HttpClient Single Instance

Question

User439975351 posted

I have been reading several articles about the best way to use HttpClient. One article suggests a single instance in the Global.asax file. Does anyone have a definitive answer as to whether this is the best way to implement? The example code in the article is:

using System.Net;
using System.Net.Http;
using System.Web.Http;
 
namespace HttpClientGuidance
{
    public class WebApiApplication : System.Web.HttpApplication
    {
        internal static HttpClient httpClientInstance;
 
        protected void Application_Start()
        {
            GlobalConfiguration.Configure(WebApiConfig.Register);            
 
            httpClientInstance = new HttpClient();
            httpClientInstance.DefaultRequestHeaders.ConnectionClose = false;
            
            ServicePointManager.FindServicePoint
            ("some uri").ConnectionLeaseTimeout = 60 * 1000;
            ServicePointManager.FindServicePoint
            ("some other uri").ConnectionLeaseTimeout = 60 * 1000;
            ServicePointManager.FindServicePoint
            ("some other other uri").ConnectionLeaseTimeout = 60 * 1000;
            //etc.....
        }
    }
}

Answer 1

User475983607 posted

The standard .NET documentation covers HttpClient in detail and includes recommended patterns.  

https://docs.microsoft.com/en-us/dotnet/api/system.net.http.httpclient?view=netframework-4.8

This code snippet copied from the docs is perfectly fine.

public class GoodController : ApiController
{
    // OK
    private static readonly HttpClient HttpClient;

    static GoodController()
    {
        HttpClient = new HttpClient();
    }
}

Other options are a Singleton pattern with one Singleton for each service consumed. 

Dependency inject which is the standard in ASP.NET Core.

Answer 2

User439975351 posted

Thanks mgebhard,

So would you say that the placement in global.asax would make sense for application wide usage? Also, do you have any suggestions regarding stale DNS/DNS caching?

Answer 3

User1724605321 posted

Hi 1jus,

You can create in Global.asax  . A Singleton HttpClient does not respect DNS changes .Re-using an instance of HttpClient means that it holds on to the socket until it is closed so if you have a DNS record update occurring on the server the client will never know until that socket is closed . One easy workaround is to set the keep-alive header to false so the socket will be closed after each request, this obviously results in sub-optimal performance but if you do not care , or you can set the `ConnectionLeaseTimeout ` which specifies how long (in ms) the TCP socket can stay open :

ServicePointManager.FindServicePoint(endpoint)  
    .ConnectionLeaseTimeout = (int)TimeSpan.FromMinutes(1).TotalMilliseconds;

You can also need to reduce the DNS cache timeout which we can do by setting the DnsRefreshTimeout on the ServicePointManager class like :

ServicePointManager.DnsRefreshTimeout = (int)1.Minutes().TotalMilliseconds;  

Best Regards,

Nan Yu

Answer 4

User439975351 posted

Thanks Nan Yu :)