Sembast Database Setup

sembast: opening, migrating and managing a database

A sembast database is opened through a DatabaseFactory; the factory decides where and how data is stored. The whole database is loaded in memory on open, so open it once at startup and keep a single Database instance for the application lifetime.

import 'package:sembast/sembast_io.dart';

Future<Database> openAppDatabase(String dir) async {
  // sembast_io.dart re-exports sembast.dart and provides databaseFactoryIo.
  return databaseFactoryIo.openDatabase('$dir/app.db', version: 1);
}

Guidelines

Choosing the factory (platform)

• Dart VM and Flutter mobile/desktop: databaseFactoryIo from package:sembast/sembast_io.dart. One JSON-lines file per database, pure Dart, no plugin. Not cross-process and not cross-isolate safe: use it from the main isolate only.
• Web and Flutter web: databaseFactoryWeb from package:sembast_web (IndexedDB, cross-tab safe). databaseFactoryIo compiles on the web but throws UnimplementedError at runtime. Select the factory with a conditional import (sembast-web-setup skill).
• Cross-process safety on VM/Flutter (several processes on the same file): sembast_sqflite (getDatabaseFactorySqflite). The Flutter package tekartik_app_flutter_sembast bundles a getDatabaseFactory() that picks the right one per platform.
• Tests and mocks: newDatabaseFactoryMemory() (a fresh isolated factory) or openNewInMemoryDatabase() (an always-empty database) from package:sembast/sembast_memory.dart. databaseFactoryMemory is a shared global memory factory: databases with the same path are the same database until closed. databaseFactoryMemoryFs simulates the io file format in memory (useful to test import/export or compact).
• createDatabaseFactoryIo(rootPath: dir) makes relative paths resolve under dir. anyFactory.sandbox(path: dir) does the same for any factory (io, memory, web) and rejects paths escaping the sandbox. factory.pathContext gives the path.Context used and factory.hasStorage is false for memory.

Opening

• factory.openDatabase(path, {version, onVersionChanged, mode, codec}). On io path is a file path (create the parent directory first, see the Flutter example). Opening an already open database returns the same instance.
• mode defaults to DatabaseMode.neverFails (create if missing, delete and recreate when corrupted). Others: create (create if missing, fail on corruption), existing (fail when missing), empty (wipe content), readOnly (fail when missing, no writes).
• db.version, db.path, await db.close(). Close before deleting: factory.deleteDatabase(path); factory.databaseExists(path) checks.
• In Flutter, build the path with path_provider (getApplicationDocumentsDirectory()) and package:path join. Never hard-code a directory. Open in main() before runApp, or lazily behind a single cached Future<Database>.
• On Flutter, wrap the open in disableSembastCooperator() / enableSembastCooperator() if the splash screen should not yield, and call disableSembastCooperator() once in flutter_test files (the cooperator's Future.delayed can hang there).
• Size guidance: io below ~100K records / 100 MB, web below ~30K records. Open time grows with record count.

Versioning and migrations

• A database created without version has version 1. Pass a constant version from day one: on creation onVersionChanged(db, 0, version) is called, on later upgrades onVersionChanged(db, oldVersion, newVersion). It is not called when the versions match.
• Inside onVersionChanged use the db argument freely (store.add(db, ...), db.transaction(...)): those calls join the transaction opened for the migration, and the new version is stored only when the callback succeeds.
• Write migrations as if (oldVersion < N) steps in increasing order so a fresh install (oldVersion 0) runs every step. Seed data (initial records) belongs in the oldVersion == 0 branch.
• Sembast is schema-less: renaming a field means reading, transforming and putting records back; changing key type means copying into a new store and oldStore.drop(db).
• Lowering version is not an error; onVersionChanged is called with oldVersion > newVersion.

