SyncKit

True offline-first sync for modern apps—without vendor lock-in

Getting Started • Documentation • Examples • Discussions • Roadmap

🎯 What is SyncKit?

Build collaborative apps in hours, not months.

SyncKit is a sync engine that gives you everything for local-first collaboration:

Rich text editing with conflict resolution (Peritext + Fugue CRDTs)
Undo/redo that syncs across tabs and sessions
Live presence and cursor sharing
Framework adapters for React, Vue, and Svelte

"Add sync.document() to your app, get real-time sync automatically."

The reality: Building sync from scratch takes months. SyncKit gives you working collaboration in 3 lines of code.

const sync = new SyncKit()
await sync.init()
const doc = sync.document<Todo>('todo-123')
await doc.update({ completed: true })
// ✨ Works offline, syncs automatically, resolves conflicts

🎬 See It In Action

⚡ Try Live Demo →

LocalWrite - Full-featured collaborative editor showcasing SyncKit's capabilities.

Quick test (30 seconds):

Open localwrite-demo.fly.dev in two browser tabs
Click "Join a Room" in both tabs (auto-joins same room)
Type in one tab → appears instantly in the other
Watch live cursors showing each user's position

What you'll see:

Character-level sync - No debouncing, no lag
Conflict-free merging - Multiple users editing simultaneously
Block-based editor - Slash commands (/h1, /list) with real-time formatting
Presence & cursors - See who's online, where they're typing
Offline-first - Disconnect your network, keep editing, auto-sync on reconnect
Word Wall - Community voting feature (bonus: shows OR-Set CRDT in action)

Real-world example:

import { SyncKit } from '@synckit-js/sdk'
import { useSyncDocument } from '@synckit-js/sdk/react'

// Initialize once
const sync = new SyncKit()
await sync.init()

// Use in components
function TaskList() {
  const [tasks, { update }] = useSyncDocument<Task[]>('tasks')

  const toggleTask = (id: string) => {
    update(tasks.map(t =>
      t.id === id ? { ...t, completed: !t.completed } : t
    ))
  }
  // Syncs across all connected clients automatically
}

✨ Why SyncKit?

🚀 Works When Internet Doesn't

True offline-first architecture—not just caching. Your app works perfectly on planes, trains, tunnels, and coffee shops with spotty WiFi.

📦 Feature-Complete

154KB gzipped - Complete local-first sync solution with everything you need.

What you get:

✅ Text editing (Fugue CRDT) - Collaborative editing that just works
✅ Rich text formatting (Peritext) - Bold, italic, links with conflict resolution
✅ Undo/redo - Syncs across tabs, persists across sessions
✅ Real-time presence - See who's online, what they're editing
✅ Cursor sharing - Watch teammates type in real-time
✅ Counters & Sets - Distributed data structures for app state
✅ Framework adapters - React, Vue, Svelte (choose what you need)
✅ Offline-first sync - Works perfectly without internet
✅ IndexedDB persistence - Unlimited local storage

Size-critical apps? Use Lite variant (46KB gzipped, basic sync only)

Every byte is justified. We chose completeness over minimal size—rich text, undo/redo, cursors, and framework adapters all work together out of the box.

🔓 Your Data, Your Rules

Open source and self-hostable. No vendor lock-in, no surprise $2,000/month bills, complete data sovereignty.

⚡ Fast by Design

<1ms local operations (~5-20μs single field update)
<100ms sync latency (10-50ms p95)
154KB bundle (complete solution), 46KB lite option
~310KB total with React (comparable to React alone)

🛡️ Designed for Data Integrity

Operations are applied and stored on the device first, then synced when a connection is available
Concurrent text edits merge via Fugue and Peritext; concurrent writes to the same document field resolve by Last-Write-Wins, so one write is chosen
Algorithm designs are model-checked with TLA+; see protocol/tla for what is and is not covered
2,100+ tests across TypeScript, Rust, Python, Go, and C# (unit, integration, chaos, load)

🚀 Quick Start

Installation

npm install @synckit-js/sdk

Your First Synced App

import { SyncKit } from '@synckit-js/sdk'
import { SyncProvider, useSyncDocument } from '@synckit-js/sdk/react'

