Why DRY (Don't Repeat Yourself) in LLD? - Purpose & Use Cases

The Big Idea

What if fixing one bug could fix it everywhere instantly?

The Scenario

Imagine you are building a system where you write the same code or logic in many places to handle user authentication. Every time you need to change the login process, you must find and update all those places manually.

The Problem

This manual approach is slow and risky. You might miss some places, causing bugs. It becomes hard to maintain and update the system. The repeated code wastes time and makes the system fragile.

The Solution

DRY means writing the logic once and reusing it everywhere. This way, you update the code in one place, and all parts of the system benefit. It makes the system cleaner, easier to maintain, and less error-prone.

Before vs After

✗ Before

if user.is_authenticated:
    show_dashboard()
# repeated in many files

✓ After

def check_auth(user):
    return user.is_authenticated

if check_auth(user):
    show_dashboard()

What It Enables

DRY enables building systems that are easier to change, less buggy, and faster to develop.

Real Life Example

Think of a website where the password rules are checked in many places. With DRY, you write the rules once and reuse them, so when rules change, you fix it in one place only.

Key Takeaways

Repeating code causes bugs and wastes time.

DRY means write once, use everywhere.

It makes systems easier to maintain and update.

Practice

1. What is the main goal of the DRY principle in system design?

easy

A. To avoid repeating code or design to keep systems simple

B. To write as much code as possible for clarity

C. To duplicate modules for faster development

D. To create multiple versions of the same component

Solution

1. Step 1: Understand the meaning of DRY

   DRY stands for "Don't Repeat Yourself," which means avoiding repeated code or design.

2. Step 2: Identify the goal of DRY

   The goal is to keep systems simple, maintainable, and reduce bugs by reusing components.

3. Final Answer:

   To avoid repeating code or design to keep systems simple -> Option A

4. Quick Check:

   DRY = Avoid repetition [OK]

Hint: DRY means write once, use many times [OK]

Common Mistakes:

• Thinking DRY means writing more code
• Believing duplication speeds development
• Confusing DRY with writing multiple versions

2. Which of the following is a correct way to follow DRY in system design?

easy

A. Creating reusable components or modules

B. Ignoring code reuse to save time

C. Writing separate code for each similar task

D. Copy-pasting the same code in multiple modules

Solution

1. Step 1: Identify DRY best practice

   DRY encourages reuse by creating components or modules that can be used multiple times.

2. Step 2: Evaluate options

   Copy-pasting or writing separate code repeats work, which breaks DRY. Ignoring reuse wastes effort.

3. Final Answer:

   Creating reusable components or modules -> Option A

4. Quick Check:

   Reuse = DRY [OK]

Hint: Reuse code with modules/components [OK]

Common Mistakes:

• Copy-pasting code instead of reusing
• Writing separate code for similar tasks
• Ignoring reuse to save time

3. Consider this pseudocode snippet following DRY:

function calculateTax(amount) {
  return amount * 0.1;
}

function calculateTotal(amount) {
  return amount + calculateTax(amount);
}

print(calculateTotal(100));

What is the output?

medium

A. 10

B. 100

C. 110

D. Syntax Error

Solution

1. Step 1: Calculate tax for amount 100

   Tax = 100 * 0.1 = 10

2. Step 2: Calculate total using tax

   Total = 100 + 10 = 110

3. Final Answer:

   110 -> Option C

4. Quick Check:

   100 + 10 = 110 [OK]

Hint: Add tax to amount using reusable function [OK]

Common Mistakes:

• Forgetting to add tax to amount
• Confusing tax amount with total
• Assuming syntax error without checking code

4. You see this code snippet violating DRY:

function sendEmailToUser1() {
  // send email logic
}

function sendEmailToUser2() {
  // send email logic
}

What is the best fix to follow DRY?

medium

A. Copy-paste send email logic in more functions

B. Create one function sendEmail(user) and reuse it

C. Keep both functions as is for clarity

D. Remove email sending completely

Solution

1. Step 1: Identify repeated code

   Both functions have the same email sending logic repeated.

2. Step 2: Refactor to reusable function

   Create one function with a parameter for user to reuse the logic.

3. Final Answer:

   Create one function sendEmail(user) and reuse it -> Option B

4. Quick Check:

   Refactor repeated code = DRY [OK]

Hint: Use parameters to reuse functions [OK]

Common Mistakes:

• Leaving duplicate functions unchanged
• Copy-pasting code more times
• Removing needed functionality

5. You design a system with multiple services needing user authentication. To follow DRY, which approach is best?

hard

A. Implement authentication logic separately in each service

B. Skip authentication to reduce complexity

C. Copy authentication code into each service for speed

D. Create a shared authentication service used by all services

Solution

1. Step 1: Identify repeated authentication logic

   Multiple services need the same authentication, so repeating code breaks DRY.

2. Step 2: Use a shared authentication service

   Centralizing authentication logic in one service allows reuse and easier maintenance.

3. Final Answer:

   Create a shared authentication service used by all services -> Option D

4. Quick Check:

   Shared service = DRY in system design [OK]

Hint: Centralize common logic in shared services [OK]

Common Mistakes:

• Duplicating authentication code in each service
• Skipping authentication for simplicity
• Copy-pasting code instead of sharing