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Design a Collaborative Document Editor

System Design

Problem

Design a Collaborative Document Editor.

Self-assessment rubric

0/16

Requirements & Scope

Clarifies concurrency scale, offline needs, rich text vs plain text, and version history needs before diving in
Defines functional scope (real-time sync, conflict resolution, presence, persistence, history) and non-functional targets (latency <300ms, scale, consistency model)

Core Architecture

Proposes WebSocket-based real-time gateway with per-document rooms/fan-out
Introduces a collaboration/sequencing service that owns op ordering per document (sharded by doc_id)
Uses a durable, ordered op log (e.g., Kafka) as source of truth, separate from periodic snapshots
Separates ephemeral presence/cursor data (Redis pub/sub) from durable document content

APIs & Data Model

Defines WebSocket op submission/broadcast protocol with version numbers for ordering/transform
Provides REST endpoints for cold-load, version history, and restore
Designs schema distinguishing metadata store, op log, snapshot/blob store, and presence cache with appropriate PK/index choices

Deep Dives & Trade-offs

Discusses OT vs CRDT explicitly and justifies a choice with concrete reasoning
Explains how total ordering is maintained per document and how failover/leader takeover works
Addresses snapshotting/compaction strategy to bound replay cost and storage growth
Covers reconnect/catch-up flow and at least mentions offline-editing trade-offs
Articulates WebSocket fan-out scaling approach for hot documents/high concurrency

Communication

Structures answer with clear phases (requirements → high-level → deep dive → wrap-up) within time budget
Summarizes key trade-offs succinctly and ties back to stated requirements at the end

Solution

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