📘 System Design

System Design is the process of planning and defining the architecture, components, modules, interfaces, and data flow of a system so that it meets the required functionality, performance, scalability, and reliability.

In simple terms, system design explains how a software system should be structured and how its parts interact to solve a problem.

1️⃣ Definition

System Design is the high-level and low-level design of a software system that transforms requirements into a structured solution.

It defines:

How different components interact
How data flows through the system
How the system handles scale, failures, and performance

2️⃣ Goals of System Design 🎯

The main objectives are:

Scalability – System should handle increasing users or data.
Reliability – System should work consistently without failures.
Availability – System should be accessible most of the time.
Performance – System should respond quickly.
Maintainability – Easy to update and fix.
Security – Protect data and system resources.

3️⃣ Types of System Design

1. High-Level Design (HLD)

This describes the overall architecture of the system.

It includes:

System architecture
Major components
Communication between services
Database selection
Technology stack

Example components:

Frontend
Backend
Database
API Gateway
Load Balancer
Cache

Example:
Designing a YouTube-like system with:

Video upload service
Video streaming service
CDN
Metadata database

2. Low-Level Design (LLD)

This focuses on detailed implementation of each component.

It includes:

Class diagrams
Data structures
Algorithms
Database schema
API definitions

Example:

User class
Video class
Authentication module

4️⃣ Key Components in System Design

1. Load Balancer ⚖️

Distributes traffic across multiple servers.

Example:
If 1 million users access a website, load balancer distributes requests across servers.

2. Database 🗄️

Stores system data.

Types:

SQL databases – MySQL, PostgreSQL
NoSQL databases – MongoDB, Cassandra

3. Cache ⚡

Stores frequently accessed data to reduce database load.

Example tools:

Redis
Memcached

4. Message Queue 📩

Used for asynchronous communication.

Examples:

Kafka
RabbitMQ
AWS SQS

5. Microservices 🧩

Large systems are divided into small independent services.

Example:

User service
Payment service
Notification service

5️⃣ Important Concepts in System Design

Scalability

Ability of system to handle increasing load.

Types:

Vertical Scaling – Increase server power.
Horizontal Scaling – Add more servers.

Availability

System should be accessible even if some servers fail.

Example:
Using multiple servers and replication.

Consistency

All users should see the same data at the same time.

Example:
Bank balance updates.

Fault Tolerance

System should continue working even if some components fail.

6️⃣ Example – Simple System Design

Designing a URL Shortener (like TinyURL)

Components:

User enters long URL
Backend generates short code
Database stores mapping
When user visits short link → redirect to original URL

Architecture:

User → API Server → Database → Redirect Service

7️⃣ Why System Design is Important

Helps build large-scale applications
Improves performance and scalability
Ensures system reliability
Makes development organized and maintainable

Examples of systems requiring good design:

Social media platforms
Banking systems
E-commerce websites
Streaming platforms

✅ One Line Summary:

System Design is the process of defining the architecture and components of a system to build scalable, reliable, and efficient software.

📚 LLD vs DSA

1️⃣ What is LLD (Low Level Design)?

Low Level Design (LLD) focuses on designing the internal structure of a software system using classes, objects, and relationships.

It describes how the system components are implemented in code.

LLD mainly deals with:

Class design
Object interactions
Design patterns
UML diagrams
Code structure

Example

Designing a Parking Lot System

You design:

Vehicle class
ParkingSpot class
ParkingTicket
Payment

You decide:

attributes
methods
relationships between classes

Example code idea:

class Car extends Vehicle {
    String licensePlate;
}

So LLD answers:

👉 How should we structure the code?

2️⃣ What is DSA (Data Structures & Algorithms)?

DSA focuses on solving computational problems efficiently using data structures and algorithms.

It deals with:

Optimizing time complexity
Optimizing memory usage
Writing efficient logic

Common Data Structures

Arrays
Linked Lists
Stacks
Queues
Trees
Graphs
Hash Maps

Algorithms

Sorting
Searching
Recursion
Dynamic Programming
Greedy algorithms
Graph algorithms

Example:

def binary_search(arr, target):

DSA answers:

👉 How to solve a problem efficiently?

