durable_queue #

Persistent, retryable task execution for Dart and Flutter.

The problem #

Apps constantly start work that must not be lost: uploading a photo, sending a message, syncing an edit. The first attempt often fails because the network drops, the server returns 503, or the user closes the app mid-request. A try/catch around the call handles one failure, but then you have to decide:

• where the work lives until it can be retried, and whether it survives a restart;
• how long to wait between attempts, and when to give up;
• how to stop a double tap from uploading the same photo twice;
• how many uploads may run at once, and which ones go first;
• what happens to step 3 when step 2 fails.

durable_queue answers those questions in one small, framework-free engine. Work is described as data (a serializable task), stored through a pluggable storage interface, and executed by a worker that owns retries, concurrency, ordering, and recovery.

Features #

• Tasks are data, so they can be stored and rebuilt after a restart
• Fixed and exponential retry with optional jitter, and a retryIf filter
• Concurrency limits and priorities
• Task dependencies, chains, and groups
• Deduplication and idempotency keys
• Cooperative cancellation, pause, resume, and stop
• Lifecycle events and queries by status or group
• Startup recovery of tasks interrupted by a crash
• An injectable clock, so retry logic is testable without waiting
• Pure Dart: no HTTP client, database, Flutter, or state-management dependency

Install #

dependencies:
  durable_queue: ^0.3.0

Quick start #

import 'package:durable_queue/durable_queue.dart';

final class UploadPhotoTask extends DurableTask {
  UploadPhotoTask({required this.path});

  final String path;

  @override
  String get type => 'upload_photo';

  @override
  Map<String, dynamic> toJson() => {'path': path};

  static UploadPhotoTask fromJson(Map<String, dynamic> json) =>
      UploadPhotoTask(path: json['path'] as String);
}

Future<void> main() async {
  final queue = DurableQueue(
    storage: MemoryQueueStorage(), // use a persistent adapter in an app
    maxConcurrentTasks: 3,
  );

  queue.register<UploadPhotoTask>(
    type: 'upload_photo',
    decoder: UploadPhotoTask.fromJson,
    handler: (task, context) async {
      await photoApi.upload(task.path); // your code
    },
  );

  await queue.start();

  await queue.enqueue(
    UploadPhotoTask(path: '/storage/avatar.jpg'),
    retryPolicy: RetryPolicy.exponential(
      maxAttempts: 5,
      initialDelay: Duration(seconds: 1),
    ),
  );
}

enqueue returns once the task is stored. The worker runs it in the background of your process and retries it if the handler throws.

How it works #

            Your app (Flutter or Dart)
                       │
         enqueue(task, retry, priority, dependsOn, group)
                       │
                       ▼
           ┌───────────────────────┐
           │     QueueStorage      │  memory, file, SQLite, ...
           │ pending · waiting ·   │  (survives restarts with a
           │ retryScheduled · ...  │   persistent adapter)
           └───────────┬───────────┘
                       │  worker claims the next ready task:
                       │  • dependencies completed
                       │  • retry time reached
                       │  • highest priority, then oldest
                       │  • a concurrency slot is free
                       ▼
           ┌───────────────────────┐
           │   your task handler   │
           └───────────┬───────────┘
              returns  │  throws
          ┌────────────┴──────────────┐
          ▼                           ▼
     completed              retryIf + RetryPolicy
          │                  ┌────────┴─────────┐
          │                  ▼                  ▼
          │           retryScheduled         failed
          │          (backoff, then           (or cancelled
          │           claimed again)           if requested)
          │                                     │
          └────────────────┬────────────────────┘
                           ▼
          tasks waiting on it are released, cancelled,
          failed, or run, per their DependencyFailurePolicy

Every state change is written to storage before the matching event is emitted. On the next launch, start recovers tasks that were left running when the process died.

Real-world use cases #

• Media upload. Retry network failures with backoff, fail fast on a rejected file, and send an idempotency key so the server never stores a photo twice. See example/image_upload.dart.
• Offline-first edits. Queue each change while offline, deduplicate repeated saves of the same record, and let the queue drain when connectivity returns.
• Multi-step workflows. Upload, then process, then notify, as a chain where a failed step cancels the rest. See example/orchestration.dart.
• Background sync. Group related sync tasks so they can be listed or cancelled together on logout.
• Priority work. Let a chat message jump ahead of analytics uploads that are already waiting.

