Realtime Sync
Overview
Architects high-concurrency, sub-50ms latency synchronization between distributed clients and servers. Covers WebTransport (HTTP/3) bidirectional streaming, transactional outbox patterns for database-to-sync consistency, CRDTs for collaborative editing, and AI token stream orchestration.
Core principles: the database is the source of truth (real-time channels notify, not persist), CRDTs eliminate locking for concurrent edits, and backpressure management prevents UI jitter from high-frequency streams.
When to use: Real-time collaborative UIs, pub/sub messaging, WebTransport/WebSocket connections, live AI token streams, presence tracking, conflict resolution with CRDTs, multiplayer applications.
When NOT to use: Batch processing pipelines, static content delivery, request-response APIs without real-time requirements, offline-only applications.
Quick Reference
| Pattern | Approach | Key Points |
|---|---|---|
| WebTransport | new WebTransport(url) + await ready | HTTP/3 multiplexed; replaces WebSockets for new projects |
| Bidirectional stream | transport.createBidirectionalStream() | Returns {readable, writable} for request-response |
| Unidirectional stream | transport.createUnidirectionalStream() | Returns a WritableStream directly for one-way pushes |
| Datagrams | transport.datagrams.writable | UDP-like unreliable delivery for high-frequency state |
| Connection stats | transport.getStats() | Returns smoothedRtt, bytesSent, packetsLost |
| Transactional outbox | DB write + outbox insert in one transaction | CDC worker pushes to channel; prevents state drift |
| Sequence tracking | Sequence IDs on every message | Rewind on reconnect to fetch missed messages |
| CRDT collaboration | Yjs (text) or Automerge (JSON state) | Conflict-free concurrent editing without locks |
| CRDT awareness | y-protocols/awareness module | Tracks cursors, selections, and user presence |
| CRDT undo/redo | Y.UndoManager with trackedOrigins | Tracks only local user operations for selective undo |
| Presence | Heartbeat-based user tracking | Epidemic broadcast for reliable zombie cleanup |
| AI stream batching | requestAnimationFrame for token rendering | Prevents UI jitter from high-frequency updates |
| Buffer-and-batch | useTransition for sync-triggered updates | Defers non-urgent re-renders during sync bursts |
| Backpressure | Buffer size limit with forced flush | Prevents memory buildup when tokens outpace rendering |
| WebSocket fallback | Detect WebTransport support first | Enterprise firewalls may block UDP/HTTP/3 |
| Serialization | Protocol Buffers or MessagePack | Avoid JSON.stringify on 100Hz+ streams |
| Web Worker transport | Handle WebTransport in a Web Worker | Prevents blocking the UI thread |
| Channel multiplexing | Subscribe to multiple channels on one socket | All subscriptions share a single transport connection |
Common Mistakes
| Mistake | Correct Pattern |
|---|---|
| Using real-time messages as the primary source of truth | State lives in the database; real-time is the notification of change |
| Using JSON.stringify on high-frequency streams (100Hz+) | Use Protocol Buffers or MessagePack for serialization |
| Ignoring sequence drift when a client misses messages | Implement Sequence IDs and a rewind mechanism on the client |
| Implementing global locks for concurrent access | Use Optimistic UI and CRDTs for conflict-free collaboration |
| Sticking with WebSockets for new projects needing multiplexing | Use WebTransport (HTTP/3) for bidirectional, multiplexed streams |
| CRDT document bloat from unbounded history | Use snapshotting (LWW) for fields that do not need history |
| Failing to close WebTransport streams on unmount | Explicitly close streams to prevent transport crashes |
Importing Awareness from yjs instead of y-protocols | Use import {Awareness} from 'y-protocols/awareness' |
Using ydoc.clientID as UndoManager tracked origin | Use custom transaction origins passed to doc.transact() |
| One React re-render per AI token received | Batch tokens with requestAnimationFrame and flush once per frame |
Not handling WebTransport closed promise | Monitor transport.closed to detect unexpected disconnections |
Delegation
If thelocal-firstskill is available, delegate local-first architecture decisions, sync engine comparison, and client storage selection to it. If theelectricsqlskill is available, delegate ElectricSQL shape-based Postgres sync patterns to it.
- Explore transport protocol options and latency benchmarks: Use
Exploreagent to compare WebTransport, SSE, and WebSocket tradeoffs - Implement transactional outbox pattern with CDC pipeline: Use
Taskagent to set up database triggers, outbox table, and background worker - Design real-time architecture for collaborative editing: Use
Planagent to map CRDT strategy, presence management, and conflict resolution flow - Build multi-stream AI orchestration component: Use
Taskagent to implement parallel token stream rendering with backpressure - Audit real-time connection lifecycle management: Use
Reviewagent to check for stream leaks, missing cleanup, and reconnection handling