static async Task<string> BasicUsage()
{
  var serviceCollection = new ServiceCollection();
  serviceCollection.AddHttpClient();
  var services = serviceCollection.BuildServiceProvider();
  var clientFactory = services.GetService<IHttpClientFactory>();

  var client = clientFactory.CreateClient();
  var request = new HttpRequestMessage(HttpMethod.Get, "https://www.github.com");

  var response = await client.SendAsync(request).ConfigureAwait(false);

  return response.StatusCode.ToString();
}

Console.WriteLine($"BasicUsage, StatusCode = {BasicUsage().GetAwaiter().GetResult()}");

public void ConfigureServices(IServiceCollection services)
{
  var fallbackResponse = new HttpResponseMessage();
  fallbackResponse.Content = new StringContent("fallback");
  fallbackResponse.StatusCode = System.Net.HttpStatusCode.TooManyRequests;

  services.AddHttpClient("cb", x =>
  {
    x.BaseAddress = new Uri("http://localhost:8000");
    x.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Test");
  })
  //fallback
  .AddPolicyHandler(Policy<HttpResponseMessage>.Handle<Exception>().FallbackAsync(fallbackResponse, async b =>
  {
    Logger.LogWarning($"fallback here {b.Exception.Message}");
  }))
  //circuit breaker
  .AddPolicyHandler(Policy<HttpResponseMessage>.Handle<Exception>().CircuitBreakerAsync(2, TimeSpan.FromSeconds(4), (ex, ts) =>
  {
    Logger.LogWarning($"break here {ts.TotalMilliseconds}");
  }, () =>
  {        
    Logger.LogWarning($"reset here ");
  }))
  //timeout
  .AddPolicyHandler(Policy.TimeoutAsync<HttpResponseMessage>(1));
   
  services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
}

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
  private static int myCount = 0;

  private readonly IHttpClientFactory _clientFactory;

  public ValuesController(IHttpClientFactory clientFactory)
  {
    this._clientFactory = clientFactory;
  }

  // GET api/values/timeout
  [HttpGet("timeout")]
  public ActionResult<IEnumerable<string Timeout()
  {
    if (myCount < 3)//模拟超时
    {
      System.Threading.Thread.Sleep(3000);
    }
    myCount++;

    return new string[] { "value1", "value2" };
  }

  // GET api/values
  [HttpGet("")]
  public async Task<string> GetAsync()
  {
    var client = _clientFactory.CreateClient("cb");
    
    var request = new HttpRequestMessage(HttpMethod.Get, "/api/values/timeout");
    var response = await client.SendAsync(request);
    var content = await response.Content.ReadAsStringAsync();

    return content;
  }
}