Introduction

Imagine building a skyscraper without blueprints. It might look impressive at first, but without a solid foundation and a well-thought-out plan, it could collapse under its own weight. Similarly, system design is the architectural blueprint for software applications. It ensures that systems like Netflix, Amazon, or Google Search can handle millions of users, deliver lightning-fast responses, and stay reliable even when things go wrong.

🧩 What is System Design?

System design is the process of defining the architecture, components, modules, and interactions of a software system to meet specific requirements. It’s about answering questions like:

• How will data be stored and retrieved?
• How do different parts of the system communicate?
• What happens when traffic suddenly spikes?

Think of it as solving a giant puzzle where every piece—databases, servers, APIs, caches—must fit together seamlessly.

It serves as the blueprint for the entire system, ensuring that all components work harmoniously to achieve the desired functionality.

Purpose of the System Design

1. Bridge Between Requirements and Implementation
   1. Converts abstract requirements into concrete structures.
   2. Acts as a blueprint for developers to follow during coding.
2. Define the System’s Architecture
   1. Describes components, modules, and data flow.
   2. Specifies how different parts interact (e.g., client-server, microservices, etc.).
3. Ensure Scalability and Performance
   1. Prepares the system to handle increased load, large datasets, or user traffic.
   2. Involves decisions like caching, load balancing, database sharding, etc.
4. Ensure Maintainability and Modularity
   1. Encourages modular design so individual parts can be updated or replaced.
   2. Simplifies debugging, testing, and future enhancements.
5. Improve System Reliability and Fault Tolerance
   1. Designs for redundancy, error handling, and graceful degradation.
   2. Ensures the system can recover from failures or continue operating in degraded mode.
6. Enable Security Planning
   1. Incorporates authentication, authorization, encryption, and data protection strategies.
   2. Prevents security flaws from being built into the architecture.
7. Facilitate Communication Across Teams
   1. Provides a common language and documentation for developers, QA, DevOps, and stakeholders.
   2. Helps align the vision of everyone working on the project.
   3. It is very interactive, you will be explaining your ideas on the fly, answering questions.
8. Cost and Resource Optimization
   1. Helps identify the most efficient infrastructure, tools, and technologies.
   2. Prevents unnecessary over-engineering.
   3. Trade off analysis.

“System design tells what to build, how to build it, and how it will work under the hood — in a way that’s efficient, scalable, and easy to maintain.”

Purpose of System Design in Interview

1. Assessing problem-solving skills of the candidate.
2. Evaluating technical knowledge.
3. Considering scalability and reliability.
4. Trade-off analysis and decision making.
5. Communication and Collaboration.

What System Design is not for

1. It's not about getting the perfect answer. In fact, there often is not one correct solution in system design.
2. It's not about memorization. Understanding the concepts is more important.

Why System Design Matters

1. Handling Growth: Apps like Instagram or Uber started small but scaled to millions of users. Good system design ensures they grow without breaking.
2. Cost Efficiency: Poorly designed systems waste resources (e.g., servers, bandwidth).
3. User Experience: Slow or unreliable apps drive users away.
4. Future-Proofing: Systems must adapt to new features, technologies, and user demands.

Components

• High-Level Design (HLD): Outlines the system's architecture and major components.
• Low-Level Design (LLD): Details the internal working of each component.

1️⃣ High-Level Design (HLD) 🏗️

Definition: HLD provides a macro-level overview of the system, focusing on architecture and major components without delving into detailed implementation.

2️⃣ Low-Level Design (LLD) 🔧

Definition: LLD delves into the micro-level details of the system, specifying the internal logic and implementation of each component.

Key Components of System Design

Every system relies on these building blocks:

1. Databases
   1. SQL (e.g., PostgreSQL): Structured data with ACID guarantees (Atomicity, Consistency, Isolation, Durability).
   2. NoSQL (e.g., MongoDB): Flexible schema for unstructured data and horizontal scaling.
   3. Use Case: A banking app needs SQL for transactions; a social media app uses NoSQL for user profiles.
2. APIs (Application Programming Interfaces)
   1. Define how clients (e.g., mobile apps) interact with the system.
   2. REST: Simple, stateless communication (e.g., GET /users).
   3. GraphQL: Lets clients request exactly the data they need.
3. Caching
   1. Stores frequently accessed data in memory (e.g., Redis) to reduce database load.
   2. Example: Twitter caches trending tweets to serve them faster.
4. Load Balancers
   1. Distribute traffic across servers to prevent overload.
   2. Algorithms: Round Robin, Least Connections, IP Hashing.
5. CDN (Content Delivery Network)
   1. Caches static content (images, videos) closer to users.
   2. Example: Netflix uses CDNs to stream videos with low latency.
6. Message Queues
   1. Enable asynchronous communication between services.
   2. Tools: Kafka, RabbitMQ.
   3. Use Case: Uber uses queues to process ride requests and payments separately.

Core Principles of System Design

1. Scalability:
   1. Vertical Scaling: Upgrade server hardware (add RAM, CPU).
   2. Horizontal Scaling: Add more servers (e.g., cloud auto-scaling).
2. Reliability:
   1. Redundancy: Backup servers, databases, and services.
   2. Failover: Automatically switch to a backup if a server crashes.
3. Availability:
   1. Aim for “five nines” (99.999% uptime = ~5 minutes of downtime per year).
4. Consistency:
   1. Strong Consistency: All users use the latest data (e.g., banking apps).
   2. Eventual Consistency: Data syncs eventually (e.g., social media likes).
5. Security:
   1. Encrypt data, authenticate users (OAuth, JWT), and guard against DDoS attacks.