Codec and encryption

• SembastCodec(signature: 'name', codec: myCodec) where myCodec is a Codec<Object?, String> turning a JSON-encodable value into a single-line string. Pass it to every openDatabase of that database; opening with a different codec throws DatabaseException with code DatabaseException.errInvalidCodec. The signature is stored (encoded by the codec itself) so a wrong password is detected at open.
• Sembast ships no cipher. Bring your own (for example on top of pointycastle); the repository's sembast_test/lib/encrypt_codec.dart is a demonstration only, not production grade.
• A codec calling a plugin or async API: extend AsyncContentCodecBase, implement decodeAsync/encodeAsync and pass it as codec.
• The codec cannot be changed later. To encrypt an existing database: exportDatabase it, importDatabase(export, factory, newPath, codec: codec), then delete the old one (or databaseMerge between two open databases).
• Custom value types are handled by JsonEncodableCodec type adapters (package:sembast/utils/type_adapter.dart); sembastCodecDefault already handles Timestamp and Blob. Do not write adapters for models: store maps.

Export, import, maintenance

• package:sembast/utils/sembast_import_export.dart: exportDatabase(db, {storeNames}) returns a JSON-encodable map, importDatabase(map, factory, path, {codec, storeNames}) creates (after deleting) a database from it and returns it open. exportDatabaseLines / importDatabaseLines / exportLinesToJsonlString are a line-based format friendlier to git; importDatabaseAny accepts either format, a JSON string or a list of strings. package:sembast/utils/import_export_io.dart adds exportDatabaseToJsonlFile and importDatabaseFromFile on the VM.
• package:sembast/utils/database_utils.dart: getNonEmptyStoreNames(db) (there is no other way to list stores) and databaseMerge(db, sourceDatabase: other, storeNames: [...]) which makes db content equal to other.
• db.compact() rewrites the io file dropping obsolete lines (automatic when 20% of lines are obsolete). db.checkForChanges() reloads external changes on web/sqflite. db.reload() / db.reOpen() reread the io file (unsafe if writes are in progress).
• DatabaseException.code values: errBadParam, errDatabaseNotFound, errInvalidCodec, errDatabaseClosed (using a closed database).

Examples

Flutter: single global database opened at startup

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

Database? _db;

Future<Database> getDatabase() async {
  if (_db != null) return _db!;
  var dir = await getApplicationDocumentsDirectory();
  await dir.create(recursive: true);
  var dbPath = join(dir.path, 'my_app.db');
  _db = await databaseFactoryIo.openDatabase(dbPath, version: 1);
  return _db!;
}

Dart VM script with a sandboxed factory

import 'package:sembast/sembast_io.dart';

Future<void> main() async {
  // All relative paths live under .local/db
  var factory = databaseFactoryIo.sandbox(path: '.local/db');
  var db = await factory.openDatabase('cache.db');
  var store = StoreRef<String, int>.main();
  await store.record('runs').put(db, (await store.record('runs').get(db) ?? 0) + 1);
  print(await store.record('runs').get(db));
  await db.close();
}

Versioned migrations with seed data

import 'package:sembast/sembast.dart';

const kDbVersion = 3;
final productStore = stringMapStoreFactory.store('product');

Future<Database> openWithMigrations(DatabaseFactory factory, String path) {
  return factory.openDatabase(
    path,
    version: kDbVersion,
    onVersionChanged: (db, oldVersion, newVersion) async {
      // oldVersion is 0 on creation: every step below runs.
      if (oldVersion < 1) {
        await productStore.record('demo_1').put(db, {'name': 'Demo 1', 'price': 10});
      }
      if (oldVersion < 2) {
        // Price change shipped in app version 2.
        await productStore.record('demo_1').update(db, {'price': 15});
      }
      if (oldVersion < 3) {
        // Tag every product whose name contains 'demo'.
        await productStore.update(
          db,
          {'tag': 'demo'},
          finder: Finder(
            filter: Filter.custom(
              (record) => (record['name'] as String).toLowerCase().contains('demo'),
            ),
          ),
        );
      }
    },
  );
}

