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Introducing Polly Dart - Resilience for the Dart Ecosystem

· 5 min read
Anirudh Singh
Anirudh Singh
Creator of Polly Dart

Today, I'm excited to announce Polly Dart - a comprehensive resilience and transient-fault-handling library for Dart applications. Inspired by the battle-tested .NET Polly library, Polly Dart brings enterprise-grade resilience patterns to the Dart ecosystem.

Why Resilience Matters More Than Ever​

In our interconnected world, applications depend on numerous external services, APIs, databases, and resources. Network hiccups, service outages, and resource contention are not exceptions—they're inevitable realities. The question isn't whether your application will encounter failures, but how gracefully it will handle them.

Consider these common scenarios:

  • A mobile app loses network connectivity while syncing data
  • A Flutter web app calls an API that's temporarily overwhelmed
  • A Dart server application hits database connection limits
  • A microservice times out due to unexpected load

Without proper resilience strategies, these situations lead to poor user experiences, data loss, and system instability.

The Polly Dart Solution​

Polly Dart provides six core resilience strategies that can be combined to create robust, fault-tolerant applications:

🔄 Reactive Strategies​

  • Retry: Automatically retry failed operations with intelligent backoff
  • Circuit Breaker: Prevent cascading failures by blocking calls to failing services
  • Fallback: Provide alternative responses when operations fail
  • Hedging: Execute multiple parallel attempts for optimized response times

⚡ Proactive Strategies​

  • Timeout: Cancel operations that take too long
  • Rate Limiter: Control operation rate and manage concurrency

Design Principles​

When creating Polly Dart, I focused on several key principles:

1. Developer Experience First​

Resilience shouldn't require complex boilerplate code. Polly Dart uses a fluent builder pattern that makes creating resilience policies intuitive and readable:

final pipeline = ResiliencePipelineBuilder()
    .addRetry(RetryStrategyOptions(maxRetryAttempts: 3))
    .addCircuitBreaker(CircuitBreakerStrategyOptions(failureRatio: 0.5))
    .addTimeout(Duration(seconds: 30))
    .addFallback(FallbackStrategyOptions.withValue('Cached data'))
    .build();

// Clean, declarative resilience
final result = await pipeline.execute(() => apiCall());

2. Composability and Flexibility​

Strategies can be combined in any order to create sophisticated resilience pipelines. Each strategy focuses on a specific concern, making the system both powerful and maintainable.

3. Type Safety and Performance​

Built with Dart's strong type system, Polly Dart provides compile-time safety while maintaining excellent runtime performance. Strategies only activate when needed, keeping overhead minimal.

4. Observability Built-In​

Every strategy includes comprehensive callbacks for monitoring, logging, and metrics collection:

.addRetry(RetryStrategyOptions(
  onRetry: (args) async {
    logger.info('Retrying operation, attempt ${args.attemptNumber + 1}');
    metrics.incrementCounter('retry_attempts');
  },
))

Real-World Impact​

Let me show you how Polly Dart transforms code from fragile to resilient:

Before: Fragile HTTP Client​

class ApiClient {
  Future<User> getUser(int id) async {
    final response = await httpClient.get('/users/$id');
    return User.fromJson(response.data);
    // What if the network fails? What if the server is slow?
    // What if the service is temporarily down?
  }
}

After: Resilient HTTP Client​

class ResilientApiClient {
  final _pipeline = ResiliencePipelineBuilder()
      .addRetry(RetryStrategyOptions(
        maxRetryAttempts: 3,
        backoffType: DelayBackoffType.exponential,
        shouldHandle: (outcome) => isTransientError(outcome),
      ))
      .addCircuitBreaker(CircuitBreakerStrategyOptions(
        failureRatio: 0.5,
        breakDuration: Duration(seconds: 30),
      ))
      .addTimeout(Duration(seconds: 15))
      .addFallback(FallbackStrategyOptions(
        fallbackAction: (args) => getCachedUser(id),
      ))
      .build();

  Future<User> getUser(int id) async {
    return await _pipeline.execute((context) async {
      final response = await httpClient.get('/users/$id');
      return User.fromJson(response.data);
    });
    // Now handles network failures, slow responses, service outages,
    // and provides graceful fallbacks - all transparently!
  }
}

Platform Support​

Polly Dart works across the entire Dart ecosystem:

  • Flutter Mobile (iOS, Android)
  • Flutter Web
  • Flutter Desktop (Windows, macOS, Linux)
  • Dart Server applications
  • Dart CLI tools

The same resilience patterns work consistently across all platforms, making it easy to share code and expertise across your entire stack.

Learning from Production Experience​

The patterns implemented in Polly Dart aren't theoretical—they're proven solutions that have helped organizations handle billions of requests reliably. The .NET Polly library has been battle-tested in production environments ranging from small startups to Fortune 500 companies.

By bringing these patterns to Dart, we're enabling the Flutter and Dart communities to build applications with the same level of resilience that enterprise systems depend on.

Getting Started​

Adding resilience to your Dart application is straightforward:

dart pub add polly_dart

Then start with a simple retry policy and gradually add more strategies as needed:

import 'package:polly_dart/polly_dart.dart';

final pipeline = ResiliencePipelineBuilder()
    .addRetry()
    .addTimeout(Duration(seconds: 30))
    .build();

final result = await pipeline.execute(() => yourOperation());

What's Next​

This initial release includes all six core resilience strategies with comprehensive configuration options. Future releases will focus on:

  • Additional strategies (bulkhead isolation, hedging variants)
  • Performance optimizations
  • Enhanced monitoring capabilities
  • Integration guides for popular Dart/Flutter packages
  • Community feedback and feature requests

Community and Contribution​

Polly Dart is open source and welcomes community contributions. Whether you're reporting bugs, suggesting features, improving documentation, or contributing code, your involvement helps make the Dart ecosystem more resilient.

Acknowledgments​

Special thanks to the .NET Polly community for pioneering these resilience patterns and creating comprehensive documentation that guided this implementation. Their work has helped countless developers build more reliable systems.

Build Resilient Applications Today​

Failures are inevitable, but with Polly Dart, they don't have to be catastrophic. Start building more resilient Dart and Flutter applications today, and give your users the reliable experience they deserve.

Try Polly Dart in your next project and see how easy it is to add enterprise-grade resilience to your applications. Your users (and your on-call schedule) will thank you.

Have questions or feedback? Open a discussion on GitHub or reach out on social media. I'd love to hear about how you're using Polly Dart in your projects!