HttpClientFactory for Resilient HTTP Requests

Mind Map Summary

• Topic: HttpClientFactory for Resilient HTTP Requests
• Problems with new HttpClient():
  • Socket Exhaustion: Creating a new HttpClient for each request can exhaust the number of available sockets.
  • DNS Changes: HttpClient does not respect DNS changes, which can cause problems in dynamic environments.
• HttpClientFactory:
  • Manages HttpClient instances: Manages the lifetime of HttpClient instances to avoid the problems with new HttpClient().
  • Named and Typed Clients: Allows you to configure named and typed clients with specific settings, such as base address and default headers.
  • Integration with Polly: Integrates with Polly to add resilience patterns, such as retry and circuit breaker.
• Polly: A .NET resilience and transient-fault-handling library that allows developers to express policies such as Retry, Circuit Breaker, Timeout, Bulkhead Isolation, and Fallback in a fluent and thread-safe manner.

Practice Exercise

Use HttpClientFactory to create a typed client for an external API. Use the Polly extension to add a transient error handling policy that retries failed requests (HTTP 5xx or network errors) up to three times with an exponential backoff.

Answer

1. Typed Client:

public class MyApiClient
{
    private readonly HttpClient _httpClient;

    public MyApiClient(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

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

2. Configure HttpClientFactory and Polly in Program.cs:

using Polly;
using Polly.Extensions.Http;

builder.Services.AddHttpClient<MyApiClient>(client =>
{
    client.BaseAddress = new Uri("https://api.example.com");
})
.AddPolicyHandler(GetRetryPolicy());

static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy()
{
    return HttpPolicyExtensions
        .HandleTransientHttpError()
        .OrResult(msg => msg.StatusCode == System.Net.HttpStatusCode.NotFound)
        .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));
}

Explanation:

• We create a typed client MyApiClient that takes an HttpClient in its constructor.
• We use AddHttpClient<MyApiClient>() to register the typed client with the HttpClientFactory.
• We use AddPolicyHandler() to add a Polly policy to the client.
• The policy handles transient HTTP errors (5xx and 408) and retries the request up to three times with an exponential backoff.