⚖️ LLD vs DSA Comparison

Aspect	LLD	DSA
Full Form	Low Level Design	Data Structures & Algorithms
Focus	Code design	Problem solving
Level	System architecture inside modules	Algorithmic logic
Used for	Designing software systems	Optimizing solutions
Concepts	OOP, design patterns, UML	Trees, graphs, DP, sorting
Interview type	Design round	Coding round
Example	Design Parking Lot	Find shortest path

🧠 Simple Analogy

Imagine building a house 🏠

DSA → How to efficiently move bricks and materials
LLD → How rooms, doors, and windows should be structured

🧑‍💻 Example Difference

Problem: Build a Ride Booking App (like Uber)

DSA Approach

Find:

shortest path between locations
nearest driver

Algorithms used:

Dijkstra
Graph search

LLD Approach

Design classes:

User
Driver
Ride
Payment
Location

Define relationships:

User -> Request Ride
Driver -> Accept Ride
Ride -> Payment

🎯 When Each is Used

DSA

Used in:

Coding interviews
Competitive programming
Performance optimization

Companies ask:

Google
Amazon
Meta
Microsoft

LLD

Used in:

Backend engineering
Software architecture interviews
Real system building

Companies ask:

Amazon
Uber
Flipkart
Swiggy

📘 Three Pillars of LLD (Low Level Design)

In Low Level Design (LLD), the foundation is built on Object-Oriented Programming (OOP) principles.

The three core pillars are:

Encapsulation
Abstraction
Polymorphism

(⚠️ Note: Some people say 4 pillars by including Inheritance separately. But commonly in LLD discussions, these three are emphasized strongly.)

1️⃣ Encapsulation 🔒

Definition:

Encapsulation means bundling data (variables) and methods (functions) together inside a class and restricting direct access to internal details.

Why?

Protect data
Improve security
Prevent accidental modification

Example:

class BankAccount {
    private double balance;

    public void deposit(double amount) {
        balance += amount;
    }

    public double getBalance() {
        return balance;
    }
}

Here:

balance is private
Can only access via methods

👉 Real-life example:
ATM machine — you cannot directly access bank database.

2️⃣ Abstraction 🎭

Definition:

Abstraction means hiding complex implementation details and showing only essential features.

You expose what to do, not how it works internally.

Why?

Reduce complexity
Improve maintainability
Loose coupling

Example:

interface Payment {
    void pay();
}

Implementation:

class CreditCardPayment implements Payment {
    public void pay() {
        // complex logic
    }
}

User only calls:

payment.pay();

User does not know internal logic.

👉 Real-life example:
Driving a car — you press accelerator, but you don’t know engine internals.

3️⃣ Polymorphism 🔄

Definition:

Polymorphism means one interface, multiple implementations.

Same method name behaves differently depending on object.

Types:

Method Overloading (Compile-time)
Method Overriding (Runtime)

Example:

class Animal {
    void sound() {
        System.out.println("Animal sound");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Bark");
    }
}

Here:
sound() behaves differently for different objects.

👉 Real-life example:
Payment method — Credit Card, UPI, Net Banking
Same pay() method → different behavior.

🎯 Why These 3 Are Important in LLD?

LLD is about:

Designing classes
Creating relationships
Writing scalable object-oriented code

These pillars help in:

Writing clean code
Applying design patterns
Making system flexible
Improving maintainability

🧠 Simple Summary

Pillar	Meaning
Encapsulation	Protect and bundle data
Abstraction	Hide complexity
Polymorphism	Same method, different behavior

📚 LLD vs HLD

1️⃣ What is HLD (High Level Design)?

High Level Design (HLD) describes the overall architecture of a system and how major components interact with each other.

It focuses on the big picture of the system.

HLD defines:

System architecture
Major modules/services
Data flow between services
Technology stack
Database selection
External integrations

Example

Designing Netflix

HLD will define:

User Service
Video Streaming Service
Recommendation Service
Database
CDN
Load Balancer

Architecture example:

User → Load Balancer → API Server → Microservices → Database

👉 HLD answers:

“How the entire system is structured?”

2️⃣ What is LLD (Low Level Design)?

Low Level Design (LLD) focuses on detailed design of individual components inside the system.

It explains how each module will be implemented in code.

