SyncKit

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🎯 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):

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
}

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✨ 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)

🛡️ Data Integrity Guaranteed

• Zero data loss at the system level: every operation is persisted and delivered
• 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
• Formally modeled with TLA+; the specs are in the repo
• 2,100+ tests across TypeScript, Rust, Python, Go, and C# (unit, integration, chaos, load)

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🚀 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) →

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🎓 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

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🏗️ 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 →

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📚 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 →

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🎯 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

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🌐 How SyncKit Fits the Ecosystem

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

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 →

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📦 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)

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🚦 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 →

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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 →

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🏢 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: danbitengo@gmail.com

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📊 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 →

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🙏 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.

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📄 License

MIT License - see LICENSE for details.

Copyright (c) 2025 Daniel Bitengo

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🔗 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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