Migrate int keys to String keys in a new store

import 'package:sembast/sembast.dart';

final fruitStoreV1 = intMapStoreFactory.store('fruit');
final fruitStore = stringMapStoreFactory.store('fruit_v2');

Future<Database> openFruits(DatabaseFactory factory, String path) {
  return factory.openDatabase(
    path,
    version: 2,
    onVersionChanged: (db, oldVersion, newVersion) async {
      if (oldVersion == 1) {
        await db.transaction((txn) async {
          var values = (await fruitStoreV1.find(txn)).values.toList();
          await fruitStore.addAll(txn, values);
          await fruitStoreV1.drop(txn);
        });
      }
    },
  );
}

Unit test with an in-memory factory

import 'package:sembast/sembast_memory.dart';
import 'package:test/test.dart';

void main() {
  test('put/get', () async {
    var factory = newDatabaseFactoryMemory();
    var db = await factory.openDatabase('test.db');
    var record = StoreRef<String, String>.main().record('key');
    await record.put(db, 'value');
    expect(await record.get(db), 'value');
    await db.close();

    // Or an always-empty database in one call:
    var db2 = await openNewInMemoryDatabase();
    expect(await record.get(db2), isNull);
    await db2.close();
  });
}

Custom (async) codec and wrong-codec detection

import 'dart:convert';

import 'package:sembast/sembast.dart';

/// Demo only: reverses the JSON text. Replace with real encryption.
class ReverseCodec extends AsyncContentCodecBase {
  String _reverse(String text) => text.split('').reversed.join();

  @override
  Future<Object?> decodeAsync(String encoded) async =>
      jsonDecode(_reverse(encoded));

  @override
  Future<String> encodeAsync(Object? input) async =>
      _reverse(jsonEncode(input));
}

Future<Database> openEncoded(DatabaseFactory factory, String path) async {
  var codec = SembastCodec(signature: 'reverse_v1', codec: ReverseCodec());
  try {
    return await factory.openDatabase(path, codec: codec);
  } on DatabaseException catch (e) {
    if (e.code == DatabaseException.errInvalidCodec) {
      // Existing file written with another codec (or password).
      rethrow;
    }
    rethrow;
  }
}

Convert an existing database to an encrypted one

import 'package:sembast/utils/sembast_import_export.dart';

Future<Database> migrateToCodec(
  DatabaseFactory factory,
  SembastCodec codec,
) async {
  const plainPath = 'my_database.db';
  const encryptedPath = 'my_database_encrypted.db';
  if (await factory.databaseExists(plainPath)) {
    var plainDb = await factory.openDatabase(plainPath);
    var export = await exportDatabase(plainDb);
    await plainDb.close();
    var db = await importDatabase(export, factory, encryptedPath, codec: codec);
    await factory.deleteDatabase(plainPath);
    return db;
  }
  return factory.openDatabase(encryptedPath, codec: codec);
}

Backup and restore as JSON lines (VM)

import 'package:sembast/sembast_io.dart';
import 'package:sembast/utils/import_export_io.dart';

Future<void> backup(Database db) =>
    exportDatabaseToJsonlFile(db, 'backup/app.jsonl');

Future<Database> restore() =>
    importDatabaseFromFile('backup/app.jsonl', databaseFactoryIo, 'restored.db');

Common mistakes

• Opening the database in several places or on every screen. Keep one instance; opening is the expensive operation.
• Using databaseFactoryIo in a Flutter web build (runtime UnimplementedError), or databaseFactoryMemory in production.
• Forgetting dir.create(recursive: true) on Flutter, or using a relative path such as 'app.db' on mobile.
• Opening without version then adding one later: the existing database is already at version 1, so creation logic guarded by oldVersion == 0 never runs for old installs. Handle oldVersion == 1 too.
• Doing migration writes with a different Database object than the db passed to onVersionChanged.
• Changing the codec of an existing database in place. Export and import.
• Deleting the database file directly while it is open; call db.close() then factory.deleteDatabase(path).

More

See references/factories.md for the factory, DatabaseMode, codec and import/export listings.