DuraQ

A robust, durable queuing system implemented in Dart, providing persistent queue management with multiple storage backend support.

Overview

DuraQ is designed to provide a reliable queuing system with:

• Type-safe queue operations
• Persistent storage options
• Multiple queue support
• Entry tracking and monitoring
• Extensible storage backends

Quick Start

1. Add to your pubspec.yaml:

dependencies:
  duraq: ^0.0.1

2. Create a queue manager with SQLite storage:

import 'package:duraq/duraq.dart';
import 'package:path/path.dart' as path;

void main() async {
  // Initialize storage
  final dbPath = path.join(Directory.current.path, 'queue.db');
  final storage = SQLiteStorage(dbPath: dbPath);

  // Create queue manager
  final manager = QueueManager(storage);

  // Get a typed queue
  final emailQueue = manager.queue<String>('emails');

  try {
    // Enqueue items
    await emailQueue.enqueue('Welcome email to user@example.com');

    // Process items
    final email = await emailQueue.dequeue();
    if (email != null) {
      await processEmail(email);
    }

    // Check queue status
    final remaining = await emailQueue.length;
    print('Remaining emails: $remaining');
  } finally {
    // Clean up resources
    storage.dispose();
  }
}

Features

• Type-safe Queues: Generic support for any data type
• Multiple Queues: Manage different queue types in one manager
• Persistent Storage: Built-in SQLite support with ACID compliance
• Entry Tracking: Monitor attempts, creation time, and status
• Transaction Support: Atomic operations with nested transaction support
• Retry Policies: Configurable retry strategies with exponential backoff
• Priority Support: Priority-based queue processing (lower value = higher priority)
• TTL Support: Automatic entry expiration
• Scheduled Execution: Delay processing until a specific time
• Dead Letter Queue: Automatic handling of permanently failed entries
• Health Checks: Monitor storage, metrics, and queue health
• Concurrent Processing: Entry-level locking for safe multi-consumer access
• Extensible: Custom storage backend support via StorageInterface

Storage Backends

SQLite Storage (Built-in)

final storage = SQLiteStorage(dbPath: 'path/to/queue.db');

Features:

• Persistent across restarts
• ACID compliant
• Automatic schema management
• FIFO guarantee
• Data integrity
• Transaction support with savepoints
• Priority-based retrieval
• TTL support

Isar Storage

// Initialize Isar with required schemas
await Isar.initializeIsarCore(download: true);
final isar = await Isar.open([
  ...IsarStorage.requiredSchemas,
  // Add your other schemas here if needed
], directory: 'path/to/db');

// Create storage with external Isar instance
final storage = IsarStorage(isar);

// Use the storage
final manager = QueueManager(storage);

// Clean up (caller manages Isar lifecycle)
await storage.dispose(); // Releases locks but doesn't close Isar
await isar.close(); // Caller closes Isar

Features:

• High-performance NoSQL database
• Persistent across restarts
• Native indexing for better performance
• Type-safe queries
• Cross-platform support
• Memory efficient with lazy loading
• Shared Isar instance support - allows multiple components to use the same database
• External lifecycle management - caller controls when to open/close the database

Note: Isar's transaction support in DuraQ is compatibility-tracked (depth counter) rather than true nested transactions. Individual Isar write operations are atomic via writeTxn, but the beginTransaction/commitTransaction API does not provide the same ACID guarantees as SQLite's savepoint-based transactions.

Upcoming Storage Options

• File system storage
• Custom storage implementations via StorageInterface

Best Practices

1. Resource Management

final storage = SQLiteStorage(dbPath: 'queue.db');
try {
  // Use storage
} finally {
  storage.dispose(); // Always dispose
}

2. Type Safety

// Prefer specific types
final emailQueue = manager.queue<EmailMessage>('emails');
final jobQueue = manager.queue<BackgroundJob>('jobs');

3. Error Handling

try {
  await queue.enqueue(item);
} catch (e) {
  // Handle storage errors
}

4. Transaction Usage

// Simple transaction
await storage.transaction(() async {
  await queue.enqueue(item1);
  await queue.enqueue(item2);
  return null;
}); // Automatically commits or rolls back

// Nested transactions (SQLite uses savepoints)
await storage.transaction(() async {
  await queue.enqueue(item1);

  await storage.transaction(() async {
    await queue.enqueue(item2);
    return null;
  }); // Inner transaction

  return null;
}); // Outer transaction

// Manual transaction control
await storage.beginTransaction();
try {
  await queue.enqueue(item1);
  await queue.enqueue(item2);
  await storage.commitTransaction();
} catch (e) {
  await storage.rollbackTransaction();
  rethrow;
}

