A simple read-optimized in-memory key-value store w/persistent file-based redo log.

kv.dart

/// A simple read-optimized in-memory key-value store w/persistent file-based redo log.
class KV {
  KV._(this._file, this._data);
  final File _file;
  final Map<String, dynamic> _data;

  static Future<KV> open(File file) async {
    final data = <String, dynamic>{};
    if (!file.existsSync()) return KV._(file, data);
    await for (final line in file //
        .openRead()
        .transform(utf8.decoder)
        .transform(const LineSplitter())
        .where((line) => line.isNotEmpty)
        .map(json.decode)) {
      if (line case [String key, String value]) {
        data[key] = value;
      } else if (line case [String key]) {
        data.remove(key);
      }
    }
    return KV._(file, data);
  }

  Future<Object?> get(String key) async => _data[key];

  Future<Object?> set(String key, Object? value) async {
    final old = _data[key];
    if (old == value) return value;
    if (value != null) {
      await _append([key, value]);
      _data[key] = value;
    } else {
      await _append([key]);
      _data.remove(key);
    }
    return old;
  }

  Stream<(String, Object)> list(String prefix) {
    return Stream.fromIterable(_data.entries.where((e) => e.key.startsWith(prefix)).map((e) => (e.key, e.value)));
  }

  Future<void> snapshot() async {
    final file = File('${_file.path},$hashCode');
    await file.writeAsString(_data.entries.map((e) => '${json.encode([e.key, e.value])}\n').join());
    await file.rename(_file.path);
  }

  Future<void> _append(List<Object> values) async {
    final sink = _file.openWrite(mode: FileMode.append);
    sink.writeln(json.encode(values));
    await sink.flush();
    await sink.close();
  }
}