Instantiation of ASP.NET Core application in production

Sachin Singh

There is a question on stackoverflow which says

When developing a .NET Core web application, I have to build it locally (by Ctrl-F5 for example). When doing that, Main method is running,So, in which way the remote web server is being informed about Main().

The accepted answer is

Program .cs and Startup .cs are running only the very first time an Http request is being made. It is this first time which configures Kestrel and informs it about everything it has to know for any subsequent request (e.g. routing).

which i think is completely wrong answer. However other answers seems logical

https://stackoverflow.com/questions/49033299/instantiation-of-asp-net-core-application-in-production

If anyone have a clear idea, how the Main() starts in production,where there is no way to start by ctrl + f5, No IIS ISAPI module, No url-rewriting module for Apache.

Note:- was not able to POST the question directly, so asking in answer. @Admin, please let me know the reason.

5 Replies

Know the answer? Post it — somebody with the same question will find it here.

Ranganath Prasad4y ago
Accepted answer
Hi Sachin,

The answer you found in stackoverflow ( which you think is wrong ) is actually the right answer - atleast, when you run your app on IIS In-process hosting model.

The answer you found in stack overflow :
Program .cs and Startup .cs are running only the very first time an Http request is being made. It is this first time which configures Kestrel and informs it about everything it has to know for any subsequent request (e.g. routing).

Proof :
I hosted a .NET Core Web Api ( 3.1 ) on IIS
Added a static test variable in both Main method and Startup constructor, and wrote an action method that returns these variable values.
public class Program
   {
       public static void Main(string[] args)
       {
           mainMethodExecAt = DateTime.Now.ToString();  // Notice this
           CreateHostBuilder(args).Build().Run();
       }

       public static IHostBuilder CreateHostBuilder(string[] args) =>
           Host.CreateDefaultBuilder(args)
               .ConfigureWebHostDefaults(webBuilder =>
               {
                   webBuilder.UseStartup();
               });

       public static string mainMethodExecAt = null;   // Notice this

   }
public class Startup
   {

       public static string startupExecAt = null;  // Notice this
       public Startup(IConfiguration configuration)
       {
           Configuration = configuration;
           startupExecAt = DateTime.Now.ToString();   // Notice this
       }
[HttpGet]
      public String Get(string title)
      {
           return $"Main Method Execution : {Program.mainMethodExecAt} and Startup Execution At : {Startup.startupExecAt}";
      }
basically, i am capturing timestamp of Main method and Startup constructor execution in a couple of static variables and then returning these variables upon calling the action method.

I started my application in IIS at 9:42:00 PM and then have issued a request to my controller action at 9:45:26 PM, and below is the reponse :

Main Method Execution : 1/14/2022 9:45:26 PM and Startup Execution At : 1/14/2022 9:45:27 PM

As you can see the values match with the first Http Request and NOT the App start time.

And who is responsible for calling these methods? Its Asp.Net Core Module. It is this module that takes the Http Request from the user and calls the Main method and the Startup if not already done.

More info here : https://docs.microsoft.com/en-us/aspnet/core/host-and-deploy/iis/?view=aspnetcore-3.1

The flow is a bit different for other hosting options. Let me know if you are interested to know.
Ranganath Prasad4y ago
You are right @Sachin. In this case ( Nginx --> Kestrel ), its our responsiblity to start the app. In my experience, i have always seen a service file doing that - which means Main and Startup gets executed during app startup in this case. There might be a way to configure to Nginx to trigger the 'dotnet ****.dll' command, but i am not aware of that ( not an expert of Nginx config :) ). And even if there is a way, i dont see any benefit. Just my thought.
Sachin Singh4y ago
@Ranganath and @Rijwan,
suppose the Nginx is configured as below (default setup)
server {
    listen        80;
    server_name   example.com *.example.com;
    location / {
        proxy_pass         http://127.0.0.1:5000;
        proxy_http_version 1.1;
        proxy_set_header   Upgrade $http_upgrade;
        proxy_set_header   Connection keep-alive;
        proxy_set_header   Host $host;
        proxy_cache_bypass $http_upgrade;
        proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header   X-Forwarded-Proto $scheme;
    }
}
so, it will forward all requests coming on port 80 to the ASP.NET Core app running the Kestrel on  127.0.0.1:5000
This works.
But for this to happen, first, the App should be running, then only the Main() will run and the Kestrel will start listening to the requests.
Now there are 2 ways:- run the App using dotnet.exe bootstrapper, manually.
or by Creating a service file using systemd , which is more convenient.
Am I correct? There is no way for Nginx or .NET SDK to fire dotnet internally when the very first request arrives.
as in the case of windows and IIS, Asp.Net Core Module can start the App.
Sachin Singh4y ago
Thanks, @Ranganath, for your detailed answer, much appriciated. Yes, can you please explain for other OS like for reverse proxy, how and when the Main() is called?
Rijwan Ansari4y ago
Check if this article can help you.
https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/web-host?view=aspnetcore-6.0