The hosting model for ASP.NET Core is dramatically different from previous versions of ASP.NET. This is also one area where I’ve seen a fair amount of misunderstanding.
ASP.NET Core is a set of libraries you can install into a project using the NuGet package manager. One of the packages you might install for HTTP message processing is a package named Microsoft.AspNetCore.Server.Kestrel. The word server is in the name because this new version of ASP.NET includes its own web servers, and the featured server has the name Kestrel.
In the animal kingdom, a Kestrel is a bird of prey in the falcon family, but in the world of ASP.NET, Kestrel is a cross-platform web server. Kestrel builds on top of libuv, a cross-platform library for asynchronous I/O. libuv gives Kestrel a consistent streaming API to use across Windows and Linux. You also have the option of plugging in a server based on the Windows HTTP Server API (Web Listener), or writing your own IServer implementation. Without good reason, you’ll want to use Kestrel by default.
An Overview of How It Works
You can configure the server for your application in the entry point of the application. There is no Application_Start event in this new version of ASP.NET, nor is there any default XML configuration files. Instead, the start of the application is a static Main method, and configuration lives in the code.
public class Program
public static void Main(string args)
var host = new WebHostBuilder()
If you are looking at the above Program class with a static Main method and thinking the code looks like what you would see in a .NET console mode application, then you are thinking correctly. Compiling an ASP.NET project still produces a .dll file, but with .NET Core we launch the web server from the command line with the dotnet command line interface. The dotnet host will ultimately call into the Main method. In this way of working, .NET Core resembles environments like Java, Ruby, and Python.
If you are working on ASP.NET Core from Visual Studio, then you might never see the command line. Visual Studio continues to do a job it has always done, which is to hide some of the lower level details. With Visual Studio, you can set the application to run with Kestrel as a direct host, or to run the application in IIS Express (the default setting). In both cases, the dotnet host and Kestrel server are in play, even when using IIS Express. This brings us to the topic of running applications in production.
ASP.NET Core Applications in Production
One you realize that ASP.NET includes a cross-platform host and web server, you might think you have all the pieces you need to push to production. There is some truth to this line of thought. Once you’ve invoked the Run method on the WebHost object in the above code, you have a running web server that will listen to HTTP requests and can work on everything from a 32 core Linux server to a Raspberry Pi. However, Microsoft strongly suggests using a hardened reverse proxy in front of your Kestrel server in production. The proxy could be IIS on Windows, or Apache or NGINX.
Why the reverse proxy? In short because technologies like IIS and Apache have been around for over 20 years and have seen all the evils the Internet can deliver to a network socket. Kestrel, on the other hand, is still a newborn babe. Also, reliable servers require additional infrastructure like careful process management to restart failed applications. Outside of ASP.NET, in the world of Java, Python, Ruby, and NodeJS web apps, you’ll see tools like Phusion Passenger and PM2 work in combination with the reverse proxy. These types of tools provide the watchdog monitoring, logging, balancing, and overall process management needed for a robust server. With ASP.NET Core on Windows you can use IIS to achieve the same goals. HTTP requests will still arrive at IIS first, and IIS can forward requests to the Kestrel application. You can have multiple applications deployed behind a single instance of IIS, and IIS will manage the application and provide logging, request filtering, URL rewrites, and many other useful features. In a way, this isn’t much different than what we’ve done in the past with ASP.NET, but once you dig behind the architectural diagrams, you’ll see the details are very different.
Deploying ASP.NET Core applications to IIS requires a web.config file. ASP.NET Core knows nothing about web.config files. The web.config file only exists to configure IIS in a reverse proxy role. A typical web.config file will look like the following.
<add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />
<aspNetCore processPath="dotnet" arguments=".\TheWebApp.dll" stdoutLogEnabled="false"
stdoutLogFile=".\logs\stdout" forwardWindowsAuthToken="false" />
The web.config file instructs IIS to send requests for all paths and verbs to a new HTTP handler named aspNetCore. This ASP.NET Core Module for IIS is a piece of software you’ll need to install on an IIS server to run ASP.NET Core applications. The ASP.NET Core Module is available with the .NET Core SDK install, or with a special Windows Server Hosting .NET Core installer.
The second bit of the web.config file configures the ASP.NET Core module with instructions on how to start your application, which is to use the same dotnet command we saw earlier. Now, instead of our ASP.NET application running inside of a w3wp.exe IIS worker process, the application will execute inside of a dotnet.exe process.
With a better idea of how ASP.NET Core runs in production, let’s talk about benefits and risks.
One benefit to Kestrel is the ability to execute across different platforms. You can author an ASP.NET application on Windows using Visual Studio and IIS Express, but deploy the application on Linux with Apache in front. In all scenarios, the server is always Kestrel and your application code doesn’t need to change.
Another benefit to Kestrel is the incredible work Microsoft has put into making a blazing fast web server with managed code. Inside the readme file for the ASP.NET Benchmarks repository, you’ll currently find benchmarks showing ASP.NET Core serving five times the number of requests per second as ASP.NET 4.6 on the same hardware. 313,001 requests per second compared to 57, 843.
Of course, benchmark code and benchmark results don’t always reflect how a specific business application will behave. It’s like seeing an F1 racing car manufactured by Toyota on the television and then thinking you’ll find a car that goes 220 mph at the local Toyota dealer. What you will find at the dealer are cars that indirectly benefit from the millions of dollars that Toyota puts into researching new technologies for their F1 cars. The benefits trickle down.
I decided to try some comparative benchmarks of my own. I created three applications with ASP.NET WebForms, ASP.NET MVC 5, and ASP.NET MVC Core. Each application delivers 3kb of HTML to the client using the typical patterns you would find in business applications. For example, using a master page in WebForms and using a Layout page in MVC. For ASP.NET MVC Core, I ran tests against a naked Kestrel server as well as a Kestrel server proxied by IIS.
In my tests, the naked Kestrel server delivered 5 times the throughput of WebForms, and over twice the throughput of MVC 5. Once I moved the Core application behind IIS however, throughput dropped below the level of MVC 5. I know these results might surprise many people who believe that ASP.NET Core is inherently faster and lighter than its predecessors. However, ASP.NET Core’s predecessors were deeply integrated into IIS and could execute inside the same worker process where sockets were open. The current recommended setup adds an additional hop.
Another surprise – switching the ASP.NET Core application to run on the full .NET framework instead of .NET Core resulted in very little change in the throughput numbers. As developers, we want to believe .NET Core is also inherently faster and lighter than the full .NET framework, but for runtime performance in this specific application, the difference appears to be negligible.
I do think we can reasonably expect the performance of ASP.NET Core with IIS to improve in the future, so we’ll leave performance as a benefit.
The biggest risk I’ve seen in the new hosting model is the confusion that results in using IIS with ASP.NET Core. There are ASP.NET related settings in IIS that have no impact on a .NET Core application. These are settings like the pipeline mode (integrated or classic), and the setting to select a version of the .NET framework for a specific AppPool.
Developers and IT operations will need to understand the new deployment strategy and understand which IIS settings are significant and which settings to ignore. Both parties will also need to learn new troubleshooting techniques and diagnose a new set of common errors. 502.5 – “Failed to start process” is a terrifying new common error. One reason for this error is that the web.config file specifies incorrect arguments for the dotnet host.
Currently, I feel the hosting area is one of the bigger areas of risk in ASP.NET Core. However, I think we will be able to mitigate the risks over time through a combination of experience, education, experimentation, and improvements from Microsoft based on customer feedback. It is important for developers and IT operations to look at how to host an ASP.NET core application early and not treat deployment as a known procedure that can wait till the end of the project.