Design a multi-tenant Slack-like messenger

Q: Design a multi-tenant Slack-like messenger

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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Question

System Design: Multi‑Tenant B2B Team Messaging (Slack‑like)

Design a multi‑tenant, enterprise‑grade team messaging platform similar to Slack.

Functional Requirements

Workspaces (organizations) with members and roles
Channels (public/private) and direct/group messages
Message fan‑out to channels with thousands of members
Real‑time presence and typing indicators
Reactions and threaded replies
Full‑text search across messages/files
File sharing and previews
Push notifications (mobile/desktop)

Enterprise Requirements

SSO/SAML and SCIM user provisioning
Role‑based access control (RBAC)
Audit logs (tamper‑evident) and eDiscovery/compliance exports
Data retention policies, legal hold
Cross‑workspace federation (shared channels)

Non‑Functional Goals

50M DAU, up to 10M concurrent connections
p99 intra‑workspace delivery under 200 ms
Durable storage with RPO = 0 across availability zones
Cost efficiency (resource utilization, tiered storage)

What to Cover

Overall architecture (edge/gateway, WebSocket vs HTTP/2, message queues)
Data modeling (users, workspaces, channels, messages, indexes)
Fan‑out patterns (write vs read; hybrids)
Ordering and idempotency
Offline delivery and pagination
Search indexing pipeline
Media storage and CDN
Rate limiting and abuse prevention
Multi‑region strategy (latency, failover, data residency)
Observability (metrics, tracing, logging)
Backfill/migration strategies
Provide key APIs and schema sketches