utils/crdt_sync_dart/README.md

crdt_sync

Shared CRDT merge scheme + GitHub-Contents-API sync transport, extracted for reuse across four personal apps that each need cross-device sync: todo, diet_guard's Flutter app, wake_alarm's phone_app, and screen-locker's workout_app. Pure Dart -- no Flutter SDK dependency (package:http only) -- so it works in both Flutter apps and plain Dart tooling. Mirrors the Python crdt-sync package module-for-module and test-for-test, so the merge algorithm canonically agrees on both sides of any sync between a Dart client and a Python one.

The merge scheme

See the Python package's README for the full rationale; in short, a generic LWW-map-with-sticky-remove:

• Hlc -- a Hybrid Logical Clock (wallTimeMs, counter, nodeId), totally ordered, monotonic even across clock skew.
• Record -- an id, a per-field last-writer-wins map (fieldName -> (value, Hlc)), and a sticky deleted flag (deleted = a.deleted || b.deleted, never the other way -- a delete can never be silently undone).
• Log -- Map<String, Record>, merged by taking the union of ids and merging shared ones.

All three merges are commutative and idempotent -- see test/log_test.dart for the concrete convergence-property tests.

Install

Consume via a git dependency on the utils monorepo:

dependencies:
  crdt_sync:
    git:
      url: https://github.com/kuhyx/utils
      ref: crdt_sync_dart-v0.2.0
      path: crdt_sync_dart

Usage

import 'package:crdt_sync/crdt_sync.dart';

final nodeId = 'phone';
final clock = Hlc.newTick(nodeId);
final record = Record(
  id: 'abc123',
  fields: {'text': ('buy milk', clock)},
);

final client = GitHubClient(owner: 'kuhyx', repo: 'my-app-sync', token: token);
final merged = await syncLog(
  client: client,
  deviceId: nodeId,
  pathPrefix: 'devices',
  localLog: {record.id: record},
  encode: (log) => jsonEncode(log.map((k, r) => MapEntry(k, r.toJson()))),
  decode: (text) => (jsonDecode(text) as Map<String, dynamic>).map(
    (k, v) => MapEntry(k, Record.fromJson(v as Map<String, dynamic>)),
  ),
);

syncLog pulls every other device's last-pushed log from <pathPrefix>/<other-device-id>/..., merges each into the local log with mergeLogs, then pushes this device's own merged result back up.

Local persistence (LogStore)

The merge scheme above is in-memory only. For an app that needs to keep its Log on disk and drive a live UI off it, this package also ships a domain-agnostic LogStore:

import 'package:crdt_sync/crdt_sync.dart';
import 'package:crdt_sync/crdt_sync_io.dart'; // FileLogPersistence (dart:io)

final store = LogStore(
  persistence: FileLogPersistence(File('$docsDir/notes.json')),
  nodeId: 'phone',
);
await store.load();

// Reactive read: re-derive a domain view whenever anything changes. Filtering,
// sorting and search live here, in the app -- the store is field-agnostic.
final Stream<List<Record>> visible = store.changes.map(
  (_) => store.values.where((r) => !r.deleted).toList(),
);

await store.upsert(Record(id: 'n1', fields: {'text': ('hi', store.nextHlc())}));
await store.delete('n1'); // sticky tombstone, not a hard remove

// After a sync tick, swap in the merged result:
await store.replaceAll(await syncLog(/* ... */, localLog: store.snapshot()));

LogStore is pure Dart (import it from the main barrel); the filesystem-backed FileLogPersistence lives behind the separate crdt_sync_io.dart entrypoint so the core stays web-safe. Querying is deliberately not in the library — a Record's fields are an opaque map, so the app filters store.values itself. The Python side mirrors the on-disk format via crdt_sync.dump_log/read_log (load/save only; no reactive stream — that's a Dart/Flutter convenience).

Development

dart pub get
dart analyze
dart test