Mock 2: design a chat system like WhatsApp

Copy this into your fork. 45-minute timer. Out loud. Lead with the connection model. Stop when it rings and grade yourself.

The brief

Design a messaging app like WhatsApp. One-to-one and group chats, delivery receipts (sent, delivered, read), presence, and offline delivery. Assume a billion registered users. You may be nudged toward media, encryption, or huge groups, so leave room, but protect the core: connection, routing, delivery, offline.

1. Requirements (5 min)

Functional:

Non-functional (name exactly-once, ordered-per-conversation, scale):

2. Scale estimate (5 min)

Sends/s (peak):

Concurrent open connections:

Storage per day:

3. API and core entities (5 min)

Events over the connection:

Entities (model the message and the per-conversation sequence):

4. High-level design (10 to 15 min)

The connection and routing layer (how A on gateway 7 reaches B on gateway 2000):

The send path (online recipient):

The offline path (recipient disconnected, then reconnects):

5. Deep dives (10 to 15 min)

The hinge I am going deep on (delivery and ordering, or group fan-out, or presence):

My answer and the tradeoff:

6. Bottlenecks and failure (5 min)

A gateway dies with its connections: what happens, and how is no message lost?

What gets shed first under load?

Self-grade (score each out of 2)

• Requirements: exactly-once, ordered-per-conversation, billion-user scale
• Scale: sends/s and connection count
• API and entities: message + per-conversation sequence
• High-level: persistent connections + routing layer, not stateless web
• Deep dive: offline sync or fan-out done properly
• Failure and cost: gateway death and reconnect, presence best-effort

Total out of 12: __ The concept I drill again: __