Tasks #

A task is a JSON payload plus a type name, like UploadPhotoTask above. The queue stores toJson(), never a closure, and rebuilds the task with the registered decoder. Register each type before enqueuing it; the type passed to register must match the task's type getter.

The handler's context carries the task id, attempt number, attempt budget, idempotency key, and whether cancellation was requested. Handlers that do not need it can ignore it.

Retry #

RetryPolicy.none();

RetryPolicy.fixed(
  maxAttempts: 3,
  delay: Duration(seconds: 2),
);

RetryPolicy.exponential(
  maxAttempts: 5,
  initialDelay: Duration(seconds: 1),
  maxDelay: Duration(minutes: 1),
  jitter: true,
);

maxAttempts includes the first run. retryIf chooses which errors are eligible:

queue.register<UploadPhotoTask>(
  type: 'upload_photo',
  decoder: UploadPhotoTask.fromJson,
  handler: upload,
  retryIf: (error, stackTrace) => error is TemporaryUploadException,
);

The default retries handler errors until the attempt budget is spent. Decode failures are not retried. This package does not inspect HTTP status codes.

Deduplication and idempotency #

await queue.enqueue(
  SyncUserTask(userId: '123'),
  deduplicationKey: 'sync-user:123',
  idempotencyKey: '123',
);

If a task with that deduplication key is already pending, waiting, running, or retryScheduled, enqueue returns the existing id and leaves it unchanged. A completed, failed, or cancelled task does not block the key.

The idempotency key is stored and exposed on TaskContext. The handler decides how to send it. The same key does not, by itself, collapse two tasks.

Priorities #

await queue.enqueue(SendMessageTask(text: 'hi'), priority: 10);
await queue.enqueue(UploadLogsTask(), priority: -5);

Among tasks that are ready to start, higher priority goes first; equal priorities keep enqueue order. The default is 0. Priority decides which task gets the next free slot. It never interrupts a handler that is already running.

Dependencies and chains #

A task can wait for other tasks:

final upload = await queue.enqueue(UploadPhotoTask(path: path));
final resize = await queue.enqueue(
  ResizePhotoTask(path: path),
  dependsOn: [upload],
);

Until every dependency is completed, the dependent task is waiting and does not use a concurrency slot. If a dependency fails, is cancelled, or no longer exists, onDependencyFailure decides what happens:

The outcome cascades: if resize is cancelled, tasks waiting on resize follow their own policy. The task's lastFailure names the dependency that blocked it.

enqueueChain stores steps that run one after another, even when maxConcurrentTasks is greater than one:

final ids = await queue.enqueueChain([
  UploadPhotoTask(path: path),
  ResizePhotoTask(path: path),
  NotifyFriendsTask(path: path),
], group: 'photo:$path');

With the default policy, a failed step cancels the rest of the chain. Dependencies must already be stored when you enqueue, so cycles cannot be created.

Groups #

await queue.enqueue(SyncContactsTask(), group: 'sync');
await queue.enqueue(SyncCalendarTask(), group: 'sync');

final syncTasks = await queue.getTasks(group: 'sync');
final cancelled = await queue.cancelGroup('sync');

A group is a label. cancelGroup cancels every active task in it, using the same rules as cancel, and returns how many tasks it affected.

Cancellation and controls #

await queue.cancel(taskId);
await queue.pause();
await queue.resume();
await queue.stop();

Cancelling queued or waiting work marks it cancelled immediately. Cancelling a running task does not abort the future. The handler can read context.isCancellationRequested and throw TaskCancelledException to stop. If it returns normally, the task is completed.

pause finishes handlers that have already started and does not start new ones. stop waits for those handlers and leaves everything else stored.

Retry, delete, and purge finished tasks #

Completed, failed, and cancelled tasks stay in storage until you remove them.

