Design a scalable real-time chat system

Q: Design a scalable real-time chat system

This is a System Design interview question from Lyft for Software Engineer roles. View the full question and solution on PracHub.

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Question

System Design: Real-Time Chat (1:1 and Groups)

Context

Design a mobile-first, globally available real-time chat system that supports both 1:1 and group messaging at large scale (target: 100M daily active users). Assume multi-region deployment, clients are primarily mobile apps with intermittent connectivity, and the system must prioritize low latency and high availability.

Requirements

Cover the following areas explicitly:

API design
- Send, receive/sync, acknowledgments (acks)
- Message ordering and idempotency semantics
Features
- Read receipts and typing indicators
- Online presence
Architecture
- Fan-out strategy (write vs. read; hybrid if applicable)
- Storage tiers (hot vs. cold) and media/attachments handling
- Indexing and data model
- Replication and sharding strategy
- Consistency vs. availability trade-offs
Delivery
- Offline delivery and retries via push notifications
Safety
- Rate limiting and spam/abuse controls
- End-to-end encryption considerations (1:1 and groups)
Scale
- Back-of-the-envelope capacity estimates for 100M DAU
Operations
- Monitoring, observability, and disaster recovery

Make reasonable assumptions where needed and call them out explicitly.

Design a scalable real-time chat system

System Design: Real-Time Chat (1:1 and Groups)

Context

Requirements

Solution (Locked)

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