Design a Slack-like real-time messaging system

Q: Design a Slack-like real-time messaging system

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

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System Design: Slack-like Messaging (DMs + Channels)

Design a Slack-like real-time messaging system that supports:

Core features

Direct Messages (DMs) between two users.
Channels with many members (from small teams to very large channels ).
Real-time delivery to online users (web/mobile).
Multi-device : the same user may be logged in on multiple devices simultaneously.

Key design topics to cover

How messages are persisted and delivered in real time.
Push vs pull delivery models, including how you would handle large channels .
Multi-device delivery semantics (e.g., once per device vs once per user) and how you track delivery/reads.
Scalability: caching, partitioning/sharding, and how the system behaves under high fanout and spikes.

Assumptions / constraints (you may refine)

Message ordering should be correct within a DM or channel .
The system should tolerate server failures and client reconnects.
Latency target: near real-time for online users (e.g., p99 < 1–2s).

Provide APIs, data model, high-level architecture, and the key trade-offs/edge cases.

Design a Slack-like real-time messaging system

System Design: Slack-like Messaging (DMs + Channels)

Core features

Key design topics to cover

Assumptions / constraints (you may refine)

Solution

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