Category Archives: .NET Framework

Automatic Reconnection in the Swift SignalR Client

As of version 0.7.0 the Swift SignalR Client supports automatic reconnection. This means that, if configured, the client will try to re-establish the connection to the server if the connection was lost. This post explains how this feature works, how to enable it and what configuration options are available.

Automatic reconnects

There are many scenarios where restoring an interrupted connection automatically is important. For instance, mobile applications very often have to be able to deal with unstable network and it’s crucial for these apps to be resilient to network issues. Conceptually the solution to the problem is simple – when the connection is lost a new connection needs to be started. In case of the Swift SignalR Client, this could always be implemented by the code consuming the client (i.e. on the application side). It turns out however, that in practice, implementing this logic is quite hard. Given that this is a common request and is the implementation is tricky it made sense to add support for automatic reconnection to the client. An important thing to note however is that the client does not offer anything more than just restoring the connection. In other words, the client will make a few attempts to restart the connection if it was stopped due to an error but will not do anything more than that. If reconnecting succeeds the server will treat the connection as a completely new connection and will assign it a new connection id. The client will also not receive messages it might have missed when it was disconnected. Anyone who used the non-Core version of SignalR can notice that this a big change to how reconnection worked in the non-Core version where, upon reconnecting, the server would recognize that the client has reconnected and resend missed messages. This functionality can no longer be implemented in the client for the Core version of SignalR because it requires cooperation from the server side (e.g. the server needs to buffer messages) and that logic does not exist in the Core version of SignalR.

Automatic reconnection is disabled by default. The main reason for this is backward compatibility – existing applications did not expect the connection to try to reconnect automatically so, they could break if the feature was enabled by default. Automatic reconnection requires also a bit of additional work and handle new lifecycle events.

Basics

The easiest way to enable automatic reconnection is to use the new .withAutoReconnect() method available on the HubConnectionBuilder class.

When automatic reconnection is enabled the application may receive two additional events:

  • connectionWillReconnect – invoked when the connection was lost
  • connectionDidReconnect – invoked when the connection was successfully restored

By default, the client will make up to four attempts to restore the connection. The first attempt will be made immediately after the connection was lost. The next three attempts will take place respectively 2, 10 and 30 seconds after the previous unsuccessful attempt. If all four attempts are unsuccessful the client will give up, close the connection and invoke the connectionDidClose event.

When the connection is being restored the client will not allow to invoke any method that tries to send data to the server.

Connection is now restartable

Adding support for automatic reconnection made the connection restartable. Before, once the connection was stopped it was necessary to create a new instance to be able to connect to the server again. This is no longer the case – the same instance can be used to restart connection to the server after it was stopped. This could be especially useful when handling background/foreground transitions.

Advanced Scenarios

The default reconnect configuration can be customized. It is possible to change the number of attempts as well as the time intervals between the attempts. The easiest way to do this is to create a new instance of the DefaultReconnectPolicy class with an array of retry intervals and pass this policy to the .withAutoReconnect() method. The number of retry intervals in the array tells the client how many reconnect attempts it should make, while the interval values indicate the time to wait between the attempts.

If the default reconnect policy is not flexible enough it is possible to go even further and create a custom reconnect policy by creating a class that conforms to the ReconnectPolicy protocol. This protocol has just one method – nextAttemptInterval that takes a RetryContext and returns the time interval telling the client when the next reconnect attempt should happen. The RetryContext instance passed to the nextAttempInterval method contains information about the current reconnect – the time when the reconnect was initiated, the number of failed attempts so far and the original error that triggered the reconnect. To stop further reconnect attempts the nextAttempInterval method should return DispatchTimeInterval.never. To put the policy to work the policy needs to be passed to the .withAutoReconnect() method when configuring the connection.

Backward Compatibility (a.k.a. Kill Switch)

As noted above, writing reconnect logic turned out to be quite tricky. It also required modifying existing code that is executed even when automatic reconnects are disabled. This created a risk of introducing issues into existing scenarios. In case of running into a bug like this it is possible to go back to the previous behavior by using the .withLegacyHttpConnection() method on the HubConnectionBuilder when creating a new hub connection.

Conclusion

These are pretty much all the details needed to be able to use automatic reconnection in the Swift SignalR Client. My hope is that automatic reconnection will make lives of developers much easier.

The SignalR for ASP.NET Core JavaScript Client, Part 2 – Outside the Browser

Last time we looked at using the ASP.NET Core SignalR TypeScript/JavaScript client in the browser. I mentioned, however, that the new client no longer has dependencies that prevent from using it outside the browser. So, today we will try taking the client outside the browser and use it in a NodeJS application. We will add a NodeJS client for the SignalR Chat service we created last time. Initially we will write the client in JavaScript and then we will convert it to TypeScript.

