Design a Chat Application

1. Problem Statement

2. Architecture Discussion Map

Use this as one discussion aid, not a single correct answer. Your design should follow from the requirements, scale, and trade-offs you establish.

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[(Message store, partitioned by conversation)]

3. Key Focus Areas

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

4. What the Interviewer Looks For

The interviewer probes these areas, and the report grades the interview against them.

• Requirements and estimation
• Connection and gateway layer
• Message flow, ordering, and delivery guarantees
• Storage and group fan-out
• Presence, multi-device, and resilience