Polly won't throw on some exceptions?

Question

I'm using Polly with .net Core. My ConfigureServices is :

private static void ConfigureServices()
{
    var collection = new ServiceCollection();
    var timeoutPolicy = Policy.TimeoutAsync<HttpResponseMessage>(3);  
    collection.AddHttpClient<INetworkService, NetworkService>(s=>
             {
                   s.BaseAddress = new Uri("http://google.com:81"); //this is a deliberate timeout url
             })
    .AddPolicyHandler((a,b)=>GetRetryPolicy(b))
    .AddPolicyHandler(timeoutPolicy); ;
    ...
}

This is the GetRetryPolicy function:

private static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy(HttpRequestMessage req)
{
    return HttpPolicyExtensions
        .HandleTransientHttpError()
        .OrResult(msg => httpStatusCodesWorthRetrying.Contains(msg.StatusCode))  // see below
        .Or<TimeoutRejectedException>()
        .Or<TaskCanceledException>()
        .Or<OperationCanceledException>()
        .WaitAndRetryAsync(3, retryAttempt =>
        {
            return TimeSpan.FromSeconds(3);
        }, onRetry: (response, delay, retryCount, context) =>
        {

            Console.WriteLine($"______PollyAttempt_____ retryCount:{retryCount}__FOR_BaseUrl_{req.RequestUri.ToString()}");
        });
}

Those are the httpcodes I want to retry :

static HttpStatusCode[] httpStatusCodesWorthRetrying = {
   HttpStatusCode.RequestTimeout, // 408
   HttpStatusCode.InternalServerError, // 500
   HttpStatusCode.BadGateway, // 502
   HttpStatusCode.ServiceUnavailable, // 503
   HttpStatusCode.GatewayTimeout // 504
};

Ok. And this is the actual invocation :

public async Task Work()
    {

        try
        {
            
            HttpResponseMessage response = await _httpClient.GetAsync("");
            Console.WriteLine("After work");

        }
        catch (TimeoutRejectedException ex)
        {
            Console.WriteLine("inside TimeoutRejectedException");
        }

        catch (Exception ex)
        {
            Console.WriteLine("inside catch main http");
        }
}

The output is :

_PollyAttempt retryCount:1__FOR_BaseUrl_http://google.com:81/
_PollyAttempt retryCount:2__FOR_BaseUrl_http://google.com:81/
_PollyAttempt retryCount:3__FOR_BaseUrl_http://google.com:81/
inside TimeoutRejectedException

(notice it throws) Which is OK. Because Polly throws after this invalid URL is timeout.

But if I change the http://google.com:81/ to an "internal server error" url : (this return 500)

https://run.mocky.io/v3/9f1b4c18-2cf0-4303-9136-bb67d54d0148

Then it doesn't throw but continues :

_PollyAttempt retryCount:1__FOR_BaseUrl_https://run.mocky.io/v3/9f1b4c18-2cf0-4303-9136-bb67d54d0148
_PollyAttempt retryCount:2__FOR_BaseUrl_https://run.mocky.io/v3/9f1b4c18-2cf0-4303-9136-bb67d54d0148
_PollyAttempt retryCount:3__FOR_BaseUrl_https://run.mocky.io/v3/9f1b4c18-2cf0-4303-9136-bb67d54d0148
After work

(notice "after work" at the end)

Question:

Why does Polly throw at timeout, But doesn't throw at another condition ? I explictly wrote : .OrResult(msg => httpStatusCodesWorthRetrying.Contains(msg.StatusCode)) and 500 is one of them.

Answer 1

As @StephenCleary said that's how the Polly works.

First let me share with you the cleaned up version of your code
then I will give you some explanation about the observed behaviours.

ConfigureServices

var timeoutPolicy = Policy.TimeoutAsync<HttpResponseMessage>(3,
    onTimeoutAsync: (_, __, ___) => {
        Console.WriteLine("Timeout has occured");
        return Task.CompletedTask;
});

services.AddHttpClient<INetworkService, NetworkService>(
    client => client.BaseAddress = new Uri("https://httpstat.us/500"))
.AddPolicyHandler((_, request) => Policy.WrapAsync(GetRetryPolicy(request), timeoutPolicy));

GetRetryPolicy

private static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy(HttpRequestMessage req)
    => HttpPolicyExtensions
        .HandleTransientHttpError()
        .Or<TimeoutRejectedException>()
        .Or<OperationCanceledException>()
        .WaitAndRetryAsync(3,
            _ => TimeSpan.FromSeconds(3),
            onRetry: (_, __, retryCount, ___) =>
                Console.WriteLine($"POLLY retryCount:{retryCount} baseUrl: {req.RequestUri}"));

---

Example http://google.com:81

1. Initial request has been sent out
2. No response has been received under 3 seconds
3. Timeout Policy triggered
4. TimeoutRejectedException is thrown
5. Retry policy is aware of that exception, so it triggers
6. Retry policy issues 3 seconds penalty
7. Retry policy issues a new request
8. No response has been received under 3 seconds
9. ...
   n. Retry policy is aware of that exception, but it has reached the max retry count
   n+1. Retry throws the exception that it could not handle, so in this case the TimeoutRejectedException

Example https://httpstat.us/500

1. Initial request has been sent out
2. A response with status code 500 has received under 3 seconds
3. Retry policy is aware of that status code, so it triggers
4. Retry policy issues 3 seconds penalty
5. Retry policy issues a new request
6. response with status code 500 has received under 3 seconds
7. ...
   n. Retry policy is aware of that status code, but it has reached the max retry count
   n+1. Retry returns with that response that it could not handle, so in this case the 500

Because there is a lack of EnsureSuccessStatusCode method call that's why no exception is being thrown.

As you can see in the second example the TimeoutPolicy is not triggered at all.

Answer 2

This is expected behavior. A delegate invocation results in either an exception or a return value. When the Polly retries are done, then it propagates whatever result was last, whether it is an exception or a return value.

In this case, the response would have a 500 status code.