// Initialize (works offline-only, no server needed!)
const sync = new SyncKit()
await sync.init()

function App() {
  return (
    <SyncProvider synckit={sync}>
      <TodoApp />
    </SyncProvider>
  )
}

function TodoApp() {
  const [todo, { update }] = useSyncDocument<Todo>('todo-1')

  if (!todo || !todo.text) return <div>Loading...</div>

  return (
    <div>
      <input
        type="checkbox"
        checked={todo.completed}
        onChange={(e) => update({ completed: e.target.checked })}
      />
      <span>{todo.text}</span>
    </div>
  )
}

That's it! Your app now:

✅ Works 100% offline
✅ Syncs across tabs automatically
✅ Persists data in IndexedDB
✅ Resolves conflicts automatically

Bundle: SyncKit (154KB gzipped) + React (156KB) = ~310KB total

Size-critical? import { SyncKit } from '@synckit-js/sdk/lite' (46KB gzipped, local-only)

Full tutorial (5 minutes) →

🎓 Features

Text Editing & Collaboration

✍️ Text CRDT (Fugue) - Collaborative editing with conflict-free convergence
🎨 Rich Text (Peritext) - Bold, italic, links with proper formatting merge
↩️ Undo/Redo - Cross-tab undo that syncs everywhere
👥 Awareness & Presence - See who's online and what they're editing
🖱️ Cursor Sharing - Real-time cursor positions with smooth animations
🔢 Counters & Sets - Distributed counters (PN-Counter) and sets (OR-Set)

Framework Integration

⚛️ React Hooks - Complete hook library for all features
🟢 Vue Composables - Idiomatic Vue 3 Composition API integration
🔶 Svelte Stores - Reactive Svelte 5 stores with runes support

Core Capabilities

🔄 Real-Time Sync - WebSocket-based instant sync across devices
📴 Offline-First - Works perfectly with zero connectivity
🗄️ Local Persistence - IndexedDB storage, unlimited capacity
🔀 Conflict Resolution - Automatic Last-Write-Wins (LWW) merge for documents, CRDTs for collaboration
⚡ Fast Operations - <1ms local updates, <100ms sync latency
📦 Production Bundle - 154KB gzipped (complete) or 46KB (lite)
🔐 Secure - JWT authentication, RBAC permissions

🏗️ Architecture

graph TD
    A[Your Application<br/>React/Vue/Svelte] --> B[SyncKit SDK<br/>TypeScript]

    B -->|Simple API| B1[document, text, counter]
    B -->|Framework adapters| B2[React/Vue/Svelte hooks]
    B -->|Offline queue| B3[Storage adapters]

    B --> C[Rust Core Engine<br/>WASM + Native]

    C -->|80% of use cases| C1[LWW Sync]
    C -->|Collaborative editing| C2[Text CRDTs]
    C -->|Advanced features| C3[Custom CRDTs<br/>counters, sets]

    C --> D[IndexedDB Storage<br/>Your local source of truth]

    D -.->|Optional| E[SyncKit Server<br/>TypeScript/Python/Go/C#]

    E -->|Real-time sync| E1[WebSocket]
    E -->|Persistence| E2[PostgreSQL/MongoDB]
    E -->|Security| E3[JWT auth + RBAC]

    style A fill:#e1f5ff,stroke:#333,stroke-width:2px,color:#1a1a1a
    style B fill:#fff4e1,stroke:#333,stroke-width:2px,color:#1a1a1a
    style C fill:#ffe1e1,stroke:#333,stroke-width:2px,color:#1a1a1a
    style D fill:#e1ffe1,stroke:#333,stroke-width:2px,color:#1a1a1a
    style E fill:#f0e1ff,stroke:#333,stroke-width:2px,color:#1a1a1a

Detailed architecture docs →

📚 Documentation

Getting Started

5-Minute Quick Start - Build your first synced app
Installation Guide - Setup instructions
API Reference - Complete API documentation

Core Concepts

Offline-First Patterns - True offline architecture
Conflict Resolution - Automatic LWW merge strategy
Performance Optimization - Bundle size, memory, sync speed
Testing Guide - Property-based tests, chaos engineering

Migration Guides

From Firebase/Firestore - Escape vendor lock-in
From Supabase - Add offline support
From Yjs/Automerge - Simplify your stack

Examples

LocalWrite Demo - Full-featured collaborative editor (live demo, see source in demo/)
Vanilla Counter - Minimal example with no build tools (just open in browser!)
Todo App - Simple CRUD with filters
Collaborative Editor - Real-time text editing with CodeMirror 6
Project Management - Full kanban app with drag-and-drop

