Design a scalable messenger
Overview
This question evaluates system design and architectural competencies for building scalable, low-latency real-time messaging platforms, testing knowledge of distributed systems, data consistency and ordering, messaging protocols, storage choices, security, and operational concerns at both conceptual and practical application levels.
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Sep 6, 2025, 12:00 AM
Software Engineer
Onsite
System Design
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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
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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).
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Real-time & Offline
- Real-time messaging with message ordering per conversation.
- Offline store-and-forward, sync across devices, and resend on reconnect.
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Presence & Notifications
- User presence (online/last seen), typing indicators.
- Push notifications for offline users.
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Search
- Search messages and users. Scope results to user’s accessible conversations.
Non-Functional Requirements
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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%.
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Consistency & Reliability
- Per-conversation message ordering.
- Delivery guarantees and idempotency.
- Consistent state across devices.
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Security
- Authentication, authorization, transport encryption; discuss E2E encryption option.
- Abuse/spam controls.
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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.
Solution
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