LLD defines:

Classes
Objects
Methods
Relationships between classes
Database schema
Algorithms used inside modules

Example

In Netflix Video Service

LLD will design classes like:

Video
User
Playlist
Subscription

Define:

attributes
methods
interactions between classes

👉 LLD answers:

“How exactly will the code be implemented?”

⚖️ LLD vs HLD Comparison

Aspect	HLD	LLD
Full Form	High Level Design	Low Level Design
Focus	System architecture	Code-level design
Level	Macro / Big picture	Micro / Detailed
Components	Services, databases, APIs	Classes, objects, methods
Diagrams	Architecture diagrams	Class diagrams, sequence diagrams
Used by	System architects	Developers
Goal	Define system structure	Implement system logic

🧠 Simple Example

Imagine building a food delivery app like Swiggy 🍔

HLD

Define modules:

User Service
Restaurant Service
Order Service
Payment Service
Delivery Tracking

Architecture:

Mobile App → API Gateway → Microservices → Database

LLD

Inside Order Service, design:

Classes:

Order
OrderItem
Cart
Payment
DeliveryAgent

Define:

attributes
methods
relationships

🏗️ Simple Analogy

Building a house 🏠

Concept	Example
HLD	House blueprint (rooms, floors, layout)
LLD	How walls, wiring, plumbing are built

🎯 When Each is Used

HLD

Used for:

System architecture planning
Large scale systems
Microservices design
Cloud architecture

LLD

Used for:

Code implementation
Class design
Design patterns
Object relationships

📚 Common Reusable Algorithms in LLD

1️⃣ Sorting Algorithms

Sorting is frequently used in many systems such as:

Leaderboards
Product listings
Ranking systems
Log processing

Common algorithms:

Quick Sort
Merge Sort
Heap Sort
Bubble Sort (for small data)

Example (Python):

def bubble_sort(arr):
    n = len(arr)
    for i in range(n):
        for j in range(0, n-i-1):
            if arr[j] > arr[j+1]:
                arr[j], arr[j+1] = arr[j+1], arr[j]

Example use case:

Sorting orders by price
Sorting users by score

2️⃣ Searching Algorithms

Used when systems need to quickly find data.

Common algorithms:

Linear Search
Binary Search
Hash-based lookup

Example:

def binary_search(arr, target):
    left, right = 0, len(arr)-1

Use cases:

Searching product by ID
Finding user records
Querying sorted datasets

3️⃣ Caching Algorithms

Used to store frequently accessed data.

Common caching strategies:

LRU (Least Recently Used)
LFU (Least Frequently Used)
FIFO

Example use cases:

Web caching
Database query caching
API response caching

Example:
Redis cache in backend systems.

4️⃣ Rate Limiting Algorithms

Used to control number of requests a user can make.

Common algorithms:

Token Bucket
Leaky Bucket
Sliding Window

Example use cases:

API request limits
Login attempt restrictions
Payment request limits

Example:
Allow only 100 requests per minute per user.

5️⃣ Scheduling Algorithms

Used for task execution and resource allocation.

Common algorithms:

First Come First Serve (FCFS)
Round Robin
Priority Scheduling

Use cases:

CPU scheduling
Job processing
Task queues

Example:
Background job systems like Celery or Kafka consumers.

6️⃣ Graph Algorithms

Used when dealing with network relationships.

Common algorithms:

Dijkstra (shortest path)
BFS (Breadth First Search)
DFS (Depth First Search)

Use cases:

Google Maps routing
Social network connections
Recommendation systems

7️⃣ Load Balancing Algorithms

Used in distributed systems.

Common algorithms:

Round Robin
Least Connections
Weighted Round Robin
Consistent Hashing

Use cases:

Distributing traffic across servers

Example:
User request → Load Balancer → Multiple servers.

8️⃣ Retry Algorithms

Used for handling temporary failures.

Common strategies:

Fixed Retry
Exponential Backoff
Circuit Breaker

Use cases:

API failures
Network retry
Microservices communication

🎯 Why Reusable Algorithms Are Important in LLD

They help in:

Reducing duplicate code
Improving efficiency
Making systems scalable
Improving maintainability

🧠 Quick Summary Table

Algorithm Type	Example	Use Case
Sorting	Merge Sort	Ranking, ordering
Searching	Binary Search	Data lookup
Caching	LRU	Fast access
Rate Limiting	Token Bucket	API protection
Scheduling	Round Robin	Task management
Graph	Dijkstra	Routing
Load Balancing	Least Connections	Server traffic

📘 UML Diagram

UML (Unified Modeling Language) is a standard visual language used to represent the design and structure of a software system.

