Use IHttpClientFactory to implement resilient HTTP requests
This content is an excerpt from the eBook, .NET Microservices Architecture for Containerized .NET Applications, available on .NET Docs or as a free downloadable PDF that can be read offline.
Issues with the original HttpClient class available in .NET
The original and well-known HttpClient class can be easily used, but in some cases, it isn't being properly used by many developers.
Though this class implements
IDisposable, declaring and instantiating it within a
using statement is not preferred because when the
HttpClient object gets disposed of, the underlying socket is not immediately released, which can lead to a socket exhaustion problem. For more information about this issue, see the blog post You're using HttpClient wrong and it's destabilizing your software.
HttpClient is intended to be instantiated once and reused throughout the life of an application. Instantiating an
HttpClient class for every request will exhaust the number of sockets available under heavy loads. That issue will result in
SocketException errors. Possible approaches to solve that problem are based on the creation of the
HttpClient object as singleton or static, as explained in this Microsoft article on HttpClient usage. This can be a good solution for short-lived console apps or similar, that run a few times a day.
Another issue that developers run into is when using a shared instance of
HttpClient in long-running processes. In a situation where the HttpClient is instantiated as a singleton or a static object, it fails to handle the DNS changes as described in this issue of the dotnet/runtime GitHub repository.
However, the issue isn't really with
HttpClient per se, but with the default constructor for HttpClient, because it creates a new concrete instance of HttpMessageHandler, which is the one that has sockets exhaustion and DNS changes issues mentioned above.
To address the issues mentioned above and to make
HttpClient instances manageable, .NET Core 2.1 introduced two approaches, one of them being IHttpClientFactory. It's an interface that's used to configure and create
HttpClient instances in an app through Dependency Injection (DI). It also provides extensions for Polly-based middleware to take advantage of delegating handlers in HttpClient.
The alternative is to use
SocketsHttpHandler with configured
PooledConnectionLifetime. This approach is applied to long-lived,
static or singleton
HttpClient instances. To learn more about different strategies, see HttpClient guidelines for .NET.
Polly is a transient-fault-handling library that helps developers add resiliency to their applications, by using some pre-defined policies in a fluent and thread-safe manner.
Benefits of using IHttpClientFactory
- Provides a central location for naming and configuring logical
HttpClientobjects. For example, you may configure a client (Service Agent) that's pre-configured to access a specific microservice.
- Codify the concept of outgoing middleware via delegating handlers in
HttpClientand implementing Polly-based middleware to take advantage of Polly's policies for resiliency.
HttpClientalready has the concept of delegating handlers that could be linked together for outgoing HTTP requests. You can register HTTP clients into the factory and you can use a Polly handler to use Polly policies for Retry, CircuitBreakers, and so on.
- Manage the lifetime of HttpMessageHandler to avoid the mentioned problems/issues that can occur when managing
HttpClient instances injected by DI can be disposed of safely, because the associated
HttpMessageHandler is managed by the factory. Injected
HttpClient instances are Transient from a DI perspective, while
HttpMessageHandler instances can be regarded as Scoped.
HttpMessageHandler instances have their own DI scopes, separate from the application scopes (for example, ASP.NET incoming request scopes). For more information, see Using HttpClientFactory in .NET.
The implementation of
DefaultHttpClientFactory) is tightly tied to the DI implementation in the
Microsoft.Extensions.DependencyInjection NuGet package. If you need to use
HttpClient without DI or with other DI implementations, consider using a
static or singleton
PooledConnectionLifetime set up. For more information, see HttpClient guidelines for .NET.
Multiple ways to use IHttpClientFactory
There are several ways that you can use
IHttpClientFactory in your application:
- Basic usage
- Use Named Clients
- Use Typed Clients
- Use Generated Clients
For the sake of brevity, this guidance shows the most structured way to use
IHttpClientFactory, which is to use Typed Clients (Service Agent pattern). However, all options are documented and are currently listed in this article covering the
How to use Typed Clients with IHttpClientFactory
So, what's a "Typed Client"? It's just an
HttpClient that's pre-configured for some specific use. This configuration can include specific values such as the base server, HTTP headers or time outs.
The following diagram shows how Typed Clients are used with
Figure 8-4. Using
IHttpClientFactory with Typed Client classes.
In the above image, a
ClientService (used by a controller or client code) uses an
HttpClient created by the registered
IHttpClientFactory. This factory assigns an
HttpMessageHandler from a pool to the
HttpClient can be configured with Polly's policies when registering the
IHttpClientFactory in the DI container with the extension method AddHttpClient.
