This is a great way to make LLD interview notes. I would keep the same style and add Top 3 Structural and Top 3 Behavioral patterns.

LLD Design Patterns (Easy Analogy Notes)

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CREATIONAL PATTERNS

1. Singleton Pattern

One Security Guard

Imagine a Ryan Square building.

There should be only one security guard.

Block A ----\
Block B ----- > Security Guard (only one security guard for all blocks)
Block C ----/

Everyone uses the same guard.

Rule

Singleton = "Only one object for everyone."

When to use

• Database connection
• Logger
• Config manager

---

2. Factory Pattern

Restaurant Kitchen

You don't cook food yourself.

You simply say:

"Give me Burger"

or

"Give me Pizza"

The kitchen decides:

• ingredients
• cooking process
• oven temperature

You only ask for the final product.

Customer
   |
   v
Kitchen (Factory)
   |
   +--> Pizza
   |
   +--> Burger

Rule

Factory = "You ask, factory creates."

When to use

When you have many object types.

Car
Bike
Truck

or

Email
SMS
Push Notification

and don't want if-else everywhere.

---

3. Builder Pattern

Build Your Own PC

You don't say:

Give me everything.

You choose step by step:

CPU → Intel i9
RAM → 32GB
SSD → 1TB
GPU → RTX 5090

Then press:

Build

You
 |
 +--> CPU
 |
 +--> RAM
 |
 +--> SSD
 |
 +--> GPU
 |
 +--> Build
       |
       v
    Computer

Rule

Builder = "Build a complex object step by step."

When to use

When an object has lots of optional fields.

Examples:

• Computer
• House
• SQL Query Builder
• API Request Builder

---

STRUCTURAL PATTERNS

1. Adapter Pattern

Travel Adapter (Power Plug Converter)

You travel from India to the USA.

Your charger cannot fit.

Indian Charger
       |
       v
Travel Adapter
       |
       v
USA Socket

The adapter makes two incompatible things work together.

Rule

Adapter = "Convert one interface into another."

When to use

• Third-party APIs
• Legacy systems
• Different data formats

Examples:

Old Payment API
New Payment API

or

XML -> JSON

---

2. Decorator Pattern

Pizza Toppings

You start with a plain pizza.

Then add toppings.

Pizza

+ Cheese

+ Mushroom

+ Olives

Plain Pizza
      |
      +--> Cheese
      |
      +--> Mushroom
      |
      +--> Olives

You keep adding features without changing the original pizza.

Rule

Decorator = "Add features without modifying the original object."

When to use

Examples:

Coffee + Milk

Coffee + Sugar

Coffee + Cream

or

Logger

Logger + Timestamp

Logger + File Logging

---

3. Facade Pattern

TV Remote

Inside a TV there are many systems.

TV

Sound System

HDMI

Display

WiFi

But you don't control them individually.

You simply use one remote.

You
 |
 v
TV Remote (Facade)
 |
 +--> TV
 +--> Sound
 +--> HDMI
 +--> WiFi

Rule

Facade = "One simple interface for a complex system."

When to use

Examples:

• Payment service
• Video processing service
• Booking service

---

BEHAVIORAL PATTERNS

1. Observer Pattern

YouTube Subscribers

You subscribe to a channel.

Whenever the creator uploads a video, everyone gets notified.

YouTube Channel
      |
      +--> User A
      |
      +--> User B
      |
      +--> User C

Rule

Observer = "One changes, many get notified."

When to use

Examples:

• Notifications
• Event systems
• Stock price updates

---

2. Strategy Pattern

Google Maps Route Selection

You want to go somewhere.

You can choose:

Car

Bike

Walk

You
 |
 v
Google Maps
 |
 +--> Car Route
 |
 +--> Bike Route
 |
 +--> Walking Route

The algorithm changes but the goal stays the same.

Rule

Strategy = "Choose a behavior at runtime."

When to use

Examples:

• Payment methods
• Sorting algorithms
• Search algorithms

---

3. Command Pattern

Restaurant Waiter

You don't go to the chef.

You tell the waiter.

Customer
   |
   v
Waiter (Command)
   |
   v
Chef

The waiter carries your request.

Rule

Command = "Convert a request into an object."

When to use

Examples:

• Undo/Redo
• Queue systems
• Task schedulers

---

30-Second Interview Cheat Sheet

CREATIONAL

Need ONE object?
→ Singleton

Need DIFFERENT objects?
→ Factory

Need a BIG object built in steps?
→ Builder


STRUCTURAL

Need to CONNECT incompatible things?
→ Adapter

Need to ADD features?
→ Decorator

Need to HIDE complexity?
→ Facade


BEHAVIORAL

Need NOTIFICATIONS?
→ Observer

Need to SWITCH algorithms?
→ Strategy

Need to EXECUTE requests?
→ Command

• Car Rental