It helps developers and architects visualize, design, and document software systems before implementing them.

UML diagrams show:

System components
Relationships between objects/classes
Data flow
Interaction between modules

In LLD and System Design interviews, UML diagrams are often used to explain system structure clearly.

🎯 Why UML Diagrams Are Important

UML diagrams help in:

Understanding system architecture
Communicating design with team members
Documenting software systems
Simplifying complex designs
Planning implementation

📊 Most Important Types of UML Diagrams

UML diagrams are mainly divided into two categories:

1️⃣ Structural Diagrams
2️⃣ Behavioral Diagrams

1️⃣ Class Diagram (Most Important for LLD)

A Class Diagram shows:

Classes
Attributes
Methods
Relationships between classes

It is the most commonly used diagram in Low Level Design.

Example

Parking System Classes

Car
-----
licensePlate
color
startEngine()

ParkingSpot
-----
spotNumber
isAvailable()

ParkingLot
-----
totalSpots
assignSpot()

Relationships between classes are also shown.

Relationships types

Association
Inheritance
Aggregation
Composition

2️⃣ Sequence Diagram

A Sequence Diagram shows how objects interact with each other over time.

It represents:

Order of operations
Message flow between objects

Example

Food Delivery Order Flow

User → App → Order Service → Payment Service → Restaurant

Sequence:

User places order
App sends request to Order Service
Payment processed
Order confirmed

This diagram focuses on interaction timeline.

3️⃣ Use Case Diagram

A Use Case Diagram shows how users interact with a system.

It identifies:

Actors (users or external systems)
System functionalities

Example

ATM System

Actors:

Customer
Bank System

Use cases:

Withdraw money
Deposit money
Check balance

Diagram concept:

Customer → Withdraw Money
Customer → Check Balance
Customer → Deposit Money

This diagram shows system functionalities from user's perspective.

4️⃣ Activity Diagram

An Activity Diagram represents the workflow of a system or process.

It is similar to a flowchart.

Example

Online Shopping Flow

Start
 ↓
Browse Products
 ↓
Add to Cart
 ↓
Make Payment
 ↓
Order Confirmation
 ↓
End

Used to visualize business processes or workflows.

5️⃣ Component Diagram

A Component Diagram shows different components of a system and how they interact.

Example system components:

Frontend
Backend API
Database
Payment Gateway
Notification Service

This is commonly used in High Level Design (HLD).

6️⃣ Deployment Diagram

A Deployment Diagram shows how software components are deployed on hardware infrastructure.

Example:

User Device
      ↓
Web Server
      ↓
Application Server
      ↓
Database Server

Used for cloud architecture and distributed systems.

🧠 Most Important UML Diagrams for Interviews

Diagram	Used in
Class Diagram	LLD interviews
Sequence Diagram	LLD + System Design
Use Case Diagram	Requirement analysis
Activity Diagram	Workflow modeling
Component Diagram	HLD
Deployment Diagram	Infrastructure design

⭐ Quick Summary

UML Diagram	Purpose
Class Diagram	Structure of classes
Sequence Diagram	Interaction between objects
Use Case Diagram	User-system interaction
Activity Diagram	Workflow
Component Diagram	System components
Deployment Diagram	System deployment

🎯 One-Line Definition

UML diagrams are visual representations used to design and document software systems and their interactions.

3 MOST IMPORTANT UML diagrams used in LLD interviews with simple examples.

1️⃣ Class Diagram
2️⃣ Sequence Diagram
3️⃣ State Machine Diagram

1️⃣ Class Diagram (Most Important in LLD)

A Class Diagram represents the structure of the system.

