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Add OPFSWriteAheadVFS documentation #400

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32 changes: 27 additions & 5 deletions client-sdks/reference/javascript-web.mdx
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Expand Up @@ -424,13 +424,35 @@ export const db = new PowerSyncDatabase({
- The system is not designed to handle multiple tab scenarios.
- The configuration is similar to `OPFSCoopSyncVFS`, but requires using `WASQLiteVFS.AccessHandlePoolVFS`.

##### OPFSWriteAheadVFS

- Uses OPFS with a write-ahead log to support **concurrent reads** alongside a single writer.
- This is the only VFS that supports opening multiple SQLite connections to the same database, allowing read queries to run in parallel.
- Requires web workers (the `useWebWorker` flag must not be set to `false`).
- The `additionalReaders` option controls how many extra read-only connections to open (defaults to `1`). Including the writer connection, `additionalReaders + 1` reads can run concurrently.
- Example configuration:

```js
import { PowerSyncDatabase, WASQLiteOpenFactory, WASQLiteVFS } from '@powersync/web';

export const db = new PowerSyncDatabase({
schema: AppSchema,
database: new WASQLiteOpenFactory({
dbFilename: 'exampleVFS.db',
vfs: WASQLiteVFS.OPFSWriteAheadVFS,
additionalReaders: 2
})
});
```

#### VFS Compatibility Matrix

| VFS Type | Multi-Tab Support (Standard Browsers) | Multi-Tab Support (Safari/iOS) | Notes |
| ------------------- | ------------------------------------- | ------------------------------ | ------------------------------------- |
| IDBBatchAtomicVFS | ✅ | ❌ | Default, some Safari stability issues |
| OPFSCoopSyncVFS | ✅ | ✅ | Recommended for multi-tab support |
| AccessHandlePoolVFS | ❌ | ❌ | Best for single-tab applications |
| VFS Type | Multi-Tab Support (Standard Browsers) | Multi-Tab Support (Safari/iOS) | Concurrent Reads | Notes |
| ------------------- | ------------------------------------- | ------------------------------ | ---------------- | ---------------------------------------- |
| IDBBatchAtomicVFS | ✅ | ❌ | ❌ | Default, some Safari stability issues |
| OPFSCoopSyncVFS | ✅ | ✅ | ❌ | Recommended for multi-tab support |
| AccessHandlePoolVFS | ❌ | ❌ | ❌ | Best for single-tab applications |
| OPFSWriteAheadVFS | ✅ | ❌ | ✅ | Best for read-heavy workloads |

**Note**: There are known issues with OPFS when using Safari's incognito mode.

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