HttpClient & HttpMessageHandler & HttpMessageHandler
Make HTTP requests using IHttpClientFactory in ASP.NET Core | Microsoft Docs
HttpClient and lifetime management
A new HttpClient
instance is returned each time CreateClient
is called on the IHttpClientFactory
.
An HttpMessageHandler is created per named client. The factory manages the lifetimes of the HttpMessageHandler
instances.
IHttpClientFactory
pools the HttpMessageHandler
instances created by the factory to reduce resource consumption. An HttpMessageHandler
instance may be reused from the pool when creating a new HttpClient
instance if its 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 (Domain Name System) changes.
The default handler lifetime is two minutes. The default value can be overridden on a per named client basis:
builder.Services.AddHttpClient("HandlerLifetime") .SetHandlerLifetime(TimeSpan.FromMinutes(5));
HttpClient
instances can generally be treated as .NET objects not requiring disposal.
Disposal cancels outgoing requests and guarantees the given HttpClient
instance can't be used after calling Dispose.
IHttpClientFactory
tracks and disposes resources used by HttpClient
instances.
Keeping a single HttpClient
instance alive for a long duration is a common pattern used before the inception of IHttpClientFactory
. This pattern becomes unnecessary after migrating to IHttpClientFactory
.
Alternatives to IHttpClientFactory
Using IHttpClientFactory
in a DI-enabled app avoids:
- Resource exhaustion problems by pooling
HttpMessageHandler
instances. - Stale DNS problems by cycling
HttpMessageHandler
instances at regular intervals.
There are alternative ways to solve the preceding problems using a long-lived SocketsHttpHandler instance.
- Create an instance of
SocketsHttpHandler
when the app starts and use it for the life of the app. - Configure PooledConnectionLifetime to an appropriate value based on DNS refresh times.
- Create
HttpClient
instances usingnew HttpClient(handler, disposeHandler: false)
as needed.
The preceding approaches solve the resource management problems that IHttpClientFactory
solves in a similar way.
- The
SocketsHttpHandler
shares connections acrossHttpClient
instances. This sharing prevents socket exhaustion. - The
SocketsHttpHandler
cycles connections according toPooledConnectionLifetime
to avoid stale DNS problems.