DRY (Don't Repeat Yourself) in LLD - Architecture Diagram

System Overview - DRY (Don't Repeat Yourself)

The DRY principle helps developers avoid repeating the same code or logic in multiple places. It encourages reusing components and centralizing logic to make systems easier to maintain and less error-prone.

Key requirements include modular design, single source of truth for logic, and easy updates without duplication.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
+-------------------+
|  Service Layer     |
|  +-------------+  |
|  | Auth Module |  |
|  +-------------+  |
|  +-------------+  |
|  | Data Module |  |
|  +-------------+  |
+-------------------+
  |
  v
Database
  |
  v
Cache

Components

User

user

Initiates requests to the system

Load Balancer

load_balancer

Distributes incoming requests evenly across servers

API Gateway

api_gateway

Routes requests to appropriate services and handles authentication

Service Layer

service

Contains reusable modules implementing business logic following DRY

Auth Module

service_module

Handles authentication logic reused across services

Data Module

service_module

Manages data access logic reused across services

Database

database

Stores persistent data

Cache

cache

Stores frequently accessed data to reduce database load

Request Flow - 11 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Service Layer

Service Layer → Cache

Cache → Service Layer

Service Layer → Database

Database → Service Layer

Service Layer → Cache

Service Layer → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Cache

Impact:Cache misses increase, causing more database queries and higher latency

Mitigation:System continues working by querying database directly; cache can be rebuilt asynchronously

Architecture Quiz - 3 Questions

Which component ensures that repeated business logic is centralized and reused?

A Load Balancer

B Cache

C Service Layer

D API Gateway

Design Principle

This architecture demonstrates the DRY principle by centralizing reusable business logic in service modules. It avoids code duplication by having a single source of truth for authentication and data access logic, improving maintainability and reducing errors.

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