Local First

---
name: local-first
description: "Use when Local-first software architecture mastery. CRDTs (Conflict-free Replicated Data Types), IndexedDB synchronization, sync engines (ElectricSQL, Replicache, PowerSync), offline-capable data fetching, optimistic UI, and SQLite in the browser (WASM). Use when building PWA offline capabilities, rapid UIs, or multiplayer collaborative tools."
version: 5.0.0
last-updated: 2026-09-13
skills:
  - local-first-architecture
  - react-specialist
  - baseline-ui
tools: Read, Grep, Glob, Bash, Edit, Write
scripts-binding:
  - .agent/scripts/lint_runner.js
  - .agent/scripts/verify_all.js
---

# Local-First Architecture — Offline-capable Mastery

---

## 🛠️ Technical Architecture & Reference Recipes

---

## Hallucination Traps (Read First)

- ❌ Assuming server data is always newer than local data -> ✅ Conflicts are inevitable; design merge strategy (LWW, CRDTs) before writing code
- ❌ Using localStorage for anything beyond 5MB -> ✅ Use IndexedDB (via Dexie or idb) for structured local storage; localStorage is synchronous and tiny
- ❌ Syncing entire datasets on every connection -> ✅ Use incremental sync with watermarks/timestamps to minimize bandwidth

---

---

## 1. Core Principles of Local-First

In a Cloud-First app (REST/GraphQL), the UI waits for the server.
In a Local-First app, the UI talks _only_ to a local database. A background engine syncs that database to the cloud when online.

1. **Fast by default**: Zero network latency because reads/writes happen locally.
2. **Offline works flawlessly**: The app bounds to a local store (SQLite via WASM, IndexedDB).
3. **Multi-device Sync**: Conflict resolution is handled natively (usually via CRDTs or central conflict ledgers).

---

## 2. Sync Engines vs traditional fetching

Do not use React Query / SWR to build local-first. They are HTTP caching mechanisms.

```typescript
// ❌ CLOUD-FIRST (React Query / Fetch)
// Fails when offline. Subject to UI latency.
const { data, isLoading } = useQuery({
  queryKey: ['todos'],
  queryFn: () => fetch('/api/todos').then(res => res.json()),
});

// ✅ LOCAL-FIRST (e.g. PowerSync / ElectricSQL / WatermelonDB)
// Resolves instantly. Data lives locally. Syncs silently in background.
import { useQuery } from '@powersync/react';

const { data, isLoading } = useQuery('SELECT * FROM todos ORDER BY created_at DESC');

// Writes are also local First
const addTodo = async (text: string) => {
  // Written to the local SQLite WASM database instantly.
  await localDb.execute('INSERT INTO todos (id, text) VALUES (uuid(), ?)', [text]);
  // Background worker syncs to Postgres later.
};
```

---

## 3. Conflict Resolution (CRDTs)

When two users edit the exact same document offline and then reconnect, how is it resolved?

**Conflict-free Replicated Data Types (CRDTs)** automatically merge data without requiring a central server to decide the "winner."

```typescript
// Yjs - The leading CRDT library for collaborative text/state
import * as Y from 'yjs';
import { WebsocketProvider } from 'y-websocket';

const ydoc = new Y.Doc();
const provider = new WebsocketProvider('wss://sync.example.com', 'room-1', ydoc);

// Shared state array
const yarray = ydoc.getArray('todos');

// Observe changes (Fires locally and when peers sync)
yarray.observe(event => {
  console.log('State updated natively without conflict:', yarray.toArray());
});

// Insert data (Instantly merges cleanly with remote peers)
yarray.insert(0, ['Buy milk']);
```

---

## 4. In-Browser Databases

Storing megabytes of relational data in `localStorage` will crash the browser.

| Technology             | Use Case      | Pros                                | Cons                                          |
| :--------------------- | :------------ | :---------------------------------- | :-------------------------------------------- |
| **IndexedDB**          | Key-Value     | Native to browser                   | Hideous callback API, weak querying           |
| **Dexie.js**           | Key-Value     | Wraps IndexedDB with clean Promises | Not relational                                |
| **SQLite WASM (OPFS)** | Relational    | True SQL in browser                 | Setup complexity (Origin Private File System) |
| **RxDB**               | NoSQL Offline | Reactive UI out-of-the-box          | Requires learning RxJS/Observables            |
| **WatermelonDB**       | Relational    | Built for React Native & Web        | Requires native module setup on mobile        |

```typescript
// Clean IndexedDB Wrapper Example (Dexie)
import Dexie, { type EntityTable } from 'dexie';

interface Friend {
  id: number;
  name: string;
  age: number;
}

const db = new Dexie('FriendsDatabase') as Dexie & {
  friends: EntityTable<Friend, 'id'>;
};

// Schema configuration
db.version(1).stores({
  friends: '++id, name, age', // Primary key and indexed props
});

// Write to DB instantly
await db.friends.add({ name: 'Alice', age: 25 });

// Live query natively drives React state without network calls
import { useLiveQuery } from 'dexie-react-hooks';
const friends = useLiveQuery(() => db.friends.where('age').above(21).toArray());
```