// Run a failed or cancelled task again with a fresh attempt budget.
await queue.retry(taskId);

// Remove one finished task.
await queue.delete(taskId);

// Remove completed tasks older than a week. Returns the number removed.
final removed = await queue.purge(
  statuses: {TaskStatus.completed},
  olderThan: Duration(days: 7),
);

retry keeps the task id, payload, keys, priority, group, and dependencies, and can take a new retryPolicy. It refuses tasks that are still active, refuses when another active task already holds the same deduplication key, and refuses when a dependency did not complete (retry that dependency first). Tasks cancelled because a dependency failed are not revived automatically. delete and purge only touch terminal tasks, and never remove a task that a waiting task still depends on.

When the queue is no longer needed, close stops it and closes the events and storageErrors streams:

await queue.close();

Observation #

queue.events.listen((event) {
  // TaskEnqueued, TaskStarted, TaskRetryScheduled,
  // TaskCompleted, TaskFailed, TaskCancelled
});

final failed = await queue.getTasks(status: TaskStatus.failed);

The event stream is broadcast and does not replay events that happened before the listener subscribed.

Worker storage errors are reported as values on queue.storageErrors and retried after storageRetryDelay (default one second). A failed result write retries persistence without running the handler again. Storage outages hold serialized operations, so enqueue, queries, cancellation, pause, and stop can wait until storage recovers. Failed startup recovery leaves the queue idle and allows another start() call.

Using it in Flutter #

The core package does not depend on Flutter. A typical app creates one queue at startup, starts it, and listens to its events:

late final DurableQueue queue;

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  final dir = await getApplicationSupportDirectory(); // path_provider
  queue = DurableQueue(storage: JsonFileStorage('${dir.path}/queue.json'));
  queue.register<UploadPhotoTask>(
    type: 'upload_photo',
    decoder: UploadPhotoTask.fromJson,
    handler: (task, context) => api.upload(task.path),
  );
  await queue.start();
  runApp(const MyApp());
}

// Anywhere in the UI:
StreamBuilder<QueueEvent>(
  stream: queue.events,
  builder: (context, snapshot) => Text('${snapshot.data ?? 'idle'}'),
);

JsonFileStorage is the reference adapter from example/json_file_storage.dart; copy it into your app or write your own (see below). The queue runs while your app's process runs. It does not wake a terminated app; call start again on the next launch. A durable_queue_flutter package for app lifecycle integration is planned.

Storage #

The engine talks to storage only through QueueStorage, so you choose where tasks live.

Adapters stay in separate packages so the core never forces a database on you.

To write an adapter, implement QueueStorage and run the shared contract tests against it:

final class MyStorage implements QueueStorage {
  // save, get, update, delete, getNextReady, getWaitingDependents, ...
}

// test/my_storage_test.dart
void main() => queueStorageContractTests(MyStorage.new);

queueStorageContractTests lives in this repository's test/storage_contract.dart; copy it into your adapter's tests. STORAGE.md lists every method, ordering rule, and atomicity requirement.

When to use durable_queue #

Use durable_queue when work must outlive a failed request or an app restart, and the app is allowed to finish it the next time it runs. Use an OS scheduler when the work must run while the app is closed; the two can be combined, with the OS job calling start. A plain try/catch is enough for work that is fine to lose. This table compares capabilities only; no performance benchmarks have been published.

Guarantees #

Execution is at least once. A crash after a side effect but before the completion write can run the task again.

start recovers a task left in running: that attempt counts, then the retry policy either schedules another try or fails the task. Handlers should be idempotent when a repeated side effect would be harmful.

When a task finishes, the tasks waiting on it are updated before its own result is written. A crash in between never leaves a task waiting forever behind a dependency that already finished. The worst case is that dependents were released or cancelled for an outcome that recovery then records again.

Persistence is not background execution. A terminated app stays terminated until something else launches it. After the next launch, call start again.

Do not put secrets in task payloads or exception text. The queue stores them as plain data. MemoryQueueStorage does not encrypt anything.

Examples #

dart run example/main.dart           # one task, one retry
dart run example/image_upload.dart   # uploads that survive a restart
dart run example/orchestration.dart  # chains, dependencies, priorities

Contributing and feedback #

• Questions, feature ideas, and storage-integration requests: GitHub Discussions.
• Bugs: issue tracker.
• Code and adapters: see CONTRIBUTING.md.