It shows:

Classes
Attributes
Methods
Relationships between classes

Basic Structure

-----------------
Class Name
-----------------
Attributes
-----------------
Methods
-----------------

Example: Parking Lot System

Vehicle
----------------
vehicleNumber
vehicleType
----------------
park()
unpark()

ParkingSpot
----------------
spotId
isAvailable
----------------
assignVehicle()
removeVehicle()

ParkingLot
----------------
totalSpots
availableSpots
----------------
addSpot()
getFreeSpot()

Relationship Example

Vehicle  ---- parked in ----> ParkingSpot
ParkingLot ---- contains ----> ParkingSpot

Types of Relationships

Relationship	Meaning
Association	One class uses another
Inheritance	Child class inherits parent
Aggregation	Weak "has-a" relationship
Composition	Strong "part-of" relationship

Example:

Car → Vehicle (Inheritance)
ParkingLot → ParkingSpot (Composition)

2️⃣ Sequence Diagram

A Sequence Diagram shows how objects interact step-by-step over time.

It focuses on message flow between components.

Key Elements

Actor
Objects
Messages
Timeline (top → bottom)

Example: Food Ordering System

Objects involved:

User
MobileApp
OrderService
PaymentService
Restaurant

Sequence Flow:

User → MobileApp : Place Order
MobileApp → OrderService : Create Order
OrderService → PaymentService : Process Payment
PaymentService → OrderService : Payment Success
OrderService → Restaurant : Send Order
Restaurant → OrderService : Order Accepted
OrderService → MobileApp : Order Confirmed

Important point:

Sequence diagrams show how the system behaves during execution.

3️⃣ State Machine Diagram

A State Machine Diagram shows different states of an object and how it changes from one state to another.

It is useful for systems where objects have lifecycle states.

Example: Order in Food Delivery

States:

Order Created
↓
Payment Completed
↓
Restaurant Preparing
↓
Out for Delivery
↓
Delivered

State diagram concept:

[Created]
     ↓
[Paid]
     ↓
[Preparing]
     ↓
[Out for Delivery]
     ↓
[Delivered]

Another example:

ATM Transaction

Idle
↓
Card Inserted
↓
PIN Verified
↓
Transaction Processing
↓
Cash Dispensed
↓
Completed

🧠 When to Use Each Diagram

Diagram	Used For
Class Diagram	System structure
Sequence Diagram	Interaction flow
State Machine	Object lifecycle

📊 Simple Real Example (Ride Booking App)

Class Diagram

Classes:

User
Driver
Ride
Payment
Location

Sequence Diagram

User → App : Request Ride
App → RideService : Find Driver
RideService → Driver : Ride Request
Driver → RideService : Accept
RideService → App : Ride Confirmed

State Diagram (Ride Status)

Requested
↓
Driver Assigned
↓
Driver Arrived
↓
Ride Started
↓
Ride Completed

⭐ Golden Rule for LLD Interviews

If asked to design a system:

Step 1 → Identify entities/classes
Step 2 → Draw Class Diagram
Step 3 → Show Sequence Diagram
Step 4 → Explain State transitions

This is exactly how Amazon / Uber / Flipkart LLD interviews are done.

Composition vs Association vs Aggregation in UML 📘

📘 1️⃣ Association

Definition

Association represents a general relationship between two classes where objects interact with each other.

It simply means one object uses another object.

Key Points

Weak relationship
Both objects can exist independently
No ownership

Example

Teacher and Student

A teacher teaches students, but both can exist independently.

Teacher -------- Student

Code Example

class Student {
    String name;
}

class Teacher {
    void teach(Student s) {
        System.out.println("Teaching student");
    }
}

Here:

Teacher uses Student
Student can exist without Teacher

📘 2️⃣ Aggregation

Definition

Aggregation is a special type of association that represents a "has-a" relationship where one class contains another class, but both can exist independently.

It is a weak ownership relationship.

Key Points

"Has-a" relationship
Objects can exist separately
Parent does not control child lifecycle

UML Symbol

Aggregation uses hollow diamond (◇)

Department ◇------ Teacher

Example

Department and Teachers

A department has teachers, but teachers can exist without the department.

Code Example

class Teacher {
    String name;
}

class Department {
    List<Teacher> teachers;
}

Here:

Department has teachers
Teachers can exist independently

📘 3️⃣ Composition

Definition

Composition is a strong form of aggregation where the child object cannot exist without the parent object.

It represents strong ownership.

Key Points

Strong "part-of" relationship
Child object depends on parent
If parent is deleted, child is also deleted

UML Symbol

Composition uses filled diamond (◆)

House ◆------ Room

Example

House and Rooms

Rooms cannot exist without a house.

