Microsoft.Extensions.Http.Resilience

Resilience mechanisms for HttpClient built on the Polly framework.

Install the package

From the command-line:

dotnet add package Microsoft.Extensions.Http.Resilience

Or directly in the C# project file:

<ItemGroup>
  <PackageReference Include="Microsoft.Extensions.Http.Resilience" Version="[CURRENTVERSION]" />
</ItemGroup>

Usage Examples

When configuring an HttpClient through the HTTP client factory the following extensions can add a set of pre-configured hedging or resilience behaviors. These pipelines combine multiple resilience strategies with pre-configured defaults.

• The total request timeout pipeline applies an overall timeout to the execution, ensuring that the request including hedging attempts, does not exceed the configured limit.
• The retry pipeline retries the request in case the dependency is slow or returns a transient error.
• The rate limiter pipeline limits the maximum number of requests being send to the dependency.
• The circuit breaker blocks the execution if too many direct failures or timeouts are detected.
• The attempt timeout pipeline limits each request attempt duration and throws if its exceeded.

Resilience

The standard resilience pipeline makes use of the above strategies to ensure HTTP requests can be sent reliably.

var clientBuilder = services.AddHttpClient("MyClient");

clientBuilder.AddStandardResilienceHandler().Configure(o =>
{
    o.CircuitBreaker.MinimumThroughput = 10;
});

Hedging

The standard hedging pipeline uses a pool of circuit breakers to ensure that unhealthy endpoints are not hedged against. By default, the selection from pool is based on the URL Authority (scheme + host + port). It is recommended that you configure the way the strategies are selected by calling the SelectPipelineByAuthority() extensions. The last three strategies are applied to each individual endpoint.

var clientBuilder = services.AddHttpClient("MyClient");

clientBuilder.AddStandardHedgingHandler().Configure(o =>
{
    o.TotalRequestTimeout.Timeout = TimeSpan.FromSeconds(10);
});

Custom Resilience

For more granular control a custom pipeline can be constructed.

var clientBuilder = services.AddHttpClient("MyClient");

clientBuilder.AddResilienceHandler("myHandler", b =>
{
    b.AddFallback(new FallbackStrategyOptions<HttpResponseMessage>()
    {
        FallbackAction = _ => Outcome.FromResultAsValueTask(new HttpResponseMessage(HttpStatusCode.ServiceUnavailable))
    })
    .AddConcurrencyLimiter(100)
    .AddRetry(new HttpRetryStrategyOptions())
    .AddCircuitBreaker(new HttpCircuitBreakerStrategyOptions())
    .AddTimeout(new HttpTimeoutStrategyOptions());
});

Known issues

The following sections detail various known issues.

Compatibility with the Grpc.Net.ClientFactory package

If you're using Grpc.Net.ClientFactory version 2.63.0 or earlier, then enabling the standard resilience or hedging handlers for a gRPC client could cause a runtime exception. Specifically, consider the following code sample:

services
    .AddGrpcClient<Greeter.GreeterClient>()
    .AddStandardResilienceHandler();

The preceding code results in the following exception:

System.InvalidOperationException: The ConfigureHttpClient method is not supported when creating gRPC clients. Unable to create client with name 'GreeterClient'.

To resolve this issue, we recommend upgrading to Grpc.Net.ClientFactory version 2.64.0 or later.

There's a build time check that verifies if you're using Grpc.Net.ClientFactory version 2.63.0 or earlier, and if you are the check produces a compilation warning. You can suppress the warning by setting the following property in your project file:

<PropertyGroup>
  <SuppressCheckGrpcNetClientFactoryVersion>true</SuppressCheckGrpcNetClientFactoryVersion>
</PropertyGroup>

Compatibility with .NET Application Insights

If you're using .NET Application Insights, then enabling resilience functionality in your application could cause all Application Insights telemetry to be missing. The issue occurs when resilience functionality is registered before Application Insights services. Consider the following sample causing the issue:

// At first, we register resilience functionality.
services.AddHttpClient().AddStandardResilienceHandler();

// And then we register Application Insights. As a result, Application Insights doesn't work.
services.AddApplicationInsightsTelemetry();

The issue is caused by the following bug in Application Insights and can be fixed by registering Application Insights services before resilience functionality, as shown below:

// We register Application Insights first, and now it will be working correctly.
services.AddApplicationInsightsTelemetry();
services.AddHttpClient().AddStandardResilienceHandler();

Feedback & Contributing

We welcome feedback and contributions in our GitHub repo.