Sembast Web Setup

sembast_web: sembast on IndexedDB

package:sembast_web provides databaseFactoryWeb, a DatabaseFactory storing each sembast database in one IndexedDB database. It works with dart:js_interop (JavaScript and WASM builds). The data API is the regular package:sembast API; only the factory changes.

import 'package:sembast_web/sembast_web.dart'; // re-exports sembast.dart

final store = intMapStoreFactory.store('product');

Future<void> demo() async {
  // The path is an IndexedDB database name, not a file path.
  var db = await databaseFactoryWeb.openDatabase('my_app.db');
  var key = await store.add(db, {'name': 'Table', 'price': 15});
  print(await store.record(key).get(db));
  await db.close();
}

Guidelines

Dependencies and imports

• Add sembast_web next to sembast in pubspec.yaml. Import package:sembast_web/sembast_web.dart; it re-exports package:sembast/sembast.dart so StoreRef, Finder, etc. are available.
• Exported factories: databaseFactoryWeb (page/main thread) and databaseFactoryWebWorker (inside a web worker). sembast_web_html.dart only holds a deprecated databaseFactoryWeb alias; do not use it.
• The import compiles on the VM and in Flutter mobile/desktop builds, but the getters throw UnimplementedError there. Select the factory with a conditional import when the app also targets io (example below), or use tekartik_app_flutter_sembast's getDatabaseFactory() which does it.

Database name and location

• openDatabase(name) takes an IndexedDB name scoped to the origin (scheme + host + port). localhost:8080 and localhost:8081 are different origins: use a fixed --web-port while debugging or the data "disappears".
• Directory-like names ('data/my.db') are allowed; they are just names. factory.sandbox(path: 'prefix') from sembast prefixes them. There is no file, no path_provider, no directory creation.
• deleteDatabase(name) and databaseExists(name) work. Close the database before deleting it.
• Storage is per browser profile and subject to the browser's IndexedDB quota and eviction; treat it as a cache unless persistence is granted. Keep it small (guideline: below ~30K records).

Cross-tab behavior

• Every tab or worker that opens the same database keeps its own in-memory copy. Writes go to IndexedDB in a journal with a revision number; other tabs are notified through a BroadcastChannel and load only the new records, so onSnapshot/onSnapshots listeners fire in every tab.
• Transactions are cross-tab safe: if another tab committed since the transaction started, the local data is refreshed and the transaction callback runs again. Transaction bodies must therefore be idempotent and free of side effects (no UI updates, no network calls, no incrementing outside variables inside the callback). Compute results from the data read inside the callback.
• await db.checkForChanges() forces an immediate refresh from IndexedDB (normally not needed; notifications handle it).
• db.compact() purges the journal history.

Data and API limitations

• Keys: int or String only, as in sembast. Auto-increment int keys and generated string keys work.
• Values: the sembast types (String, num, bool, null, Map<String, Object?>, List<Object?>, Timestamp, Blob). DateTime and Uint8List are still rejected: use Timestamp and Blob.
• When compiled to JavaScript (not WASM) an int is a JS number: values and keys above 2^53 lose precision. Store 64-bit ids as String.
• Timestamp round-trips at microsecond precision on the VM but the platform DateTime only has millisecond precision on JavaScript; do not rely on sub-millisecond values in web code.
• codec: is accepted (sync codecs and AsyncContentCodecBase) and the signature is checked, but the README states codecs are not supported on the web and browser-side encryption gives no real protection: keep secrets off the client or encrypt individual fields yourself.
• Everything else (queries, listeners, triggers, import/export, onVersionChanged migrations) behaves as in sembast.

Testing

• Unit tests that must run on the VM: use newDatabaseFactoryMemory() from package:sembast/sembast_memory.dart, or databaseFactoryMemoryJdb from package:sembast/utils/jdb.dart to reproduce the journal semantics (re-run transactions) without a browser.
• Browser tests: dart test -p chrome with databaseFactoryWeb; delete the database at the start of each test.

