Resilience in .NET: Mastering Polly for Fault Tolerance

2 min read 556 words

1. Introduction

In a distributed system, failures are inevitable. A database might be momentarily busy, or an external API might be down for a few seconds. If your application isn’t prepared for these “transient failures,” it will crash or provide a poor user experience.

Polly is a .NET resilience and transient-fault-handling library that allows developers to express strategies such as Retry, Circuit Breaker, Timeout, and Fallback in a fluent and thread-safe manner.

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2. Core Resilience Strategies

Polly provides several “policies” to handle failures.

1. Retry

The most basic strategy. If an operation fails, try again.

var retryPolicy = Policy
    .Handle<HttpRequestException>()
    .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));
    
await retryPolicy.ExecuteAsync(() => _httpClient.GetAsync("https://api.example.com"));

Note: Using “Exponential Backoff” (increasing the wait time) prevents overwhelming the failing service.

2. Circuit Breaker

If a service fails repeatedly, the “circuit opens,” and all subsequent calls fail immediately for a set period. This gives the failing service time to recover.

var circuitBreaker = Policy
    .Handle<HttpRequestException>()
    .CircuitBreakerAsync(5, TimeSpan.FromMinutes(1)); // Break after 5 failures

3. Fallback

Provide a “plan B” if the operation fails even after retries.

var fallback = Policy<string>
    .Handle<Exception>()
    .FallbackAsync("Default Value");

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3. Policy Wrapping

You can combine multiple policies using Policy.Wrap. For example: “Retry 3 times, but only if the operation finishes within 2 seconds (Timeout).”

var wrap = Policy.WrapAsync(retryPolicy, timeoutPolicy);

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4. Integration with HttpClientFactory

In modern .NET apps, you don’t manually wrap every call. You register Polly policies directly in your Program.cs using the Microsoft.Extensions.Http.Polly package.

builder.Services.AddHttpClient("ExternalApi")
    .AddTransientHttpErrorPolicy(policy => policy.WaitAndRetryAsync(3, _ => TimeSpan.FromSeconds(2)));

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5. .NET 8 Resilience Pipelines

With .NET 8, Microsoft introduced a new way to handle resilience using the Microsoft.Extensions.Resilience library, which is built on top of Polly v8 but offers a more integrated experience.

builder.Services.AddResiliencePipeline("my-pipeline", builder =>
{
    builder.AddRetry(new RetryStrategyOptions
    {
        MaxRetryAttempts = 3,
        BackoffType = DelayBackoffType.Exponential
    });
});

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6. Conclusion

Resilience is not an afterthought; it’s a core requirement for “solid and robust products.” By implementing Polly strategies, you ensure that your application doesn’t just “fail hard” but instead handles the chaos of the network gracefully.

Pick one external API in your current project and add a Retry policy today—your users (and your logs) will thank you!