juice_sync

An offline outbox / mutation queue as a Juice bloc — durably persist writes, then flush them to a backend when online, with ordering, retries, and dead-lettering.

What it owns

The durable queue of pending writes and their delivery. It does not own the transport (the MutationExecutor seam), offline reads / caching (that's juice_network), or your optimistic local read model (your repository layer).

Install

dependencies:
  juice_sync: ^0.1.0
  juice_storage: ^1.2.0   # for the durable StorageSyncStore

Use

final sync = SyncBloc.withConfig(SyncConfig(
  store: StorageSyncStore(storageBloc),        // durable (survives app kill)
  executor: (m) => api.replay(m),              // your transport
  onlineSignal: connectivityOnlineStream,      // adapt ConnectivityBloc.isOnline
));

// Returns once DURABLY queued; throws if it couldn't be persisted.
await sync.enqueue('createTodo', {'title': 'Buy milk'});

When offline, mutations queue durably; the queue auto-flushes on the next false→true online edge.

The executor seam (your backend)

juice_sync is transport-free — you wire ~10 lines:

Future<void> myExecutor(Mutation m) async {
  try {
    await dio.request(
      '/api/${m.type}',
      data: m.payload,
      options: Options(method: 'POST', headers: {'Idempotency-Key': m.id}),
    );
  } on DioException catch (e) {
    final code = e.response?.statusCode ?? 0;
    if (code >= 400 && code < 500 && code != 429) {
      throw PermanentSyncError('rejected: $code');   // → dead-letter
    }
    rethrow;                                          // → retry with backoff
  }
}

Delivery is at-least-once. The adapter must send m.id as an idempotency key and the server must dedupe on it — a crash between a successful send and the durable delete causes a replay.

Ordering

Strict FIFO per orderingKey; independent keys (or null) don't block each other:

await sync.enqueue('updateDoc', {...}, orderingKey: 'doc:42'); // ordered with…
await sync.enqueue('updateDoc', {...}, orderingKey: 'doc:42'); // …this one
await sync.enqueue('ping', {});                                // independent

A transient failure holds back only its own partition (bounded by maxAttempts); a different doc keeps flowing.

Retries & dead-letter

Retryable failures back off exponentially up to maxAttempts (default 8), then move to the dead-letter set (state.failed). retryFailed(id?) revives them; discard(id) drops one for good. A poison mutation can never wedge the queue.

Crash-safety

inFlight is persisted before the send and the durable delete completes before the queue head advances. On the next launch the queue is reloaded (seq-ordered) and any recovered inFlight is re-sent. If the store can't be read, the bloc enters an error status — it never silently starts with an empty queue.

State

Field / getter	Meaning
pending	queued + retrying (seq order)
failed	dead-letter
status	loading / idle / syncing / error
online	last onlineSignal value
processedCount	sent this session
pendingCount / failedCount / isSyncing / hasFailures	derived

Rebuild groups: sync:status, sync:queue, sync:failed, sync:mutation:<id>. (Per-mutation groups target discrete events cleanly; within a single flush burst the touched mutations' rebuilds coalesce.)

License

MIT License — see LICENSE.