Browse all docs →

🎯 Use Cases

Tier 1: Simple Object Sync (LWW)

Perfect for: Task apps, CRMs, project management, note apps (80% of applications)

import { SyncKit } from '@synckit-js/sdk'
import { useSyncDocument } from '@synckit-js/sdk/react'

// Initialize once
const sync = new SyncKit()
await sync.init()

// Use anywhere
const doc = sync.document<Project>('project-123')
await doc.update({ status: 'completed' })
// Conflicts resolved automatically with Last-Write-Wins

Tier 2: Collaborative Text Editing

Perfect for: Collaborative editors, documentation, notes

import { useSyncText } from '@synckit-js/sdk/react'

const [text, { insert, delete: del }] = useSyncText('document-456')
await insert(0, 'Hello ')
// Character-level sync, conflict-free convergence

Tier 3: Counters & Sets

Perfect for: Likes, votes, tags, participants

import { useCounter, useSet } from '@synckit-js/sdk/react'

const [count, { increment, decrement }] = useCounter('likes-789')
await increment()  // Conflict-free counter

const [tags, { add, remove }] = useSet<string>('post-tags')
await add('typescript')  // Observed-remove set

🌐 How SyncKit Fits the Ecosystem

Different libraries make different trade-offs. Here's how SyncKit compares:

Feature	SyncKit	Firebase	Supabase	Yjs	Automerge
Bundle Size (gzipped)	154KB (46KB lite)	~150–200KB (typical client)	~45KB (JS client)	65KB (core)	300KB+ (JS/WASM)
Text CRDT	✅ Fugue	❌ No	❌ No	✅ Y.Text	✅ Yes
Rich Text	✅ Peritext	❌ No	❌ No	⚠️ Limited	✅ Yes
Undo/Redo	✅ Cross-tab	❌ No	❌ No	⚠️ Basic	✅ Yes
Awareness/Cursors	✅ Built-in	❌ No	❌ No	⚠️ Extension	❌ No
Framework Adapters	✅ React/Vue/Svelte	❌ No	❌ No	⚠️ Community	❌ No
True Offline-First	✅ Native	⚠️ Limited (cache + persistence)	❌ No native support	✅ Full	✅ Full
Works Without Server	✅ Yes	❌ No	❌ No	✅ Yes	✅ Yes
Self-Hosted	✅ Yes	❌ No	✅ Yes	✅ Yes	✅ Yes
TypeScript Support	✅ Native	✅ Good	✅ Good	⚠️ Issues	✅ Good
Production Status	✅ v0.3.0	✅ Mature	✅ Mature	✅ Mature	⚠️ Stable core, evolving ecosystem

When to Choose SyncKit

Choose SyncKit if:

✅ You need rich text, undo/redo, cursors, and framework adapters included
✅ You want Vue or Svelte support (not just React)
✅ You value shipping fast over optimizing every byte
✅ You want true offline-first without vendor lock-in

Choose alternatives if:

Firebase/Supabase: You need a full backend-as-a-service (auth, storage, functions) and offline sync isn't critical
Yjs: Minimal bundle size is your #1 priority and you're okay wiring up separate plugins for undo, presence, and framework support.
Automerge: You need JSON patching or unique Automerge features (and can accept 300KB+ bundle)

See detailed migration guides →

📦 Packages

Core

@synckit-js/sdk - Core SDK (TypeScript) + WASM engine
@synckit-js/sdk/react - React hooks and components (export from SDK)
@synckit-js/sdk/vue - Vue 3 composables (export from SDK)
@synckit-js/sdk/svelte - Svelte 5 stores with runes (export from SDK)
@synckit-js/sdk/lite - Lightweight version (local-only, 46KB gzipped)

Servers

@synckit-js/server - Bun + Hono TypeScript server (stable)
Python Server - FastAPI implementation (stable, v0.3.0)
Go Server - High-performance goroutine-based server (stable, v0.3.0)
C# Server - ASP.NET Core implementation (stable, community-contributed)

🚦 Status

Current Version: v0.3.0

Production Ready ✅

What the core sync engine ships today:

