Table of content:

• Functional Requirements
• Non Functional Requirements
• Capacity Estimation
• High Level System Design
• Algorithms

Messaging applications such as WhatsApp/Facebook Messengers provide instant messaging services to users.

Functional Requirements

1. System should be able to support one-to-one/group messaging
2. Users should be able to see live status
3. System should support file sharing(on receive deletion)

Non-Functional Requirements

1. System should highly reliable/scalable
2. System should be highly available with eventual consistency

Capacity Estimation

Throughput

• DAU : 50M users and each user watch 10 messages/day
• 500 million messages/day -> 5000 RPS
• 10% of users upload files-> 50 million files/day

Storage

• 50 M X100 = 5GB/day

High Level System Design

• User send messages to user/group which is handled by chat server
• Chat server extracts files (if any) upload into media storage.
• Event triggered message queue and notification send to user/group via notification server
• Presence server uses heartbeat protocol to check whether user active/not
• Communication between chatServer and user happens through WebSocket(Duplex)/ SSE/Long Polling

Algorithms

• WebSockets
• Server Sent Events
• Long Polling
• HeartBeat Protocols