Have you ever copied the same validation logic into multiple classes?

Or written the same loop, query, or helper method across several files?

Or worse, updated a piece of business logic in one place but forgot it existed in two others?

If so, you have likely violated one of the most fundamental principles in software engineering: the DRY Principle, which stands for "Don’t Repeat Yourself."

This chapter explains the DRY principle through real-world examples, explores the problems caused by repetition, and offers practical advice to help you write cleaner and more maintainable code.

What Is the DRY Principle?

“Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.” — The Pragmatic Programmer

The DRY principle encourages you to avoid duplicating logic, behavior, or knowledge.

It applies not only to code, but also to:

• Business rules
• Configuration
• Data models
• Documentation
• Tests

Whenever the same concept appears in more than one place, you introduce redundancy. Redundancy makes your system harder to maintain and more prone to bugs.

A Real-World Example

Imagine you are building a system to manage users across three modules: authentication, payments, and messaging.

Each module contains the same logic to validate email addresses:

Now suppose the business changes the rule. Email addresses must now end with ".com" or ".org".

If this logic is duplicated across modules, you face a major problem. You need to update every location where the validation code exists. If you miss even one, the system becomes inconsistent, and bugs start to appear. You have violated the DRY principle and created technical debt.

Why Repetition Is a Problem

1. Harder to Maintain

When a rule or piece of logic changes, you must find and update every occurrence. Missing even one leads to inconsistent behavior.

2. Higher Risk of Bugs

More copies mean more chances to introduce errors. A typo or mismatch in one version of the logic can cause unexpected failures.

3. Bloated Codebase

Redundant logic adds noise to your codebase, making it harder to understand what is unique versus what is shared.

4. Poor Test Coverage

When logic is repeated in many places, each version needs its own set of tests. This increases the testing effort and complexity.

How to Apply DRY

Let’s refactor the email validation example by extracting the common logic.

Step 1: Create a Utility Class

Step 2: Use It Across Modules

Now the email validation logic lives in one place. You have a single source of truth. Any future updates only need to be made once, and all modules remain consistent.

DRY Before and After

Without DRY

With DRY Applied

By centralizing the validation logic, your code becomes more consistent and easier to maintain.

When It Is Okay to Repeat

The DRY principle is a guideline, not a strict rule. There are times when repetition is acceptable.

1. Avoid Premature Abstractions

Do not extract shared code too early. Let duplication reveal itself first. Abstractions created too soon can be misleading or hard to maintain.

“Duplication is far cheaper than the wrong abstraction.” — Sandi Metz

2. Keep Tests Readable

In some cases, repeating a bit of test code improves clarity. Tests should be easy to read and understand.

3. Keep It Simple

If a line of code is extremely simple and unlikely to change, it may be better to repeat it rather than create a new layer of indirection.

Final Thoughts

The DRY principle is more than just a tip for cleaner code. It is a mindset that helps you reduce risk, improve consistency, and write software that can evolve gracefully.

The next time you find yourself copying code, stop and ask: Can I extract this instead?

Following the DRY principle might take a bit more time up front, but it saves you much more time and effort in the long run.