Don't Repeat Yourself (DRY, also known as Once and Only Once or Single Point of Truth (SPOT)) is a process philosophy aimed at reducing duplication, particularly in computing. The philosophy emphasizes that information should not be duplicated, because duplication increases the difficulty of change, may decrease clarity, and leads to opportunities for inconsistency. DRY is a core principle of Andy Hunt and Dave Thomas's book The Pragmatic Programmer. They apply it quite broadly to include "database schemas, test plans, the build system, even documentation." ^[1] When the DRY principle is applied successfully, a modification of any single element of a system does not change other logically-unrelated elements. Additionally, elements that are logically related all change predictably and uniformly, and are thus kept in sync. Originally, the word computing was synonymous with counting and calculating, and a science and technology that deals with the original sense of computing mathematical calculations. ... Information is the result of processing, manipulating and organizing data in a way that adds to the knowledge of the person receiving it. ... Consistency has three technical meanings: In mathematics and logic, as well as in theoretical physics, it refers to the proposition that a formal theory or a physical theory contains no contradictions. ... Andy (Andrew) Hunt was a Professional Football player for 10 years in England, playing for Newcastle United, West Bromwich Albion and finally for Charlton Athletic, scoring 135 goals in 361 appearances for his respective teams. ... Dave Thomas speaking at the Pasadena Rails Studio This page is about Dave (David) Thomas, the computer programmer and author/editor, and not about Dave Thomas, founder of IBM OTI Labs. ... book cover The Pragmatic Programmer: from journeyman to master is a book about software engineering by Andrew Hunt and David Thomas. ... The term database originated within the computer industry. ... A test plan is a systematic approach to testing a system such as a machine or software. ... In the field of computer software, the term software build refers either to the process of converting source code files into executable code or the result of doing so. ... Software Documentation or Source Code Documentation is written text that accompanies computer software. ... Synchronization is coordination with respect to time. ...

Why DRY may not be applicable

• In some small-scale contexts, the effort required to design around DRY may be far greater than the effort to maintain two separate copies of the data.
• Imposing standards aimed at strict adherence to DRY could stifle community involvement in contexts where it is highly valued, such as wikis.
• Configuration management and version control allow multiple working copies of the same code without setting aside DRY. For example, good practice generally involves setting up development, testing, and production environments so that ongoing development and testing do not affect production code.
• Human-readable documentation (from code comments to printed manuals) are typically a restatement of something in the code with elaboration and explanation for those who do not have the ability or time to read and internalize the code. However, DRY holds that if the human-readable document adds no value except the change in format, then it should be generated rather than written.

A wiki (IPA: <WICK-ee> or <WEE-kee>[1]) is a type of Web site that allows the visitors themselves to easily add, remove, and otherwise edit and change some available content, sometimes without the need for registration. ... In information technology and telecommunications, the term configuration management or configuration control has the following meanings: The management of security features and assurances through control of changes made to hardware, software, firmware, documentation, test, test fixtures and test documentation of an automated information system, throughout the development and operational life... Revision control is an aspect of documentation control wherein changes to documents are identified by incrementing an associated number or letter code, termed the revision level, or simply revision. It has been a standard practice in the maintenance of engineering drawings for as long as the generation of such drawings... Software testing is the process used to help identify the correctness, completeness, security, and quality of developed computer software. ...

See also

• Separation of concerns
• Database normalization
• Transaction processing (related methodology to prevent internal inconsistency)

In computer science, separation of concerns (SoC) is the process of breaking a program into distinct features that overlap in functionality as little as possible. ... This article or section is in need of attention from an expert on the subject. ... In computer science, transaction processing is information processing that is divided into individual, indivisible operations, called Each transaction must succeed or fail as a complete unit; it cannot remain in an intermediate state. ...

External links

• Orthogonality and the DRY Principle
• List of Pragmatic Programmer rules
• c2.com the original
• Once and Only Once (c2.com)

References

1. ^ Dave Thomas, interviewed by Bill Venners (2003-10-10). Orthogonality and the DRY Principle. Retrieved on 2006-12-01.