Document in-memory pool

client-sdks/reference/javascript-web.mdx

@@ -488,6 +488,40 @@ export const db = new PowerSyncDatabase({
 });
 ```
 
+**Multi-Threaded In-Memory VFS**
+
+Since version 2.2.0 of the `@powersync/web` package, a per-tab in-memory file system can be configured to run with multiple worker threads.
+This VFS uses a design similar to `OPFSWriteAheadVFS`, allowing concurrent reads and a writer to operate on the database in parallel.
+Instead of writing files to OPFS, it uses `SharedArrayBuffer` to coordinate an in-memory database across workers.
+
+<Note>This setup is experimental and might change in the future.</Note>
+
+This configuration does not share data across tabs. Each tab gets a unique database and sync client, and databases opened with this
+approach cannot be named. Thus, this VFS is primarily relevant when all of the following apply:
+
+1. You need highly concurrent, high-performance queries in your app.
+2. At the same time, the overall database size (or at least the actively synced part of the database) is relatively small, as it gets
+   synced every time a tab is opened.
+3. You don't need persistence.
+4. You can enable [cross-origin isolation](https://web.dev/articles/cross-origin-isolation-guide) by using the appropriate headers.
+   Without cross-origin isolation and shared array buffers, constructing the pool will throw.
+5. You don't need multiple tabs to share offline state.
+
+We don't expect this setup to be relevant for most users. Hence, the VFS is not bundled with the web SDK to avoid any impact on bundle size.
+To use it, import `@powersync/web/in-memory-wal-experiment`:
+
+```js
+import { InMemoryWriteAheadLogPool } from '@powersync/web/in-memory-wal-experiment';
+import { PowerSyncDatabase } from '@powersync/web';
+
+export const db = new PowerSyncDatabase({
+  schema: AppSchema,
+  opened: new InMemoryWriteAheadLogPool({
+    numWorkers: 3 // Uses one writer, two additional workers for reads.
+  }),
+});
+```
+
 #### VFS Compatibility Matrix
 
 | VFS Type            | Multi-Tab (Standard) | Multi-Tab (Safari/iOS) | Concurrent Reads | Best For                                       |