5. Retry Policies

// Create a queue with exponential backoff retry
final queue = Queue<String>(
  'my-queue',
  storage,
  retryPolicy: ExponentialBackoff(
    baseDelay: Duration(milliseconds: 100),
    maxDelay: Duration(seconds: 30),
    maxAttempts: 5,
  ),
);

// Process items with automatic retry
try {
  await queue.processNext((data) async {
    await processItem(data);
    // If this throws, the entry is retried according to policy
    // After maxAttempts, the entry moves to the dead letter queue
  });
} catch (e) {
  print('Processing failed: $e');
}

Examples

Basic Queue Operations

// Create a queue
final queue = manager.queue<String>('notifications');

// Add items
await queue.enqueue('Notification 1');
await queue.enqueue('Notification 2');

// dequeue() returns the raw data (T?), not a QueueEntry
while (true) {
  final item = await queue.dequeue();
  if (item == null) break;

  await processNotification(item);
}

Multiple Queue Types

// Email queue
final emailQueue = manager.queue<EmailMessage>('emails');
await emailQueue.enqueue(EmailMessage(...));

// Job queue
final jobQueue = manager.queue<BackgroundJob>('jobs');
await jobQueue.enqueue(BackgroundJob(...));

Transaction Support

DuraQ provides robust transaction support (ACID guarantees with SQLite):

1. Atomicity: All operations in a transaction either succeed or fail together
2. Consistency: The database remains in a valid state before and after the transaction
3. Isolation: Concurrent transactions don't interfere with each other
4. Durability: Once committed, changes persist even after system failures

Transaction Methods

• transaction<T>(Future<T> Function() operations): Executes operations in a transaction
• beginTransaction(): Starts a manual transaction
• commitTransaction(): Commits a manual transaction
• rollbackTransaction(): Rolls back a manual transaction

Features

• Automatic commit/rollback handling
• Nested transaction support using savepoints (SQLite)
• Manual transaction control when needed
• Error handling with automatic rollback

Example Use Cases

1. Atomic Multi-Queue Operations

await storage.transaction(() async {
  await emailQueue.enqueue(welcomeEmail);
  await notificationQueue.enqueue(welcomeNotification);
  await analyticsQueue.enqueue(userSignupEvent);
  return null;
});

2. Batch Processing with Rollback

await storage.transaction(() async {
  for (final job in jobs) {
    if (!isValid(job)) {
      throw ValidationError(); // Rolls back all enqueued jobs
    }
    await jobQueue.enqueue(job);
  }
  return null;
});

3. Complex Queue Operations

await storage.transaction(() async {
  // Process high-priority queue
  final highPriorityItem = await highPriorityQueue.dequeue();
  if (highPriorityItem != null) {
    await processQueue.enqueue(highPriorityItem);
  }

  // Process normal queue
  final normalItem = await normalQueue.dequeue();
  if (normalItem != null) {
    await processQueue.enqueue(normalItem);
  }

  return null;
});

Retry Policies

DuraQ includes built-in retry support with configurable policies:

1. Exponential Backoff

// Configure retry policy
final retryPolicy = ExponentialBackoff(
  baseDelay: Duration(milliseconds: 100), // Initial delay
  maxDelay: Duration(seconds: 30),        // Maximum delay
  maxAttempts: 5,                         // Maximum retry attempts
);

// Create queue with retry policy
final queue = Queue<String>('retry-queue', storage, retryPolicy: retryPolicy);

// processNext handles retry logic automatically
await queue.processNext((data) async {
  await processWithRetry(data);
});

2. Custom Retry Policies

class CustomRetryPolicy implements RetryPolicy {
  @override
  int get maxAttempts => 3;

  @override
  bool shouldRetry(int attempts, [Object? error]) {
    // Custom retry logic
    return attempts < maxAttempts && error is RetryableError;
  }

  @override
  Duration getRetryDelay(int attempts) {
    // Custom delay calculation
    return Duration(seconds: attempts * 5);
  }

  @override
  bool shouldMoveToDeadLetter(int attempts, [Object? error]) {
    // Custom dead letter queue logic
    return attempts >= maxAttempts || error is NonRetryableError;
  }
}

3. Error Handling with Retries

try {
  await queue.processNext((data) async {
    if (!await processItem(data)) {
      throw RetryableError('Processing failed, will retry');
    }
  });
} catch (e) {
  // Failed attempts are automatically retried
  // After maxAttempts, moves to dead letter queue
  print('Processing failed: $e');
}

Dead Letter Queue

DuraQ provides built-in dead letter queue support for handling failed entries:

