sync_engine

Pure-Dart, immutable CRDT primitives for flutter_sync_engine. It can be tested with dart test and deliberately has no Flutter dependency.

Getting started

Create a Flutter application and add the core package, generator, and build runner:

flutter create my_app
cd my_app
flutter pub add sync_engine
flutter pub add --dev build_runner sync_engine_generator

Define a model in lib/todo.dart. The part directive is required so the generated adapter is included in the library:

import 'package:sync_engine/sync_engine.dart';

part 'todo.sync.dart';

@Syncable()
class Todo {
	const Todo({required this.id, required this.title});

	@Id()
	final String id;

	@ConflictStrategy(ConflictType.lastWriteWins)
	final String title;
}

Create build.yaml at the application root:

targets:
	$default:
		builders:
			sync_engine_generator|syncable:
				generate_for:
					- lib/**.dart

Generate the adapter:

dart run build_runner build --delete-conflicting-outputs

The generated TodoSyncAdapter is then registered with SyncEngine:

final sync = SyncEngine(
	storage: storage,
	transport: transport,
	nodeId: 'device-a',
	adapters: {Todo: const TodoSyncAdapter()},
);

storage must implement SyncStorage, and transport must implement SyncTransport. For durable Flutter persistence, use DriftSyncStorage from sync_engine_drift. Its README contains a complete database and storage setup.

LWW conflict resolution

LWWRegister uses vector clocks for causal order. If two writes are concurrent, it selects the value from the lexicographically greater node ID. That rule is deterministic but arbitrary; applications needing domain-specific resolution should provide it above this primitive.

SyncEngine serializes sync cycles, validates transport acknowledgements, and keeps the outbox cursor from advancing when a push fails. Durable persistence is provided by sync_engine_drift; the core package ships the outbox contract and in-memory implementation.