# Polly — .NET Resilience and Transient Fault Handling

> A .NET library that lets developers express retry, circuit breaker, timeout, bulkhead isolation, and fallback policies in a fluent, thread-safe API.

## Install

Save in your project root:

# Polly — .NET Resilience and Transient Fault Handling

## Quick Use
```bash
dotnet add package Polly
```
```csharp
var pipeline = new ResiliencePipelineBuilder()
    .AddRetry(new RetryStrategyOptions { MaxRetryAttempts = 3 })
    .AddTimeout(TimeSpan.FromSeconds(10))
    .Build();
await pipeline.ExecuteAsync(async ct => await httpClient.GetAsync(url, ct));
```

## Introduction
Polly is a .NET resilience and transient fault handling library. It provides a fluent API for defining policies such as retry, circuit breaker, timeout, rate limiter, and fallback that wrap unreliable calls to external services, databases, or any operation that can fail transiently.

## What Polly Does
- Retries failed operations with configurable backoff strategies
- Implements circuit breaker patterns to stop cascading failures
- Enforces timeouts on long-running operations
- Provides bulkhead isolation to limit concurrent resource usage
- Combines multiple strategies into resilience pipelines

## Architecture Overview
Polly v8 introduces a pipeline-based architecture built on ResiliencePipelineBuilder. Each strategy (retry, circuit breaker, timeout) is a composable middleware layer. The pipeline executes strategies in order, wrapping the inner delegate. The library uses a non-allocating execution path for hot paths and integrates with Microsoft.Extensions.Resilience for dependency injection in ASP.NET Core.

## Self-Hosting & Configuration
- Install the core package: `dotnet add package Polly`
- For DI integration: `dotnet add package Microsoft.Extensions.Resilience`
- Register pipelines in Program.cs via `builder.Services.AddResiliencePipeline()`
- Configure strategies through option objects or appsettings.json
- Works with .NET Standard 2.0+ and .NET 6 through .NET 9

## Key Features
- Fluent builder API for composing resilience strategies
- Built-in telemetry and metering via .NET metrics
- Hedging strategy for speculative parallel execution
- Rate limiter integration with System.Threading.RateLimiting
- Thread-safe and designed for high-throughput scenarios

## Comparison with Similar Tools
- **Microsoft.Extensions.Http.Resilience** — built on Polly; provides opinionated HttpClient resilience defaults
- **Resilience4j (Java)** — similar patterns for the JVM ecosystem; no .NET support
- **Steeltoe** — cloud-native .NET framework with circuit breaker; broader scope, less focused
- **Hystrix (.NET port)** — Netflix circuit breaker; no longer actively maintained
- **Custom retry loops** — ad hoc try/catch patterns; no composability, harder to test

## FAQ
**Q: What changed between Polly v7 and v8?**
A: Polly v8 replaced the policy-based API with a pipeline-based architecture using ResiliencePipelineBuilder. It is simpler, more performant, and integrates natively with Microsoft.Extensions.

**Q: Can I use Polly with HttpClient?**
A: Yes. The Microsoft.Extensions.Http.Resilience package provides AddStandardResilienceHandler() that configures HttpClient with Polly-based retry, circuit breaker, and timeout out of the box.

**Q: Does Polly support async operations?**
A: Yes. All resilience pipelines support both synchronous and asynchronous execution via ExecuteAsync with CancellationToken support.

**Q: How do I monitor Polly behavior in production?**
A: Polly v8 emits .NET metrics and supports TelemetryListeners. You can export these to OpenTelemetry, Application Insights, or any metrics backend.

## Sources
- https://github.com/App-vNext/Polly
- https://www.thepollyproject.org/

---
Source: https://tokrepo.com/en/workflows/asset-40a83ed4
Author: AI Open Source