How to use the retry-after header to poll API using asp.net http client

user_6787447 0
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I'm kind of new to RESTful consumption using http client in .net and i'm having trouble understanding how to use the retry-after header when polling an external API.

This is what i have to poll currently:

HttpResponseMessage result = null;
var success = false;
var maxAttempts = 7;
var attempts = 0;

using (var client = new HttpClient()) 
{
    do
    {
        var url = "https://xxxxxxxxxxxxxxx";
        result = await client.GetAsync(url);

        attempts++;

        if(result.StatusCode == HttpStatusCode.OK || attempts == maxAttempts) 
           success = true;
    }
    while (!success);
}

return result;

As you can see, i keep polling the endpoint until i've either got an OK response or the max attempts has been reached (to stop continuous looping).

How can i use the retry-after header from the response i get to dictate how long i wait between each of the calls in the loop?

I just cant work out how to apply this to my situation.

Thanks,

httpclient http .net c#
1 Answers

HttpClient is intended to be instantiated once per application, rather than per-use

private static HttpClient client = new HttpClient();

The method (updated with HTTP Host Header usage)

private static async Task<string> GetDataWithPollingAsync(string url, int maxAttempts, string host = null)
{
    using (HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, url))
    {
        if (host?.Length > 0) request.Headers.Host = host;
        for (int attempt = 0; attempt < maxAttempts; attempt++)
        {
            TimeSpan delay = default;
            using (HttpResponseMessage response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead).ConfigureAwait(false))
            {
                if (response.IsSuccessStatusCode)
                    return await response.Content.ReadAsStringAsync().ConfigureAwait(false);
                delay = response.Headers.RetryAfter.Delta ?? TimeSpan.FromSeconds(1);
            }
            await Task.Delay(delay);
        }
    }
    throw new Exception("Failed to get data from server");
}

Usage

try
{
    string result = await GetDataWithPollingAsync("http://some.url", 7, "www.example.com");
    // received
}
catch (Exception ex)
{
    Debug.WriteLine(ex.Message);
    // failed
}
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