Examples

Factory selection with a conditional import

lib/db/db_factory.dart:

import 'package:sembast/sembast.dart';

import 'db_factory_stub.dart'
    if (dart.library.js_interop) 'db_factory_web.dart'
    if (dart.library.io) 'db_factory_io.dart' as impl;

/// The database factory for the current platform.
DatabaseFactory get appDatabaseFactory => impl.appDatabaseFactory;

/// Resolves the database path or name for the current platform.
Future<String> appDatabasePath(String name) => impl.appDatabasePath(name);

lib/db/db_factory_web.dart:

import 'package:sembast_web/sembast_web.dart';

DatabaseFactory get appDatabaseFactory => databaseFactoryWeb;

// IndexedDB name: nothing to resolve.
Future<String> appDatabasePath(String name) async => name;

lib/db/db_factory_io.dart (Flutter, with path_provider):

import 'package:path/path.dart';
import 'package:path_provider/path_provider.dart';
import 'package:sembast/sembast_io.dart';

DatabaseFactory get appDatabaseFactory => databaseFactoryIo;

Future<String> appDatabasePath(String name) async {
  var dir = await getApplicationDocumentsDirectory();
  await dir.create(recursive: true);
  return join(dir.path, name);
}

lib/db/db_factory_stub.dart:

import 'package:sembast/sembast.dart';

DatabaseFactory get appDatabaseFactory =>
    throw UnsupportedError('No sembast factory for this platform');

Future<String> appDatabasePath(String name) =>
    throw UnsupportedError('No sembast factory for this platform');

Usage, identical on every platform:

import 'package:sembast/sembast.dart';

import 'db/db_factory.dart';

Future<Database> openAppDatabase() async {
  var path = await appDatabasePath('my_app.db');
  return appDatabaseFactory.openDatabase(path, version: 1);
}

Idempotent transaction (safe when re-run after a concurrent write)

import 'package:sembast_web/sembast_web.dart';

final counters = StoreRef<String, int>('counters');

/// Increments atomically even with several tabs; the body may run twice.
Future<int> nextSequence(Database db, String name) {
  return db.transaction((txn) async {
    var record = counters.record(name);
    var value = (await record.get(txn) ?? 0) + 1;
    await record.put(txn, value);
    return value; // Derived from data read inside the transaction only.
  });
}

Listening across tabs

import 'dart:async';

import 'package:sembast_web/sembast_web.dart';

final notes = stringMapStoreFactory.store('note');

StreamSubscription<List<RecordSnapshot<String, Map<String, Object?>>>>
    watchNotes(Database db) {
  // Also fires when another tab adds, updates or deletes a note.
  return notes
      .query(finder: Finder(sortOrders: [SortOrder('updatedAt', false)]))
      .onSnapshots(db)
      .listen((snapshots) => print('${snapshots.length} notes'));
}

VM test reproducing journal (re-run) semantics

import 'package:sembast/sembast.dart';
import 'package:sembast/utils/jdb.dart';
import 'package:test/test.dart';

void main() {
  test('idempotent transaction', () async {
    var factory = databaseFactoryMemoryJdb;
    await factory.deleteDatabase('test');
    var db = await factory.openDatabase('test');
    var record = StoreRef<String, int>.main().record('count');
    await db.transaction((txn) async {
      await record.put(txn, (await record.get(txn) ?? 0) + 1);
    });
    expect(await record.get(db), 1);
    await db.close();
  });
}

Common mistakes

• Using databaseFactoryIo in a web build (UnimplementedError) or databaseFactoryWeb in a mobile build; use the conditional import.
• Treating the database name as a file path (getApplicationDocumentsDirectory does not exist on the web).
• Running non-idempotent code inside db.transaction (network calls, counters in closures): it can execute twice on the web.
• Debugging Flutter web on a random port and losing the IndexedDB data.
• Storing 64-bit integers or DateTime values; use String ids and Timestamp.
• Relying on browser storage as the only copy of important data.