Collaborative Document Editor: Architecture Review

Failure Modes Analysis

1. WebSocket Connection Failures

Issue: Client connections can fail due to network issues or server restarts, causing lost updates. Solution: Implement automatic reconnection with change buffering

• Store pending changes in client memory with periodic localStorage backups
• Implement exponential backoff for reconnection attempts
• Add offline mode indicators and sync status UI
• Maintain client session context to resume collaboration properly

Trade-offs: Increases client-side complexity; potential for complex conflict resolution when reconnecting after extended offline periods

2. Single-Server Communication Silo

Issue: Changes only broadcast to clients connected to the same server Solution: Replace polling with a proper pub/sub system

• Implement Redis pub/sub channels for cross-server communication
• Each server publishes changes to a common channel and subscribes to receive others
• Eliminates the 2-second polling delay and reduces database load

Trade-offs: Adds Redis as another critical component; requires handling Redis outages

3. Database Single Point of Failure

Issue: If PostgreSQL fails, the entire system becomes unusable Solution: Implement multi-level persistence strategy

• Primary/replica configuration with automatic failover
• Write-ahead log in memory or separate queue before database commit
• Circuit breakers to gracefully degrade functionality during database issues
• Consider separating read and write paths with CQRS pattern

Trade-offs: Significantly increases operational complexity; eventual consistency challenges

Race Conditions

1. Client Clock Timestamps

Issue: Using client clocks for conflict resolution is unreliable Solution: Implement proper distributed sequence tracking

• Replace client timestamps with Lamport timestamps or vector clocks
• Consider full Operational Transformation (OT) or Conflict-free Replicated Data Types (CRDTs)
• Server-assigned sequence numbers for total ordering of operations

Trade-offs: OT/CRDTs introduce significant implementation complexity; requires more metadata with each change

2. Coarse-grained Conflict Resolution

Issue: Paragraph-level "last-write-wins" causes data loss during concurrent edits Solution: Implement finer-grained conflict resolution

• Character-level operations rather than paragraph-level
• Use differential synchronization or OT for merging concurrent edits
• Separate structure from content in the document model

Trade-offs: Increases computational overhead; more complex data model

3. JWT Token Management

Issue: localStorage tokens vulnerable to XSS; no revocation mechanism Solution: Improve token security

• Move to HttpOnly cookies for tokens (with CSRF protection)
• Implement token revocation via Redis blacklist
• Add refresh token rotation for improved security
• Consider shorter token lifetimes (1 hour instead of 24)

Trade-offs: More complex authentication flow; additional Redis dependency

Scaling Bottlenecks

1. PostgreSQL as Synchronization Point

Issue: Database becomes bottleneck with high concurrent editing Solution: Distributed event sourcing architecture

• Use specialized append-only event store for document operations
• Implement document-specific sharding/partitioning strategy
• Consider purpose-built databases for collaborative editing (e.g., FaunaDB)

Trade-offs: Increases system complexity; potentially higher infrastructure costs

2. Inefficient Document Storage

Issue: Full HTML snapshots are storage-inefficient and risky Solution: Hybrid storage approach

• Store document operations as an append-only log (event sourcing)
• Generate snapshots at logical points (not time-based), like after major changes
• Store documents as structured data (e.g., JSON) instead of raw HTML
• Implement incremental document loading for large documents

Trade-offs: More complex recovery and rendering logic; requires migration strategy

3. CDN Caching of Dynamic Content

Issue: 5-minute API caching conflicts with real-time editing Solution: Targeted caching strategy

• Cache only static assets and immutable resources
• Use cache-control headers to prevent caching of document content
• Implement cache invalidation for resources that do change occasionally

Trade-offs: Reduced CDN effectiveness; potentially higher origin load

4. WebSocket Connection Distribution

Issue: Uneven connection distribution across servers Solution: Dedicated real-time tier

• Separate WebSocket handling from API servers
• Implement consistent hashing for connection distribution
• Consider specialized WebSocket services (e.g., Pusher, Socket.io service)

Trade-offs: Additional infrastructure component; higher operational complexity

Recommended Priority Fixes

1. Implement proper real-time synchronization (Redis pub/sub)
2. Replace client clock timestamps with server-based logical clocks
3. Upgrade document storage strategy to incremental operations
4. Improve WebSocket reliability with reconnection handling
5. Fix authentication security issues with proper token management

These changes would significantly improve reliability, correctness, and performance of your collaborative editing system.