To configure the above structure, add IHttpClientFactory in your application by installing the
Microsoft.Extensions.Http NuGet package that includes the AddHttpClient extension method for IServiceCollection. This extension method registers the internal
DefaultHttpClientFactory class to be used as a singleton for the interface
IHttpClientFactory. It defines a transient configuration for the HttpMessageHandlerBuilder. This message handler (HttpMessageHandler object), taken from a pool, is used by the
HttpClient returned from the factory.
In the next snippet, you can see how
AddHttpClient() can be used to register Typed Clients (Service Agents) that need to use
//Add http client services at ConfigureServices(IServiceCollection services)
Registering the client services as shown in the previous snippet, makes the
DefaultClientFactory create a standard
HttpClient for each service. The typed client is registered as transient with DI container. In the preceding code,
AddHttpClient() registers CatalogService, BasketService, OrderingService as transient services so they can be injected and consumed directly without any need for additional registrations.
You could also add instance-specific configuration in the registration to, for example, configure the base address, and add some resiliency policies, as shown in the following:
builder.Services.AddHttpClient<ICatalogService, CatalogService>(client =>
client.BaseAddress = new Uri(builder.Configuration["BaseUrl"]);
In this next example, you can see the configuration of one of the above policies:
static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy()
.OrResult(msg => msg.StatusCode == System.Net.HttpStatusCode.NotFound)
.WaitAndRetryAsync(6, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));
You can find more details about using Polly in the Next article.
Each time you get an
HttpClient object from the
IHttpClientFactory, a new instance is returned. But each
HttpClient uses an
HttpMessageHandler that's pooled and reused by the
IHttpClientFactory to reduce resource consumption, as long as the
HttpMessageHandler's lifetime hasn't expired.
Pooling of handlers is desirable as each handler typically manages its own underlying HTTP connections; creating more handlers than necessary can result in connection delays. Some handlers also keep connections open indefinitely, which can prevent the handler from reacting to DNS changes.
HttpMessageHandler objects in the pool have a lifetime that's the length of time that an
HttpMessageHandler instance in the pool can be reused. The default value is two minutes, but it can be overridden per Typed Client. To override it, call
SetHandlerLifetime() on the IHttpClientBuilder that's returned when creating the client, as shown in the following code:
//Set 5 min as the lifetime for the HttpMessageHandler objects in the pool used for the Catalog Typed Client
Each Typed Client can have its own configured handler lifetime value. Set the lifetime to
InfiniteTimeSpan to disable handler expiry.
Implement your Typed Client classes that use the injected and configured HttpClient
As a previous step, you need to have your Typed Client classes defined, such as the classes in the sample code, like 'BasketService', 'CatalogService', 'OrderingService', etc. – A Typed Client is a class that accepts an
HttpClient object (injected through its constructor) and uses it to call some remote HTTP service. For example:
public class CatalogService : ICatalogService
private readonly HttpClient _httpClient;
private readonly string _remoteServiceBaseUrl;
public CatalogService(HttpClient httpClient)
_httpClient = httpClient;
public async Task<Catalog> GetCatalogItems(int page, int take,
int? brand, int? type)
var uri = API.Catalog.GetAllCatalogItems(_remoteServiceBaseUrl,
page, take, brand, type);
var responseString = await _httpClient.GetStringAsync(uri);
var catalog = JsonConvert.DeserializeObject<Catalog>(responseString);
The Typed Client (
CatalogService in the example) is activated by DI (Dependency Injection), which means it can accept any registered service in its constructor, in addition to
A Typed Client is effectively a transient object, that means a new instance is created each time one is needed. It receives a new
HttpClient instance each time it's constructed. However, the
HttpMessageHandler objects in the pool are the objects that are reused by multiple
Use your Typed Client classes
Finally, once you have your typed classes implemented, you can have them registered and configured with
AddHttpClient(). After that you can use them wherever services are injected by DI, such as in Razor page code or an MVC web app controller, shown in the below code from eShopOnContainers:
public class CatalogController : Controller
private ICatalogService _catalogSvc;
public CatalogController(ICatalogService catalogSvc) =>
_catalogSvc = catalogSvc;
public async Task<IActionResult> Index(int? BrandFilterApplied,
var itemsPage = 10;
var catalog = await _catalogSvc.GetCatalogItems(page ?? 0,
//… Additional code
Up to this point, the above code snippet only shows the example of performing regular HTTP requests. But the 'magic' comes in the following sections where it shows how all the HTTP requests made by
HttpClient can have resilient policies such as retries with exponential backoff, circuit breakers, security features using auth tokens, or even any other custom feature. And all of these can be done just by adding policies and delegating handlers to your registered Typed Clients.
HttpClient guidelines for .NET
Using HttpClientFactory in .NET
Using HttpClientFactory in ASP.NET Core
HttpClientFactory source code in the
Polly (.NET resilience and transient-fault-handling library)