HLD-407·Messaging & Fan-out
Expert · 2,000 ptsGlobal Multiplayer Document Revision Log & OT/CRDT Sequencer
Design Google Docs / Notion’s collaborative editing backend sequencing concurrent character-level OT/CRDT operations, live comments, and lossless revision history.
Max p99
≤ 90 ms
Availability
≥ 99.5%
Budget
≤ $11,300
Functional Requirements
- FR-1POST /docs/{doc_id}/ops — Sequences transformation against head revision and appends immutable op log.
- FR-2GET /docs/{doc_id}/snapshot — Serves compacted document model + tail revisions to collaborators.
- FR-3Retention & Lifecycle (doc_id) — Enforces TTL retention windows, deduplication, or archival compaction without blocking live reads or writes.
- FR-4Strict serializable revision numbering per doc_id with zero lost keystrokes when primary storage crashes.
Workload Profile
Workday Editing68,000 Doc Read/Sync RPS / 13,000 Keystroke Op Batch RPS
Incident Postmortem Live-Edit Surge175,000 Read RPS / 36,000 Op Write RPS
Active Working Set44 GB active open document buffers & op rings
Consistency GateTotal order revision sequence + RPO = 0 durability
Consistency Model
Strict monotonic revision sequencing per doc_id + synchronous/quorum WAL so transformed ops never diverge.
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