Design WhatsApp

End-to-end system design interview walkthrough. WebSocket APIs, message durability, offline inboxes, media attachments, pub/sub fanout, multi-device sync, ordering, presence, and failure handling for global chat.

Problem Statement & Requirements

Requirements & Scope Control

API & Interface

WebSocket Commands & Acknowledgements

The Basics

Chat Metadata & Real-Time Delivery

Deep Dive: Durability

Offline Delivery with Inbox Tables

Deep Dive: Media

Attachments, Object Storage & Presigned Uploads

Deep Dive: Scale

Fanout Routing, Pub/Sub & Partitioning

Deep Dive: Multi-Device

Clients, Per-Device Inbox & Sync

Deep Dive: Reliability

Heartbeats, Missed Messages & Ordering

Interview Expectations

Last Seen, Trade-offs & Leveling Bar