DRY Principle

Eliminate knowledge duplication through abstraction. DRY is about knowledge duplication, not code duplication—similar-looking code representing different concepts should NOT be merged.

When to Use

• Same logic appears in 3+ places (Rule of Three)
• Changing a rule requires updating multiple files
• Shared configuration or constants duplicated across modules

Don't use for:

• Code that looks similar but represents different business concepts
• Premature abstraction (apply after seeing duplication, not speculatively)
• Trivial one-off operations

Critical Patterns