What is System Design?

System design is the process of how we build large-scale applications.

Term	Simple Meaning	Example (WhatsApp)
Client	The device that makes requests	Your phone sending a chat message
Server	The computer that handles requests	WhatsApp’s backend that delivers the message
Database (DB)	Where data is stored	Stores your chat history and contacts
API	Rules for communication between systems	WhatsApp API that sends/receives messages
Latency	Delay between request and response	Time it takes for your message to be delivered
Throughput	Number of requests handled per second	WhatsApp handling millions of messages per sec
Load Balancer	Distributes requests across multiple servers	Ensures no single WhatsApp server crashes
Cache	Temporary fast storage	Stores recently sent messages for quick access
Scalability	Ability to grow with more users/data	WhatsApp working for millions of users at once
Redundancy	Backup systems to prevent failure	Duplicate servers in case one goes down
Fault Tolerance	Keeps working even if part fails	Message still sends even if one server fails
Microservices	Small independent services instead of one big app	WhatsApp splitting chat, call, and media services
Monolith	Single big application with all features inside	One large WhatsApp codebase (not split)
Horizontal Scaling	Adding more servers to handle growth	WhatsApp adding more servers worldwide
Vertical Scaling	Making one server more powerful (RAM/CPU)	Upgrading a WhatsApp server’s hardware