Building Resilient .NET Applications with Polly

Introduction

In today's fast-paced digital world, ensuring that your applications are resilient and can handle unexpected failures is crucial. This article explores how you can leverage Polly, a .NET library, in conjunction with HttpClient to build robust applications that can gracefully handle retries, timeouts, and transient faults.

What is Polly?

Polly is a .NET library that provides resilience and transient-fault handling capabilities. It allows developers to define policies such as retry, circuit breaker, timeout, bulkhead isolation, and fallback to manage the reliability of their applications.

Why Use Polly with HttpClient?

HttpClient is a powerful tool for making HTTP requests in .NET applications. However, network communication is inherently unreliable, and applications need to handle potential failures gracefully. By integrating Polly with HttpClient, you can:

• Retry failed requests: Automatically retry requests that fail due to transient faults.
• Implement timeouts: Ensure that requests do not hang indefinitely by setting appropriate timeouts.
• Handle circuit breaking: Prevent your application from repeatedly trying operations that are likely to fail.

Setting Up Polly with HttpClient

To get started with Polly, you need to install the Polly NuGet package. You can do this via the Package Manager Console:

Install-Package Polly

Once installed, you can define your resilience policies. Here’s a simple example of using Polly to retry a failed HTTP request:

var retryPolicy = Policy
    .Handle<HttpRequestException>()
    .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));

var httpClient = new HttpClient();

await retryPolicy.ExecuteAsync(async () =>
{
    var response = await httpClient.GetAsync("https://api.example.com/data");
    response.EnsureSuccessStatusCode();
});

Implementing Advanced Policies

Circuit Breaker

A circuit breaker policy prevents an application from performing an operation that is likely to fail. Here’s how you can implement it:

var circuitBreakerPolicy = Policy
    .Handle<HttpRequestException>()
    .CircuitBreakerAsync(2, TimeSpan.FromMinutes(1));

Timeout

Timeout policies ensure that operations do not run indefinitely:

var timeoutPolicy = Policy
    .TimeoutAsync<HttpResponseMessage>(10); // 10 seconds timeout

Conclusion

By using Polly with HttpClient, you can significantly improve the resilience of your .NET applications. Whether you are handling retries, implementing timeouts, or managing circuit breakers, Polly provides a flexible and powerful way to enhance your application's reliability.

Further Reading

• Polly Documentation
• HttpClient Best Practices