Design a real-time messenger

Q: Design a real-time messenger

This question evaluates system-design and distributed-systems competencies for building a large-scale, low-latency real-time messaging platform, covering topics such as real-time protocols, delivery semantics, multi-device synchronization, data partitioning, persistence and search, and privacy/security considerations.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Onsite rounds at Meta.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Question

Design a Real-Time Messaging System (1:1 and Group)

Context

You are designing a consumer-scale messaging platform supporting both direct and group conversations across multiple user devices. The system must provide low-latency real-time delivery, offline sync, and strong privacy controls while operating at large scale.

Scale targets: 100M MAU, 10M DAU
Latency SLO: p99 send-to-receive under 150 ms (same-region)

Requirements

Features
- 1:1 and group chats
- Multi-device sync (multiple active devices per user)
- Presence and typing indicators
- Read receipts and delivery receipts
- Media attachments (images/video/docs) with previews
- Message search
Delivery Semantics and Ordering
- At-least-once delivery with deduplication
- Message IDs and ordering guarantees per conversation
Offline and Sync
- Offline storage and background sync for clients
- Server-side backfill and out-of-order handling
APIs (define schemas and behavior)
- SendMessage
- Ack (delivery/read/typing)
- Sync (incremental state)
- FetchHistory (backfill)
- SubscribePresence (presence/typing)
Architecture
- Edge gateways and real-time fanout (e.g., WebSockets)
- Message persistence and indexing for search
- Cold storage tiering
- Partitioning by user/conversation
Non-Functional Concerns
- Exactly-once UX despite at-least-once transport
- End-to-end encryption options
- Abuse/spam mitigation, rate limits/quotas
- Data retention/TTL and GDPR deletion
- Observability (metrics, logs, tracing, SLOs)

Describe the end-to-end design and trade-offs. Include key data flows and how you meet latency and scale targets.

Design a real-time messenger

Quick Overview

Design a Real-Time Messaging System (1:1 and Group)

Context

Requirements

Solution

Comments (0)