired into programming code. However, critics of the relational approach suggest it does not integrate well enough with behaviour. The level of coupling that should be supplied between behavior and data is a contentious