HTTP Client and Consuming External APIs in ASP.NET Core

Introduction

ShopNest Weather widget calls OpenWeather API — never new HttpClient() per request (socket exhaustion). Use IHttpClientFactory with typed clients and Polly resilience.

After this article you will

• Register typed HttpClient with IHttpClientFactory
• Deserialize JSON with System.Text.Json
• Add retry, timeout, and circuit breaker
• Pass API keys via configuration securely
• Mock HttpClient in unit tests

Prerequisites

• Article 40 — Pagination, Filtering and Sorting
• ShopNest with EF Core and auth from Modules 2–4

Concept deep-dive

public interface IWeatherClient
{
    Task<WeatherDto?> GetCurrentAsync(string city, CancellationToken ct = default);
}

public class OpenWeatherClient : IWeatherClient
{
    private readonly HttpClient _http;
    public OpenWeatherClient(HttpClient http, IOptions<WeatherOptions> options)
    {
        _http = http;
        _http.BaseAddress = new Uri("https://api.openweathermap.org/data/2.5/");
    }
    public async Task<WeatherDto?> GetCurrentAsync(string city, CancellationToken ct)
    {
        var response = await _http.GetAsync($"weather?q={Uri.EscapeDataString(city)}&appid=...", ct);
        response.EnsureSuccessStatusCode();
        return await response.Content.ReadFromJsonAsync<WeatherDto>(cancellationToken: ct);
    }
}

builder.Services.AddHttpClient<IWeatherClient, OpenWeatherClient>(client =>
{
    client.Timeout = TimeSpan.FromSeconds(10);
})
.AddStandardResilienceHandler(); // .NET 8 Polly integration

Hands-on — ShopNest Weather Integration App

1. WeatherOptions with ApiKey in User Secrets.
2. WeatherController returns cached forecast for ShopNest homepage.
3. Simulate 503 — verify retry then circuit opens.
4. Unit test with MockHttpMessageHandler.

Common errors & best practices

• new HttpClient() in using block per call — DNS/socket issues under load.
• API key in query string logged — prefer header where provider allows.
• No timeout — hung threads block request pool.

Interview questions

Q: Why IHttpClientFactory?
A: Manages HttpClient lifetime and handler rotation — avoids socket exhaustion.

Q: Typed vs named client?
A: Typed binds interface to implementation; named uses string key — typed is cleaner.

Q: Circuit breaker?
A: Stops calling failing external API temporarily — fail fast protects ShopNest.

Summary

• IHttpClientFactory is the correct HttpClient pattern
• Typed clients + options for OpenWeather integration
• Resilience handlers add retry and circuit breaker
• Mock handlers enable fast unit tests without network

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