Let’s start from creating a new folder in the SignalRChat repo and adding a new node project:

mkdir SignalRChatNode
cd SignalRChatNode
npm init

We will call the application signarlchatnode and we will leave all other options set to default values. (6425ec1)

Our application will read messages typed by the user and send them to the server. To handle user input we will use node’s readline module. To see that things, work, let’s just add code to prompts the user for the name and displays it in the console. We will use it a starting point of our application (34bc493).

const readline = require('readline');
let rl = readline.createInterface(process.stdin, process.stdout)

rl.question('Enter your name: ', name => {
console.log(name);
  rl.close();
});

To communicate with the SignalR server we need to add the SignalR JavaScript client to the project using the following command (7875c07):

npm install @aspnet/signalr-client --save

We can now try starting the connection like this (3228a10):

const readline = require('readline');
const signalR = require('@aspnet/signalr-client');

let rl = readline.createInterface(process.stdin, process.stdout);

rl.question('Enter your name: ', name => {
  console.log(name);

  let connection = new signalR.HubConnection('http://localhost:5000/chat');
  connection.start()
  .catch(error => {
    console.error(error);
    rl.close();
  });
});

The code looks good but if you try running it, it will immediately fail with the following error:

Error: Failed to start the connection. ReferenceError: XMLHttpRequest is not defined
ReferenceError: XMLHttpRequest is not defined

What happened? The new JavaScript client no longer depends on the browser but still uses standard libraries like XmlHttpRequest or WebSocket to communicate with the server. If these libraries are not provided the client will fail. Fortunately, the required functionality can be easily polyfilled in the NodeJS environment. For now, we will just stick the polyfills on the global object. It’s not beautiful by any means but will do the trick. We are discussing how to make it better in the future but at the moment this is the way to go.

Depending on the features of SignalR you plan to use you will need to provide appropriate polyfills. Currently the absolute minimum is XmlHttpRequest. SignalR client uses it to send the initial OPTIONS HTTP request which initializes the connection on the server side and for the long polling transport. So, if use the long polling transport only, XmlHttpRequest is the only polyfill you will need to provide . If you want to use the WebSockets transport you will need a WebSocket polyfill in addition to XmlHttpRequest. (We are thinking about skipping sending the OPTIONS request for WebSockets. If this is implemented you will not need the XmlHttpRequest polyfill when using the WebSockets transport.) For ServerSentEvents transport you will need an EventSource polyfill. Finally, if you happen to use binary protocols (e.g. MessagePack) over the ServerSentEvent transport you will need polyfills for atob/btoa functions. For simplicity, we will use the WebSocket transport in our application so we will add only polyfills for XmlHttpRequest and WebSockets:

npm install websocket xmlhttprequest --save

and make them available globally via:

XMLHttpRequest = require('xmlhttprequest').XMLHttpRequest;
WebSocket = require('websocket').w3cwebsocket;

If we run the code now we will see something like this:

moozzyk:~/source/SignalRChat/SignalRChatNode$ node index.js
Enter your name: moozzyk
moozzyk
Information: WebSocket connected to ws://localhost:5000/chat?id=0d015ce4-3a78-4313-9343-cb6183a5e8ea
Information: Using HubProtocol 'json'.

which tells us that the client was able to connect successfully to the server. (946f85d)

Now, we need to add some code to handle user input and interact with the server and our Node SignalR Chat client is ready. (I admit that the user interface is not very robust but should be enough for the purpose of this post). You can now talk to browser clients from your node client and vice versa (0f7f71f):

Screen Shot 2017-09-30 at 6.57.14 PM

Now let’s convert our client to TypeScript. We will start from creating a new TypeScript project with tsc --init. In the generated tsconfig.json file we will change the target to es6. We will also add an empty index.ts file and delete the existing index.js file (we will no longer need the index.js file since we will now be generating one by compiling the newly created index.ts). (b83cf92) If you now run tsc you should see an empty index.js file created as a result of compiling the index.ts file.  The last thing to do is to actually convert our JavaScript code to TypeScript. We could just translate it one-to-one but we can do a little better. TypeScript supports async/await which makes writing asynchronous code much easier. Since many of SignalR client methods return Promises we can just await these calls instead of using .then/.catch functions. Here is how our node SignalRChat client written in TypeScript looks like (2a6d0e9):

import * as readline from "readline"
import * as signalR from "@aspnet/signalr-client"

(<any>global).XMLHttpRequest = require("xmlhttprequest").XMLHttpRequest;
(<any>global).WebSocket = require("websocket").w3cwebsocket;

let rl = readline.createInterface(process.stdin, process.stdout);

rl.question("Enter your name: ", async name => {
  console.log(name);
  let connection = new signalR.HubConnection("http://localhost:5000/chat");

  connection.on("broadcastMessage", (name, message) => {
    console.log(`${name}: ${message}`);
    rl.prompt(true);
  });

  try {
    await connection.start();
    rl.prompt();

    rl.on("line", async input => {
      if (input === "!q") {
        console.log("Stopping connection...");
        connection.stop();
        rl.close();
        return;
      }
      await connection.send("send", name, input);
    });
  }
  catch (error) {
    console.error(error);
    rl.close();
  }
});

You can run it by executing the following commands:
tsc
node index.js

Today we learned how to use the ASP.NET Core SignalR client in the NodeJS environment. We created a small node JavaScript application that was able to communicate with browser clients which. Finally, we converted the JavaScript code to TypeScript and learn a little bit about the TypeScript’s async/await feature.

