Polly - Policy Handler which evaluates transient failures from HTTP response content

Question

I'm using Refit, with Polly and Polly.Extensions.Http in an ASP.NET Core Web API to call some external APIs.

I'm trying to add a policy handler using Polly which can retry depending on the body content as the external API I am working with, doesn't use HTTP Status codes to return operation results, but instead puts operation results in a common returned body object. So it will always return 200 OK but then it could indicate it failed with a body that has Success = false and some errors in the Errors property. Here is the response object properties that are always returned:

public abstract class ApiResultResponse 
{
    public string RequestId { get; set; }
    public List<string> Warnings { get; set; }
    public List<Error> Errors { get; set; }
    public bool Success { get; set; }
}

Trying to add an asynchronous retry policy here using the AddPolicyHandler method call.

But I am not sure how to properly create the retryPolicy object with Polly that will create the desired functionality.

services.AddRefitClient<ISomeApi>(refitSettings)
    .ConfigureHttpClient(c => {
        c.BaseAddress = someApiBaseUrl;
    })
    .AddPolicyHandler(retryPolicy)

I want to add this functionality to a bunch of APIs as one common situation is that calling these APIs may result in rate limiting failures, but I need to evaluate the ApiResultResponse object Errors property to determine if the cause was rate limiting for the failure. I want to handle these at a global level and automatically retry, and not handle it downstream in services.

You might also need to consider to use [Circuit Breaker](https://github.com/App-vNext/Polly/wiki/Circuit-Breaker) to avoid sending requests while you are throttled. — Peter Csala, Oct 05 '21 at 07:18

score 0 · Answer 1 · answered Oct 05 '21 at 07:10

0

With the HandleResult you can specify the trigger, which basis on the decorated method's returned value.

AsyncRetryPolicy<ApiResultResponse> retryPolicy = Policy
       .HandleResult<ApiResultResponse>(r => r.Success == false && r.Errors?.Count > 0)
       .WaitAndRetryAsync(2, _ => TimeSpan.FromSeconds(2));

References: [1], [2]

answered Oct 05 '21 at 07:10

Peter Csala

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.AddPolicyHandler(), to add a policy at the HTTPClient level, only supports an IAsyncPolicy. Handle Result, which is documented in many cases but this one, does not appear to be a solution here. – xceled Oct 07 '21 at 14:05
@xceled You should not need to register the `ISomeApi` with the policy (`AddPolicyHandler`). You can surround the downstream API call with the policy inside the `ISomeApi` implementation. – Peter Csala Oct 07 '21 at 15:12
@xceled Shall I amend my post to demonstrate what I meant? – Peter Csala Oct 08 '21 at 06:28
Yeah, that is the implementation everyone keeps pointing to. I don't want my downstream code to be concerned with retry logic. I want the it to be at the HttpClient level, which is where I already have my other retry logic. – xceled Oct 08 '21 at 16:03
@xceled As you said AddPolicyHandler was designed for HttpClient, not for refit. So, there is no built-in solution to register your custom retry during ConfigureServices. – Peter Csala Oct 08 '21 at 16:09
@xceled Please check out [this SO thread](https://stackoverflow.com/questions/72370383/using-an-ilogger-in-a-polly-policy-attached-to-a-refit-client) – Peter Csala May 26 '22 at 06:59

Polly - Policy Handler which evaluates transient failures from HTTP response content

1 Answers1