Introduction

Handling HTTP requests in ASP.NET Core applications can be complex, especially when managing HttpClient correctly. IHttpClientFactory, introduced in .NET Core 2.1, addresses many challenges, making HTTP requests more reliable, efficient, and easier to configure.

Risks of Misusing HttpClient

Reusing a Singleton or Static HttpClient

It’s tempting to declare a single HttpClient instance as static or within a singleton, as this avoids the overhead of creating new instances. However, this approach has significant downsides:

• DNS Changes: HttpClient caches DNS lookups, meaning if the target domain changes its IP, your application may keep sending requests to an outdated address unless restarted.
• Socket Exhaustion: Persistent HttpClient instances maintain long-lived connections, which can lead to socket exhaustion in high-traffic applications. This issue arises from TCP connections staying in a TIME_WAIT state, rendering them unusable for new connections.

Disposing HttpClient Too Often

A common alternative mistake is disposing of HttpClient after every use, typically within a using block. This seems efficient but can also create issues:

• Port Exhaustion: Disposing HttpClient doesn’t close its underlying TCP connection immediately. The system leaves connections in a TIME_WAIT state, which can lead to running out of available ports under heavy load.

Why Use IHttpClientFactory?

IHttpClientFactory provides several benefits over manually managing HttpClient instances:

1. Automatic Connection Management: By pooling and recycling HttpMessageHandler instances, IHttpClientFactory ensures that connections are reused efficiently, avoiding common pitfalls like DNS caching issues and socket exhaustion.

2. Simplified Configuration: IHttpClientFactory allows you to configure clients globally or on a per-client basis. You can define default headers, base URLs, and timeouts in a central place, simplifying the management of HttpClient instances.

3. Resilience and Fault Tolerance: You can integrate policies like retries and circuit breakers through libraries like Polly, enhancing the resilience of your application’s HTTP requests. This makes handling transient failures or network outages much cleaner.

How to Use IHttpClientFactory

Named and Typed Clients

With IHttpClientFactory, you can define named clients for different services. This helps separate configuration based on use case, such as different APIs:

services.AddHttpClient("GitHub", client =>
{
    client.BaseAddress = new Uri("https://api.github.com/");
    client.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");
});

You can also use typed clients, where the configuration is injected into the service directly, reducing the need to manage names and strings:

services.AddHttpClient<MyTypedClient>();

Getting a Client After Configuration

For named clients:

public class GitHubService
{
    private readonly IHttpClientFactory _clientFactory;
    public GitHubService(IHttpClientFactory clientFactory)
    {
        _clientFactory = clientFactory;
    }

    public async Task<string> GetRepositoryInfoAsync(string owner, string repo)
    {
        var client = _clientFactory.CreateClient("GitHub");
        var response = await client.GetAsync($"/repos/{owner}/{repo}");
        return await response.Content.ReadAsStringAsync();
    }
}

For typed clients:

public class MyTypedClient
{
    private readonly HttpClient _client;
    public MyTypedClient(HttpClient client)
    {
        _client = client;
    }

    public async Task<string> GetDataAsync()
    {
        return await _client.GetStringAsync("api/data");
    }
}

Conclusion

IHttpClientFactory is essential for modern ASP.NET Core applications, helping manage HttpClient instances efficiently and avoiding issues like DNS caching and socket exhaustion. Its features—connection pooling, simplified configuration, and enhanced resilience—make it a vital tool for developers building reliable, high-performance applications.

For more detailed information, see the Microsoft Learn documentation on using IHttpClientFactory.