I configured IIS ApplicationInitialization as recommended in the documentation add initializationPage='/warm-up' - asp.net-core

Question

I configured IIS ApplicationInitialization as recommended in the documentation
add initializationPage='/warm-up'.
I implemented a /warm-up endpoint on my app deploy it to both staging and production slots.
When the app starts/restarts/swap the endpoint is NOT called because I can't see it in the logs.
When I hit the endpoint manually, it works fine!
What I'm trying to achieve is:
When I start/restart/swap my app
I want a page (/warm-up) to be called in order to preload the app
So the first call from a real client doesn't have to suffer from the app loading time
Currently, I implemented a service that runs when the app starts (IStartupfilter)
But the app, hence the filter, is not running before a first request hits the server!
So I want to hit the server instance as soon as possible with appInit
We have more than 5 instances at some time of the day

Answer 1

initializationPage is an IIS thing, it will not help you start up the ASP.NET Core application before the first request hits.
Instead, what you will need to do is configure the Application Initialization Module for ASP.NET Core. According to this documentation, you will need to enable the IIS Application Initialization module (which you probably already did if you could configure the initializationPage) and then modify the generated web.config to include the applicationInitialization node to the webServer section:
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<location path="." inheritInChildApplications="false">
<system.webServer>
<handlers>
<add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModuleV2" resourceType="Unspecified" />
</handlers>
<aspNetCore processPath="dotnet" arguments=".\MyApp.dll" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" hostingModel="inprocess" />
<applicationInitialization doAppInitAfterRestart="true" />
</system.webServer>
</location>
</configuration>
This should start the ASP.NET Core application as soon as the IIS website starts, so there should be no delay. You will not need an initialization page then and can just initialize the ASP.NET Core application as the host starts.