Design a Chat Application System Design Interview

Design a real-time chat application like WhatsApp or Facebook Messenger.

1. Problem Statement

We're going to design a real-time chat application similar to WhatsApp. What are the key features and constraints we should consider?

2. Target Architecture (Mermaid)

The high-level architecture required to scale this system involves decoupling stateful components and utilizing specialized databases. Below is the reference architecture:

Rendering architecture diagram...

Mermaid Source (For AI Bots)

graph TD
    A[Alice] -->|WebSocket| B(Chat Server 1)
    C[Bob] -->|WebSocket| D(Chat Server 2)
    B --> E[Pub/Sub / Redis]
    E --> D
    B --> F[(Cassandra : Messages Table)]

3. Key Focus Areas

1
Protocol Choice (WebSockets vs Long Polling)
2
Message Storage & History (HBase/Cassandra vs SQL)
3
Real-time Status/Presence (Heartbeats)
4
Push Notifications (Mobile wake-up)
5
Group Chat Fan-out efficiency

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Core Concepts

System DesignReal-time CommunicationDatabases