The Kestrel web server is relatively a new web server which is part of ASP.NET Core. It is now becoming the preferred web server for all new ASP.NET applications. 

In this article, I will briefly review / explain what is kestrel web server, how to use it, and the differences between Kestrel vs IIS.

Why Do We Really Need the New Kestrel Web Server When We Already Have IIS?

1. A Great Opportunity to Start All Over

If you have been developing ASP.NET applications for a while, you are probably familiar with Internet Information Services (IIS). This web server does literally anything and everything as a web server such as having the ability to be configured with ASP.NET handlers & modules via the ASP.NET integrated pipeline and having a very robust management APIs for configuration and deployment. In addition to the above, IIS also serve as FTP server.

However, without disputing the fact that IIS does everything, it is not the fastest web server around. Lightweight web servers like Node.js and Netty are extremely faster than IIS; This resulted to the Creation of a simple and fast web server for Asp.net Core known as Kestrel web server from scratch by the .NET community. Kestrel is more than just a new web server. ASP.NET Core & Kestrel combined are a whole new request pipeline for how ASP.NET requests work. Things like HTTP modules & handlers have been replaced with simple middleware. The entire System.Web namespace is gone. Another big advantage of designing this web server was to take advantage of asynchronous programming. This makes Performance a cool feature of ASP.NET application.

2. Built for High Performance (Speed)

One of the big problems with IIS and the existing ASP.NET pipeline was its performance. For most real-world applications, the performance of ASP.NET application is perfectly fine. However, it lagged way behind in benchmarks. The combination of Kestrel & ASP.NET Core has been shown to be many times faster.

Please note that benchmarking an ASP.NET request that displays “hello world” is not comparable to most real applications that do multiple SQL queries, cache calls, and web service calls in a single request. ASP.NET makes it easy to do most I/O operations asynchronously. ASP.NET Core & Kestrel have been designed from the scratch to take advantage of async/await.

2. Cross Platform

If the goal was to get ASP.NET application to run on a Linux platform, that meant porting IIS to Linux or making ASP.NET work without IIS; this was impossible in the past until. But with kestrel, this problem is solved. As a developer, I can write my ASP.NET application and deploy it to Windows or Linux either one. Kestrel works as my web server on both. However, it is still recommended to use IIS, Apache, or NGINX as a reverse proxy in front of it.

Comparison Between Kestrel Web Server and IIS

IIS does almost everything no doubt. Kestrel However, does as little as possible. Because of this, Kestrel is much faster but also lacks a lot of vital functionality. I would think of Kestrel as really more of an application server. It is recommended to use another web server in front of it for public web applications. Kestrel is designed to run ASP.NET as fast as possible. But  it still relies heavily on a full-fledged web server like IIS, Nginx, or Apache to do deal with things like security, management, etc.

Microsoft suggests to always use another web server in front of a Kestrel web server for public websites.

Feature Comparison for Kestrel vs IIS

Here is an IIS vs Kestrel comparison of some key features. This should help you better understand the limitations of Kestrel. You can overcome these limitations by pairing it up with IIS or NGINX.

From the above snippet, Port Sharing is a big limitation of Kestrel web server! This means that you can’t host “webite1.destrotech.com.ng” and “webite2.destrotech.com.ng” both on the same port 80 and as well as on the same web server running different ASP.NET applications.

In addition, Kestrel Only works with internal SSL traffic between the reverse proxy web server (IIS or Nginx for example) and ASP.NET application.

See below link to learn more about ASP.NET server hosting technologies on the Microsoft docs: Web server implementations in ASP.NET Core