WhatsApp — Mock Interview

Medium

A real-time messaging system: push 1:1 and group messages to hundreds of millions of persistent connections, and deliver reliably even when the recipient is offline.

Interviewer

You're designing WhatsApp for a live system-design interview. Users send 1:1 and group messages, see presence (online / last seen), and get sent / delivered / read receipts — all end-to-end encrypted. Messages must arrive in real time when the recipient is online and be reliably delivered the moment an offline recipient reconnects.

Scale assumptions:

~2B registered users; ~1B daily active, each sending ~50 messages/day.
At peak, ~500M users hold a live connection simultaneously.
Groups up to ~1,000 members; media (images, voice notes) sent as encrypted blobs.
Targets: in-region delivery well under 1 s p99; no message ever silently lost.

We'll move through requirements, estimation, architecture, the delivery + connection-management deep dive, and trade-offs. Think out loud and state your assumptions.

The interviewer grades each stage against a hidden rubric and asks a probing follow-up, mirroring a real system design round. Sessions are capped at 30 turns.