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Document OPFSWriteAheadVFS for the JavaScript Web SDK #414

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Document OPFSWriteAheadVFS for the JavaScript Web SDK #414

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40 changes: 35 additions & 5 deletions client-sdks/reference/javascript-web.mdx
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Expand Up @@ -424,13 +424,43 @@ 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

<Note>
Available from `@powersync/web` version 1.36.0.
</Note>

- This OPFS-based implementation is the only VFS that supports **concurrent reads** alongside writes by hosting one or more read-only SQLite connections in additional dedicated workers.
- Use this VFS when you have read-heavy workloads and want long-running queries to run in parallel without blocking writes (or each other).
- Configure the number of additional read-only worker connections via the `additionalReaders` option (default: `1`). The writer connection can also serve reads, so `additionalReaders: 1` allows two concurrent readers in practice.
- Like other OPFS-based VFS implementations, it requires a browser that supports OPFS and `FileSystemSyncAccessHandle`, and uses a dedicated web worker (`useWebWorker: true`).
- 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,
// Open one additional read-only connection for concurrent reads.
// The writer connection also serves reads, so this enables two concurrent readers in total.
additionalReaders: 1
})
});
```

The `additionalReaders` option is ignored on other VFS types.

#### 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, single-tab workloads |

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

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