Improve performance of your ASP.NET Core application

ASP.NET Core is running on Kestrel, ASP.NET Core’s internal web server. Although really fast Kestrel is not a full blown web server. The idea is that you put another web server(on Windows typically IIS) in front of it that acts as a proxy and forwards the requests to Kestrel.

To get this working in IIS, an AspNetCoreModule exists that achieves this goal. Here is the web.config that configures all of this:

	<?xml version="1.0" encoding="utf-8"?>
	<configuration>
	<!--
	Configure your application settings in appsettings.json. Learn more at https://go.microsoft.com/fwlink/?LinkId=786380
	-->
	<system.webServer>
	<handlers>
	<add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified"/>
	</handlers>
	<aspNetCore processPath="%LAUNCHER_PATH%" arguments="%LAUNCHER_ARGS%" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" forwardWindowsAuthToken="false"/>
	<httpProtocol>
	<customHeaders>
	<!--Is set in startup.cs with services.addcors, so we can't set it here also (Xavier)-->
	<!--<add name="Access-Control-Allow-Origin" value="*" />-->
	<!--<add name="Access-Control-Allow-Methods" value="GET,PUT,POST,DELETE,OPTIONS" />-->
	<!--<add name="Access-Control-Allow-Headers" value="*" />-->
	</customHeaders>
	</httpProtocol>
	</system.webServer>
	</configuration>

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If you look at the configuration, you see that by default all request(path="*" verb="*") are forwarded to Kestrel. This isn't ideal because Kestrel is a lot slower in handling static files than IIS.

A better solution is to only forward the requests that should be handled by ASP.NET Core(by example; path="api/*" verb="*") and let other requests be served by IIS.

The magic trick is to use IIS URL Rewrite rules to forward requests to the wwwroot folder of your ASP.NET Core app:

	<system.webServer>
	<rewrite>
	<rules>
	<rule name="wwwroot-static">
	<match url="([\S]+[.](html|htm|js|css))" />
	<action type="Rewrite" url="wwwroot/{R:1}" />
	</rule>
	</rules>
	</rewrite>
	<handlers>
	<add name="StaticFileModuleHtml" path="*.htm*" verb="*" modules="StaticFileModule" resourceType="File" requireAccess="Read" />
	<add name="StaticFileModuleJs" path="*.js" verb="*" modules="StaticFileModule" resourceType="File" requireAccess="Read" />
	<add name="StaticFileModuleCss" path="*.css" verb="*" modules="StaticFileModule" resourceType="File" requireAccess="Read" />
	<add name="aspNetCore" path="api/*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />
	</handlers>
	<aspNetCore processPath="%LAUNCHER_PATH%" arguments="%LAUNCHER_ARGS%"
	stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" forwardWindowsAuthToken="false"/>
	</system.webServer>

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UPDATE: Rick Strahl did a more complete blog post where he took this approach a few steps further: https://weblog.west-wind.com/posts/2017/Apr/27/IIS-and-ASPNET-Core-Rewrite-Rules-for-AspNetCoreModule