✅ Document Sync - LWW conflict resolution with vector clocks
✅ Text CRDT (Fugue) - Collaborative text editing with conflict-free convergence
✅ Rich Text (Peritext) - Bold, italic, links with formatting conflict resolution
✅ Counters & Sets - PN-Counter and OR-Set CRDTs
✅ Offline-First Architecture - Works perfectly without internet
✅ WebSocket Protocol - Real-time server synchronization
✅ Core Rust Engine - Memory-safe WASM with zero unsafe blocks
✅ WASM Compilation - 154KB gzipped (46KB lite), optimized performance
✅ TypeScript SDK - Document, Text, RichText, Counter, Set APIs
✅ Storage Adapters - IndexedDB, Memory, and OPFS
✅ Multi-Language Servers - TypeScript, Python, Go, and C# (all stable)
✅ Undo/Redo - Cross-tab undo with persistent history
✅ Awareness & Presence - Real-time user tracking with cursor sharing
✅ Cross-Tab Sync - BroadcastChannel-based synchronization
✅ Framework Adapters - React hooks, Vue 3 composables, Svelte 5 stores
✅ Quill Integration - QuillBinding for Quill editor
✅ Snapshot API - Document snapshots with automatic scheduling
✅ Benchmark Suite - Cross-server performance comparison
✅ 2,100+ Tests - 100% pass rate across TypeScript, Rust, Python, Go, and C#
✅ Example Applications - Todo app, collaborative editor, project management

What's Next 🚧

🚧 Rust Server - Native Rust server implementation
🚧 SQLite Storage - For Node.js, Bun, and Electron
🚧 SQL Sync - Multi-table relational sync
🚧 Conflict UI - Visual conflict resolution interface

Full roadmap →

How this was built

An LLM wrote much of the code. The decisions were mine: Fugue and Peritext over the alternatives, the TLA+ models, the API surface, the wire protocol.

Design first, generation under direction, review before ship. None of it asks for trust: the TLA+ specs are in the repo, and the test suites run in CI across five languages.

🤝 Contributing

We welcome contributions from the community!

Ways to contribute:

🐛 Bug Reports - Open an issue
📚 Documentation - Improve guides, fix typos
🧪 Tests - Add test coverage
🌐 Servers - Implement Rust server
💡 Features - Propose new features in discussions

Contributing guide →

🏢 Enterprise

Need enterprise support?

🎯 Managed Hosting - We host SyncKit servers for you
🔒 Priority Support - 24/7 support, SLA guarantees
📊 Monitoring & Analytics - Dashboard, alerts, insights
🎓 Training & Consulting - Onboarding, architecture review

Contact: [email protected]

📊 Benchmarks

Bundle Size (gzipped)

SyncKit (lite):      46 KB ████████
Yjs (assembled):     65 KB ███████████
SyncKit (default):  154 KB ████████████████████████████
Firebase:           150 KB ████████████████████████████
Automerge:          300 KB ████████████████████████████████████████████████████████

Sync Performance

Local update:       <1 ms  ████
Cross-tab sync:     <1 ms  ████
Network sync:    10-50 ms  ████████
Firebase (cold):  2-30 s   ████████████████████████████████

Memory Usage (10K documents)

SyncKit:       3 MB  ████
Yjs:           8 MB  █████████
Automerge:   180 MB  ████████████████████████████████████████

Detailed benchmarks →

🙏 Acknowledgments

Research & Algorithms

SyncKit's core algorithms are based on published research:

Fugue CRDT - Matthew Weidner & Martin Kleppmann, "The Art of the Fugue: Minimizing Interleaving in Collaborative Text Editing" (2023). Solves the interleaving problem in collaborative text editing.
Peritext - Geoffrey Litt, Sarah Lim, Martin Kleppmann & Peter van Hardenberg, "Peritext: A CRDT for Rich-Text Collaboration" (2021). Provides conflict-free rich text formatting.

Inspiration & Community

Built with inspiration from:

Loro - Production Fugue+Peritext implementation that influenced our Rust architecture
Yjs - Text CRDT concepts and performance optimization
Automerge - CRDT theory and formal verification
Linear - Pragmatic approach to sync
Figma - Custom sync architecture patterns
RxDB - Local-first database patterns
Ink & Switch - Research lab pioneering local-first software

Special thanks to the local-first community for pioneering this movement.

📄 License

MIT License - see LICENSE for details.

🔗 Links

Documentation - Complete guides and API reference
GitHub - Source code
Issues - Bug reports and features
Roadmap - Development timeline
Discussions - Community discussions
LinkedIn - Connect and follow updates

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