Code Example

class Room {
}

class House {
    private Room room;

    House() {
        room = new Room();
    }
}

If House is destroyed → Room is destroyed.

📊 Comparison Table

Feature	Association	Aggregation	Composition
Relationship type	General	Has-a	Strong part-of
Ownership	No ownership	Weak ownership	Strong ownership
Dependency	Independent	Independent	Dependent
Lifecycle	Separate	Separate	Child depends on parent
UML Symbol	Line	Hollow Diamond ◇	Filled Diamond ◆

🧠 Simple Real-Life Examples

Relationship	Type
Doctor treats Patient	Association
Team has Players	Aggregation
House has Rooms	Composition

🎯 Interview Tip

Interviewers often ask:

Difference between Aggregation and Composition

Best answer:

Aggregation → weak "has-a" relationship
Composition → strong "part-of" relationship where child cannot exist without parent

Example:

Car ◇ Engine  → Aggregation
Human ◆ Heart → Composition

⭐ One-Line Summary

Association → objects are related
Aggregation → objects have weak ownership
Composition → objects have strong ownership and lifecycle dependency

SOLID = 5 design principles that help write maintainable, scalable, and flexible code.

S → Single Responsibility Principle
O → Open Closed Principle
L → Liskov Substitution Principle
I → Interface Segregation Principle
D → Dependency Inversion Principle

1️⃣ Single Responsibility Principle (SRP)

Definition

A class should have only one reason to change.

This means a class should do only one job.

If a class handles multiple responsibilities, it becomes hard to maintain.

❌ Bad Example (Violates SRP)

class Report:
    def generate_report(self):
        print("Generating report")

    def save_to_file(self):
        print("Saving report to file")

    def send_email(self):
        print("Sending report via email")

Problem:

Report class is doing three responsibilities:

Generating report
Saving report
Sending email

If email logic changes → this class must change.

✅ Good Example (Follow SRP)

class Report:
    def generate(self):
        print("Generating report")


class ReportSaver:
    def save(self, report):
        print("Saving report")


class EmailSender:
    def send(self, report):
        print("Sending email")

Now each class has one responsibility.

Benefits:

Easier testing
Easier maintenance
Better modular design

2️⃣ Open Closed Principle (OCP)

Definition

Software entities should be:

Open for extension
Closed for modification

Meaning:

You should add new functionality without modifying existing code.

❌ Bad Example

class PaymentProcessor:
    def process(self, payment_type):
        if payment_type == "credit":
            print("Processing credit card")
        elif payment_type == "paypal":
            print("Processing PayPal")

Problem:
If a new payment method comes (UPI), you must modify this class.

✅ Good Example (Using Polymorphism)

class Payment:
    def process(self):
        pass


class CreditCardPayment(Payment):
    def process(self):
        print("Processing credit card")


class PaypalPayment(Payment):
    def process(self):
        print("Processing PayPal")

Usage:

def make_payment(payment: Payment):
    payment.process()

Now adding new payment:

class UpiPayment(Payment):
    def process(self):
        print("Processing UPI")

No existing code changed ✅

3️⃣ Liskov Substitution Principle (LSP)

Definition

Subclasses should be replaceable with their parent class without breaking the program.

If class B inherits class A, then B should behave like A.

❌ Bad Example

Classic example: Square vs Rectangle

class Rectangle:
    def set_width(self, width):
        self.width = width

    def set_height(self, height):
        self.height = height


class Square(Rectangle):
    def set_width(self, width):
        self.width = self.height = width

Problem:

Square changes behavior of Rectangle.

This breaks expectations.

✅ Good Example

Separate classes.

class Shape:
    def area(self):
        pass


class Rectangle(Shape):
    def __init__(self, w, h):
        self.w = w
        self.h = h

    def area(self):
        return self.w * self.h


class Square(Shape):
    def __init__(self, s):
        self.s = s

    def area(self):
        return self.s * self.s

Now both can replace Shape safely.

4️⃣ Interface Segregation Principle (ISP)

Definition

Clients should not be forced to depend on methods they do not use.

Better to have multiple small interfaces rather than one large interface.

Python does not have strict interfaces like Java, but we use abstract classes.

❌ Bad Example

class Worker:
    def work(self):
        pass

    def eat(self):
        pass

Now consider:

HumanWorker → works + eats
RobotWorker → works only

Robot doesn’t need eat().

