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 production-ready 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 production-ready 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): 1. Open localwrite-demo.fly.dev in two browser tabs 2. Click "Join a Room" in both tabs (auto-joins same room) 3. Type in one tab → appears instantly in the other 4. 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.

📦 Production-Ready, 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)

🛡️ Data Integrity Guaranteed

• Zero data loss with automatic conflict resolution (Last-Write-Wins)
• Formal verification with TLA+ (3 bugs found and fixed)
• 2,100+ comprehensive 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 

Loading...



  return (



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



  )
}

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

React/Vue/Svelte] --> B[SyncKit SDK

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

WASM + Native]

    C -->|80% of use cases| C1[LWW Sync]
    C -->|Collaborative editing| C2[Text CRDTs]
    C -->|Advanced features| C3[Custom CRDTs

counters, sets]

    C --> D[IndexedDB Storage

Your local source of truth]

    D -.->|Optional| E[SyncKit Server

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 - Production-grade 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:

(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 (production-ready)
• Python Server - FastAPI implementation (production-ready, v0.3.0)
• Go Server - High-performance goroutine-based server (production-ready, v0.3.0)
• C# Server - ASP.NET Core implementation (production-ready, community-contributed)

🚦 Status

Current Version: v0.3.0

Production Ready