Design a scalable messenger

Q: Design a scalable messenger

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

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Question

System Design: Real-Time Messenger (1:1 and Group Chats)

Context

Design a production-grade real-time messaging system that supports both one-to-one and group conversations. Assume mobile and web clients, multi-device usage, and global scale. Optimize for low latency, reliability, and cost efficiency.

Functional Requirements

Conversations
- 1:1 chats and group chats (create, invite, remove, leave, mute, block).
- Message types: text, emoji, media (images, video, files), and metadata (typing, presence).
- Read receipts (per-recipient), delivery status, message edits/deletes (soft delete acceptable).
Real-time & Offline
- Real-time messaging with message ordering per conversation.
- Offline store-and-forward, sync across devices, and resend on reconnect.
Presence & Notifications
- User presence (online/last seen), typing indicators.
- Push notifications for offline users.
Search
- Search messages and users. Scope results to user’s accessible conversations.

Non-Functional Requirements

Scale and Performance
- Target: 100M MAU, 10M DAU, peak 2M concurrent connections/region.
- P50 send-to-deliver < 200 ms within a region; P99 < 1 s.
- Availability: 99.95%.
Consistency & Reliability
- Per-conversation message ordering.
- Delivery guarantees and idempotency.
- Consistent state across devices.
Security
- Authentication, authorization, transport encryption; discuss E2E encryption option.
- Abuse/spam controls.
Operations
- Observability (metrics, logs, traces), rate limiting, backpressure.
- Cost-aware design and capacity planning.

Deliverables

Clarify assumptions and requirements.
High-level architecture: storage choices and messaging protocols.
APIs and data models.
Message ordering and delivery guarantees; read receipts.
Presence, push notifications, media handling, and search.
Scalability: sharding, caching, queues; consistency across devices.
Multi-region deployment, failover, offline/slow network scenarios.
Security, observability, and cost trade-offs.