Polly Timeout: How to utilize optimistic timeout on all HttpClient.GetAsync calls when using HttpClientFactory

Summary: What are you wanting to achieve?

This is somewhat related to #907, but within the context of HttpClientFactory. If a CancellationToken is required to be passed to the HttpClient instance for optimistic timeout, how would you go about doing that when nearly everything about the policy setup with HttpClientFactory is implicit? There is no CancellationToken object in sight to reference or pass around!

What code or approach do you have so far?

I have two separate source files here: the main program where all the DI-framework magic with HttpClientFactory happens, and the GitHubService class that actually makes the HTTP calls.

In Program.cs:

var config = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json")
    .Build();
var jitterDelay = Backoff.DecorrelatedJitterBackoffV2(TimeSpan.FromSeconds(1), 6);
var retryPolicy = Policy
    .Handle<RateLimitRejectedException>()
    .OrTransientHttpError()
    .WaitAndRetryAsync(jitterDelay);
var timeoutPolicy = Policy
    .TimeoutAsync<HttpResponseMessage>(10);
var host = new HostBuilder()
    .ConfigureServices(services =>
    {
        services.AddHttpClient<IGitHubService, GitHubService>((sp, client) =>
        {
            client.BaseAddress = new Uri("https://api.github.com");
            client.DefaultRequestHeaders.Add(
                HeaderNames.Accept, "application/vnd.github.v3+json");
            client.DefaultRequestHeaders.Add(
                HeaderNames.UserAgent, "HttpRequestsSample");
            client.Timeout = TimeSpan.FromSeconds(60); // overall timeout
            // pass config file as second argument in GitHubService constructor
            ActivatorUtilities.CreateInstance<GitHubService>(sp, client, config);
        })
        .AddPolicyHandler(retryPolicy);
        .AddPolicyHandler(timeoutPolicy);
    })
    .Build();
var gitHubService = host.Services.GetRequiredService<GitHubService>();
var gitHubZen = await gitHubService.GetZenAsync();
Console.WriteLine(gitHubZen);

In GitHubService.cs:

public class GitHubService : IGitHubService
{
    private readonly HttpClient _httpClient;
    // other private fields to populate via config

    public GitHubService(HttpClient httpClient, IConfiguration config)
    {
        _httpClient = httpClient;
        // populate private fields via config
    }

    public async Task<string> GetZenAsync()
    {
        using var response = await _httpClient.GetAsync("/zen");
        response.EnsureSuccessStatusCode();
        return await response.Content.ReadAsStringAsync();
    }
}

public interface IGitHubService
{
}

Even if I modified GetAsync() to include a CancellationToken, where would it come from? Should it be included in the constructor for GitHubService?

[fsgarin-lacity]

Regarding documentation, we have #853 to update any out-of-date documentation, but the HttpClientFactory integration is a bit of a grey area as the implementation is actually in the ASP.NET Core repo, and there’s further documentation of their own here and here.

Oof, so that explains why a lot of the documentation regarding HttpClientFactory keeps squawking about crap I don’t care about such as PageModel. My use case is simply just a console app that gets scheduled to run at specific times via Task Manager in Windows Server and needs to communicate to API endpoints via HTTP. I also like the idea of having a well-behaved client app that talks to API endpoints, which is how I stumbled upon Polly and HttpClientFactory, which solves the exhausted socket and stale DNS issues.

Anyway, I didn’t realize until you told me that CancellationTokenSource.Token exists. I might just use that for per-call timeouts instead while leaving the other policies in place. I have another use case where the overall timeout in HttpClient is set to 5 minutes and the per-call timeout has to be 1 minute, so I’m guessing that will change the calculus a bit when deciding between pessimistic vs. optimistic.

Thanks for the discussion. As I continue to work through this console app, I’ll make note on which parts of the documentation aren’t clear and give feedback (if any) to #853.

[martincostello]

I’ll make note on which parts of the documentation aren’t clear and give feedback (if any) to https://github.com/App-vNext/Polly/issues/853.

Great, thanks!