Practice: Don't Repeat Yourself (DRY)


To eliminate information repetition of all kinds.

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system. Andrew Hunt and David Thomas (2000) The Pragmatic Programmer

This does not only apply to code:

  • Copy-paste (surface features of the code are the same)
  • Logical / Functional (the same logical process, but surface features vary)
  • Data (the same information in two different places)

But also to:

  • Documentation (requirements, for users etc.)
  • Software architecture
  • The software development process

To help you remember this practice, write out 100 times, “I will not repeat myself”.


  • Repetition in process calls for automation
  • Repetition in logic calls for abstraction

Alternative Names

  • Single Source of Truth (SSOT)
  • Single Point of Truth (SPOT)

Forces Against

  • Ignorance of parts of the system (I didn’t know it was already there!)
  • Reliance on surface information rather then deep understanding. Assumption that is different because it looks different.
  • Reuse of existing information from several non-colsolidated sources.

Problem Overcome

  • Duplication requires that all copies must be maintained
  • Leads to larger code / data base, making finding defects more difficult


  • Automated derivation (eg. caches, generated code / documentation) where the original source is well know.
  • Certain optimisation techniques

Part of Development Lifecycle

  • All

Related Practices



  • Code reuse: Classes, subroutines, code libraries
  • Data Normalisation
  • Design Patterns


  • N/A