Design Microsoft Teams' Real-Time Messaging System

Problem

Design the core messaging backend for Microsoft Teams supporting real-time chat in channels and 1:1 DMs.

Requirements

Functional:

• Send and receive messages in real time
• Support channels (up to 10,000 members) and DMs
• Message history (infinite scroll, paginated)
• Read receipts and typing indicators
• File/image attachment support

Non-functional:

• 300M daily active users
• Message delivery p99 < 200ms
• 99.99% availability
• Messages must never be lost

Discussion points

1. WebSocket connection management and fan-out at scale
2. Message storage schema and partitioning strategy
3. Handling offline users — push notifications vs pull on reconnect
4. Read receipts without thundering herd
5. How Azure Service Bus or Kafka fits in the pipeline