1. Automatic Failed Entry Movement

// Create a queue with retry policy
final queue = Queue<String>(
  'my-queue',
  storage,
  retryPolicy: ExponentialBackoff(maxAttempts: 3),
);

// Failed entries move to dead letter queue after max attempts
try {
  await queue.processNext((data) async {
    if (!await processItem(data)) {
      throw ProcessingError('Failed to process item');
    }
  });
} catch (e) {
  // Entry has been moved to dead letter or scheduled for retry
}

2. Dead Letter Queue Management

// Create a dead letter queue for a specific queue
final dlq = DeadLetterQueue<String>('my-queue', storage);

// List dead letter entries with pagination
final entries = await dlq.list(limit: 10, offset: 0);
for (final entry in entries) {
  print('Failed entry: ${entry.data}');
  print('Error: ${entry.errorMessage}');
  print('Attempts: ${entry.attempts}');
}

// Get dead letter queue length
final count = await dlq.length;
print('Dead letter entries: $count');

3. Entry Recovery

// Retry a specific entry (resets attempts to 0, moves back to pending)
final deadLetter = await dlq.retrieve();
if (deadLetter != null) {
  await dlq.retry(deadLetter.id);
}

// Remove an entry permanently
await dlq.remove(deadLetter.id);

4. Cleanup and Maintenance

// Purge old entries
final purged = await dlq.purgeOldEntries(Duration(days: 7));
print('Purged $purged old entries');

Features

• Automatic movement of failed entries after retry exhaustion
• Pagination support for listing entries
• Entry retry capability (resets attempt counter)
• Permanent entry removal
• Automatic cleanup of old entries
• Error message preservation
• Attempt count tracking

Best Practices

1. Regular Monitoring

// Check dead letter queue periodically
final dlq = DeadLetterQueue<String>('my-queue', storage);
final count = await dlq.length;
if (count > 0) {
  // Alert operations team
  notifyOps('Dead letter queue has $count entries');
}

2. Cleanup Strategy

// Implement regular cleanup
final cleanupJob = Timer.periodic(Duration(days: 1), (_) async {
  final dlq = DeadLetterQueue<String>('my-queue', storage);

  // Keep entries for 30 days
  final purged = await dlq.purgeOldEntries(Duration(days: 30));

  // Log cleanup results
  logger.info('Purged $purged old dead letter entries');
});

3. Error Analysis

// Analyze failed entries
final entries = await dlq.list();
final errorCounts = <String, int>{};

for (final entry in entries) {
  errorCounts[entry.errorMessage ?? 'Unknown'] =
    (errorCounts[entry.errorMessage] ?? 0) + 1;
}

// Report error distribution
for (final error in errorCounts.entries) {
  print('${error.key}: ${error.value} occurrences');
}

4. Retry Strategy

// Implement selective retry
final entries = await dlq.list();
for (final entry in entries) {
  if (entry.errorMessage?.contains('Temporary failure') ?? false) {
    // Retry temporary failures (resets attempts to 0)
    await dlq.retry(entry.id);
  } else {
    // Remove permanently failed entries
    await dlq.remove(entry.id);
  }
}

Scheduled Execution

DuraQ supports scheduling entries for future processing:

// Schedule an entry for future processing using enqueueEntry
final scheduledTime = DateTime.now().add(Duration(hours: 1));
final entry = QueueEntry<String>(
  id: 'scheduled-task-1',
  data: 'Process me later',
  createdAt: DateTime.now(),
  scheduledFor: scheduledTime,
);

await queue.enqueueEntry(entry);

// The entry won't be retrieved until the scheduled time
final nextItem = await queue.dequeue(); // Returns null if no entries are ready

Features

• Schedule entries for future processing
• Entries remain in queue but won't be retrieved before their scheduled time
• Combines with priority support (priority order applies once scheduled time is reached)
• Perfect for:
  • Delayed email notifications
  • Scheduled background jobs
  • Time-based workflow triggers
  • Deferred processing

Example Use Cases

1. Delayed Notifications

final reminderTime = DateTime.now().add(Duration(days: 1));
await notificationQueue.enqueueEntry(QueueEntry(
  id: 'reminder-1',
  data: 'Follow-up reminder email',
  createdAt: DateTime.now(),
  scheduledFor: reminderTime,
));

2. Scheduled Tasks

final midnightTonight = DateTime.now().copyWith(
  hour: 0,
  minute: 0,
  second: 0,
  millisecond: 0,
).add(Duration(days: 1));

await jobQueue.enqueueEntry(QueueEntry(
  id: 'nightly-job-1',
  data: 'Run nightly maintenance',
  createdAt: DateTime.now(),
  scheduledFor: midnightTonight,
));

