DRY (Don't Repeat Yourself) in LLD - System Design Guide

Problem Statement

When the same code or logic is copied in multiple places, it becomes hard to maintain. Fixing a bug or updating a feature requires changes in many spots, increasing the chance of errors and inconsistencies.

Solution

DRY solves this by ensuring every piece of knowledge or logic exists in only one place. Instead of copying code, you create reusable functions or classes and call them wherever needed, so updates happen in one spot only.

Architecture

Repeated

Code Block 1

↓

Single Function

(Reusable logic called by both places)

This diagram shows two places with repeated code replaced by calls to a single reusable function, centralizing the logic.

Trade-offs

✓ Pros

→

Reduces bugs by having a single source of truth for logic.

→

Simplifies maintenance since changes happen in one place.

→

Improves code readability by avoiding clutter from repeated code.

✗ Cons

→

May introduce extra function calls, slightly impacting performance in critical paths.

→

Over-abstraction can make code harder to follow if taken too far.

→

Requires upfront design effort to identify reusable parts.

Use DRY when the same logic or code appears in two or more places, especially in medium to large codebases where maintenance cost is high.

Avoid DRY in very small scripts or one-off code where abstraction adds unnecessary complexity and overhead.

Real World Examples

Amazon

Amazon uses DRY principles in their payment processing code to ensure all payment validation logic is centralized, reducing errors and speeding up updates.

Netflix

Netflix applies DRY in their streaming service code to reuse authentication checks across multiple microservices, ensuring consistent security.

Stripe

Stripe uses DRY to centralize billing calculations, so all products and services use the same pricing logic, avoiding discrepancies.

Code Example

The before code repeats the multiplication logic in two functions. The after code extracts multiplication into a single function 'multiply' and reuses it, following DRY.

LLD

### Before applying DRY (repeated code)
def calculate_area_rectangle(width, height):
    return width * height

def calculate_area_square(side):
    return side * side

# Both functions repeat multiplication logic

### After applying DRY (reusable function)
def multiply(a, b):
    return a * b

def calculate_area_rectangle(width, height):
    return multiply(width, height)

def calculate_area_square(side):
    return multiply(side, side)

Alternatives

WET (Write Everything Twice)

WET allows code duplication without abstraction, leading to repeated logic in multiple places.

Use when: Choose WET only for very small or experimental projects where speed of writing is more important than maintainability.

Copy-Paste Programming

Directly copying code blocks instead of abstracting them into reusable units.

Use when: Use only in quick prototyping or throwaway scripts where code reuse is not a concern.

Summary

DRY prevents bugs and eases maintenance by centralizing repeated logic into one place.

It encourages reusable functions or classes instead of copying code.

Applying DRY requires balancing abstraction with code clarity.