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ISSUE_831_IMPLEMENTATION_SUMMARY.md

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Issue #831: Real-Time Collaborative Editing with CRDTs

Overview

Implemented a backend-first real-time collaboration system that supports concurrent edits, conflict-free merges, offline sync replay, and low-latency multi-user synchronization.

Core Architecture

  • CRDT model: RGA-style text CRDT using tombstones for conflict-free insert/delete merge
  • LWW registers: Deterministic Last-Writer-Wins merges for note fields and spreadsheet cells
  • Versioned operation log: Server-versioned operations for offline replay and delta sync
  • Vector clock tracking: Per-actor counters in document state for merge awareness
  • Optimistic concurrency: Retry logic for concurrent save races

Added Files

  • models/CollaborativeDocument.js
  • services/realtimeCollaborationService.js
  • routes/realtimeCollaboration.js

Updated Files

  • server.js

REST APIs

  • POST /api/realtime-collab/documents - create collaborative document
  • GET /api/realtime-collab/documents/:id - get snapshot/state
  • POST /api/realtime-collab/documents/:id/sync - submit offline or realtime ops
  • GET /api/realtime-collab/documents/:id/changes?sinceVersion=N - fetch deltas
  • POST /api/realtime-collab/documents/:id/presence - update online presence

Socket.IO Events

  • Client → Server:
    • collab:join
    • collab:operations
    • collab:leave
  • Server → Client:
    • collab:snapshot
    • collab:operations
    • collab:ack
    • collab:presence
    • collab:error

Distributed Real-Time Sync

  • Redis pub/sub channel: expenseflow:collab
  • Cross-instance operation fan-out with serverInstanceId dedupe to avoid local echo duplication

Offline Editing Support

Clients can queue operations while offline and replay them via /sync. The server performs idempotent merges using operation IDs and returns authoritative versions and deltas.

Conflict Resolution Guarantees

  • Concurrent text inserts at same position resolve deterministically by operation ID ordering
  • Deletes are tombstone-based and idempotent
  • Cell/field updates resolve deterministically by (lamport, actorId, opId) ordering

Scalability/Latency Notes

  • Room-based websocket broadcasting minimizes unnecessary fan-out
  • Redis pub/sub enables horizontal scaling across multiple API instances
  • Versioned change feed allows efficient incremental sync