HLD-310·Messaging & Fan-out
Hard · 1,500 pts1-on-1 & Small-Group E2EE Messaging, Offline Queue & Receipts
Design WhatsApp / Signal’s encrypted message relay, per-device offline inbox store-and-forward queue, and delivered/read receipt state machine.
Max p99
≤ 60 ms
Availability
≥ 99.5%
Budget
≤ $8,300
Functional Requirements
- FR-1POST /messages/{conversation_id} — Persists encrypted envelope into recipient offline inbox until ACKed.
- FR-2GET /inbox/{user_id} — Drains ordered pending E2EE envelopes and delivery receipts on reconnect.
- FR-3PATCH & DELETE /messages/{conversation_id} — Updates, cancels, or deletes user_id records and immediately invalidates or updates cached state (read-after-write).
- FR-4Read-after-write ordering required per conversation_id; zero dropped messages on node failure.
Workload Profile
Steady Global Chat48,000 Sync Read RPS / 12,000 Message Write RPS
Midnight Holiday Surge132,000 Read RPS / 30,000 Write RPS
Active Working Set32 GB ephemeral un-ACKed device queues & session routes
Latency GateSub-60ms p99 relay delivery across node failover
Consistency Model
Per-conversation monotonic ordering + read-after-write inbox visibility until recipient device ACKs.
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