Design a scalable real-time chat system supporting 1:1 and group messaging. Cover: API design (send, receive, ack), message ordering and idempotency, read receipts and typing indicators, online presence, fan-out architecture (write vs read), storage (hot vs cold, media handling), indexing and data model, replication and sharding strategy, consistency vs availability trade-offs, offline delivery and retries via push notifications, rate limiting and spam/abuse controls, end-to-end encryption considerations, back-of-the-envelope capacity estimates for 100M DAU, monitoring, observability, and disaster recovery.

This question evaluates skills in designing scalable real-time distributed systems, covering API design, message ordering and idempotency, fan-out strategies, storage tiers, indexing, sharding and replication, consistency vs.

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Design a scalable real-time chat system

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.

Context

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

Quick Overview

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

Context

Requirements

Solution

Comments (0)

Design a scalable real-time chat system

Quick Overview

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

Context

Requirements

Solution

Comments (0)