3. Time-based Workflow

final now = DateTime.now();
final workflow = [
  QueueEntry(
    id: 'step1',
    data: 'Send welcome email',
    createdAt: now,
    scheduledFor: now,
  ),
  QueueEntry(
    id: 'step2',
    data: 'Send follow-up survey',
    createdAt: now,
    scheduledFor: now.add(Duration(days: 7)),
  ),
  QueueEntry(
    id: 'step3',
    data: 'Send engagement reminder',
    createdAt: now,
    scheduledFor: now.add(Duration(days: 14)),
  ),
];

await storage.transaction(() async {
  for (final step in workflow) {
    await workflowQueue.enqueueEntry(step);
  }
  return null;
});

Concurrent Processing

DuraQ provides built-in support for concurrent processing with entry-level locking:

// Multiple consumers can safely process queue entries
final consumer1 = QueueManager(storage);
final consumer2 = QueueManager(storage);

// Each consumer gets a different entry - entries are locked while processing
final item1 = await consumer1.queue<String>('jobs').dequeue();
final item2 = await consumer2.queue<String>('jobs').dequeue();

Features

• Entry-level locking for safe concurrent processing
• Automatic lock cleanup for expired locks (default: 5 minute timeout)
• Combines with other features:
  • Priority-based processing
  • Scheduled execution
  • Transaction support
  • Retry policies

Example Use Cases

1. Multiple Workers

void startWorker(String name, SQLiteStorage storage) async {
  final queue = Queue<String>('jobs', storage);

  while (true) {
    try {
      final processed = await queue.processNext((data) async {
        await processJob(data);
      });
      if (!processed) {
        await Future.delayed(Duration(seconds: 1));
      }
    } catch (e) {
      // processNext handles retry/dead-letter automatically
      print('$name: Processing failed: $e');
    }
  }
}

// Start multiple workers sharing the same storage
final storage = SQLiteStorage(dbPath: 'queue.db');
startWorker('worker1', storage);
startWorker('worker2', storage);
startWorker('worker3', storage);

2. Safe Concurrent Processing

final queue = Queue<String>('orders', storage);

// Multiple processors can safely run in parallel
await Future.wait([
  processOrders(queue),
  processOrders(queue),
  processOrders(queue),
]);

Future<void> processOrders(Queue<String> queue) async {
  while (true) {
    final processed = await queue.processNext((data) async {
      await fulfillOrder(data);
    });
    if (!processed) break; // No more items
  }
}

3. Health Monitoring

// Periodically check for stuck entries using SQLiteStorage directly
final sqliteStorage = storage as SQLiteStorage;
final stuckEntries = await sqliteStorage.getEntriesByStatus(
  'jobs',
  EntryStatus.processing,
);

for (final entry in stuckEntries) {
  final processingTime = DateTime.now().difference(entry.lastUpdatedAt);
  if (processingTime > Duration(hours: 1)) {
    // Alert operations about stuck entry
    notifyOps('Entry ${entry.id} stuck in processing');
  }
}

Note: getEntriesByStatus() is available on SQLiteStorage and IsarStorage directly, but is not part of the StorageInterface contract.

Health Checks

DuraQ provides comprehensive health monitoring capabilities to ensure your queue system is operating correctly. The health check system monitors three main components:

1. Storage Health: Verifies that the storage backend is responsive and functioning properly.
2. Metrics Health: Ensures the metrics collection system is operational.
3. Queue Health: Monitors queue performance metrics like error rates and processing times.

Usage

// Create health checks
final healthChecks = HealthCheckAggregator([
  StorageHealthCheck(storage),
  MetricsHealthCheck(metrics),
  QueueHealthCheck(
    storage,
    metrics,
    errorRateWindow: Duration(minutes: 5),
    maxErrorRate: 0.1, // 10% threshold
  ),
]);

// Check overall health
final status = await healthChecks.getOverallStatus();
if (status == HealthStatus.healthy) {
  print('All systems operational');
}

// Get detailed health information (checks run in parallel)
final results = await healthChecks.checkAll();
for (final result in results.values) {
  print('${result.component}: ${result.status}');
  print('Message: ${result.message}');
  print('Details: ${result.details}');
}

Health Status Levels

• Healthy: All components are functioning normally
• Degraded: System is operational but showing signs of stress (e.g., high error rate)
• Unhealthy: One or more components have failed

Monitored Metrics

• Error rates
• Queue size
• Average processing time
• Storage responsiveness
• Metrics system status

Contributing

Contributions are welcome! Please read our contributing guidelines and submit pull requests to our repository.

License

MIT

See Also

• Priority Queues
• Status Tracking
• TTL Support
• API Documentation
• GitHub Repository