✅ Good Example

Split interfaces.

class Workable:
    def work(self):
        pass


class Eatable:
    def eat(self):
        pass

Now:

class Human(Workable, Eatable):
    def work(self):
        print("Working")

    def eat(self):
        print("Eating")


class Robot(Workable):
    def work(self):
        print("Working")

Robot no longer forced to implement eat().

5️⃣ Dependency Inversion Principle (DIP)

Definition

High-level modules should not depend on low-level modules.

Both should depend on abstractions.

❌ Bad Example

class MySQLDatabase:
    def connect(self):
        print("Connecting to MySQL")


class Application:
    def __init__(self):
        self.db = MySQLDatabase()

Problem:
Application depends directly on MySQL.

If you switch to PostgreSQL → code must change.

✅ Good Example

class Database:
    def connect(self):
        pass

Implementations:

class MySQLDatabase(Database):
    def connect(self):
        print("MySQL connection")


class PostgresDatabase(Database):
    def connect(self):
        print("Postgres connection")

Application:

class Application:
    def __init__(self, db: Database):
        self.db = db

    def start(self):
        self.db.connect()

Usage:

db = MySQLDatabase()
app = Application(db)
app.start()

Now database can be swapped easily.

📊 SOLID Summary Table

Principle	Meaning
SRP	One class → one responsibility
OCP	Extend without modifying
LSP	Child classes must behave like parent
ISP	Prefer small interfaces
DIP	Depend on abstractions

🧠 Real World Example (Web Backend)

Principle	Example
SRP	UserService handles user logic only
OCP	Add new payment methods without modifying core code
LSP	All shapes can be treated as Shape
ISP	Separate interfaces for payment/refund
DIP	Application depends on database interface

⭐ One-Line Memory Trick

S → Single responsibility
O → Open for extension
L → Liskov substitution
I → Interface segregation
D → Dependency inversion

System Design - 1

📘 System Design

1️⃣ Definition

2️⃣ Goals of System Design 🎯

3️⃣ Types of System Design

1. High-Level Design (HLD)

2. Low-Level Design (LLD)

4️⃣ Key Components in System Design

1. Load Balancer ⚖️

2. Database 🗄️

3. Cache ⚡

4. Message Queue 📩

5. Microservices 🧩

5️⃣ Important Concepts in System Design

Scalability

Availability

Consistency

Fault Tolerance

6️⃣ Example – Simple System Design

Designing a URL Shortener (like TinyURL)

7️⃣ Why System Design is Important

📚 LLD vs DSA

1️⃣ What is LLD (Low Level Design)?

Example

2️⃣ What is DSA (Data Structures & Algorithms)?

Common Data Structures

Algorithms

⚖️ LLD vs DSA Comparison

🧠 Simple Analogy

🧑‍💻 Example Difference

Problem: Build a Ride Booking App (like Uber)

DSA Approach

LLD Approach

🎯 When Each is Used

DSA

LLD

📘 Three Pillars of LLD (Low Level Design)

1️⃣ Encapsulation 🔒

Definition:

Why?

Example:

2️⃣ Abstraction 🎭

Definition:

Why?

Example:

3️⃣ Polymorphism 🔄

Definition:

Types:

Example:

🎯 Why These 3 Are Important in LLD?

🧠 Simple Summary

📚 LLD vs HLD

1️⃣ What is HLD (High Level Design)?

HLD defines:

Example

2️⃣ What is LLD (Low Level Design)?

LLD defines:

Example

⚖️ LLD vs HLD Comparison

🧠 Simple Example

HLD

LLD

🏗️ Simple Analogy

🎯 When Each is Used

HLD

LLD

📚 Common Reusable Algorithms in LLD

1️⃣ Sorting Algorithms

2️⃣ Searching Algorithms

3️⃣ Caching Algorithms

4️⃣ Rate Limiting Algorithms

5️⃣ Scheduling Algorithms

6️⃣ Graph Algorithms

7️⃣ Load Balancing Algorithms

8️⃣ Retry Algorithms

🎯 Why Reusable Algorithms Are Important in LLD

🧠 Quick Summary Table

📘 UML Diagram

🎯 Why UML Diagrams Are Important

📊 Most Important Types of UML Diagrams