The SignalR for ASP.NET Core JavaScript Client, Part 1 – Web Applications

The first official release of SignalR for ASP.NET Core – alpha1 – was just released. In this release, all SignalR components were rewritten to make SignalR simpler, easier to use and more reliable.

The SignalR JavaScript client has always been a fundamental part of SignalR. Unfortunately, it has a few limitations which made it hard to extend or use outside the browser. The rewrite allowed to introduce changes which allow to take the client outside the browser (no more dependency on jQuery, YAY!) and open new scenarios. And this is what this blog post will focus on. I split the post to two parts. In the first part I will show how to use the client in a web application from both JavaScript and TypeScript. In the second, part we will look at NodeJS.

The plan for this part is to recreate the chat application from the tutorial on the previous version of SignalR and then to convert it to use the new SignalR Server and JavaScript client. The sample is simple enough to allow us to focus on SignalR aspects rather than on application intricacies. As a bonus, we will see what the experience of porting an application from the previous version of SignalR is. I created a github repo for the application where each commit is a step described in this post. I will refer to particular commits from this post to show changes for a given step.

Setting up the Server

Let’s start from creating an empty ASP.NET Core application. We can do that from command line by running the dotnet new web command. (See this step on github).

Once the application is created we can start the server with dotnet run and make sure it works by navigating to http://localhost:5000 from a browser.

After we ensured that the application runs we can add SignalR server components. First, we need to add a reference to the SignalR package to the SignalRChat.csproj file (See this step on github).

Now we can add the Chat Hub class – we will just copy the code from tutorial and tweak a few things. This is how the hub class looks after the changes:

using System;
using Microsoft.AspNetCore.SignalR;
namespace SignalRChat
{
    public class ChatHub : Hub
    {
        public void Send(string name, string message)
        {
            // Call the broadcastMessage method to update clients.
            Clients.All.InvokeAsync("broadcastMessage", name, message);
        }
    }
}

The changes we made were only cosmetic – we removed the reference to the System.Web namespace, added 'Core' to the Microsoft.AspNet.SignalR so that it reads Microsoft.AspNetCore.SignalR. We also changed how we invoke the client-side method by passing the method name as the first parameter to the InvokeAsync call. (See this step on github).

Now that we created a hub we need to configure the application to be aware of SignalR and to forward SignalR related messages to our hub. It’s as easy as calling AddSignalR extension method in the ConfigureServices method of our Startup class and mapping the hub with the UseSignalR method. We will also add the static files middleware which will be responsible for serving static files. The Startup class should look like this:

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddSignalR();
    } 

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseFileServer();

        app.UseSignalR(routes =>
        {
            routes.MapHub<ChatHub>("chat");
        });
    }
}

(See this step on github).

And this is all the work we had to do create a functional SignalR chat server. Now we can focus on the client side.

The JavaScript Client

In the new version of SignalR the JavaScript client is distributed using npm. The npm module contains a version of the client that can be just included in a web page using the tag, as well as, typings and modules that can be consumed from TypeScript. To get the client to your machine you need to install npm if you haven’t already and run:

npm install @aspnet/signalr-client

The client will be installed in the node_modules folder and you can find the necessary files to include in the node_modules/@aspnet/signalr-client/dist/browser folder. You may wonder why there are so many files in this folder and what purpose they serve. Let’s go over them then and explain.

First, you will find that there are two sets of files – files that contain ES5 in the names and files that do not contain ES5 in the names. SignalR JavaScript uses ES6 (a.k.a EcmaScript 2015) features like Promises or arrow functions. Not all browsers however, support ES6 (looking at you Internet Explorer). The files without ES5 in the names are meant to be used in browsers that support ES6. The files that contain ES5 in the names are the ES6 files transpiled to ES5. They are ES5 compatible and include all required dependencies. The downside of the ES5 files is that they are much bigger than ES6 files.

Another interesting set of files are files containing msgpackprotocol in the name. The new version of SignalR supports custom hub protocols – including binary protocols – and has built-in support for a binary protocol based on MessagePack. The JavaScript implementation of the MessagePack based hub protocol (using the msgpack5) turned out to be quite big so we moved it to a separate file. This way you can include the MessagePack hub protocol only if you want to use it and will not pay the price if you don’t care.

You will also find that each file has a min counterpart. These are just minified versions of the corresponding files. You will want to use the minified versions in production but debugging is much easier with non-minified files so you may want to use non-minified versions during development.

Finally, there is also the third-party-notices.txt file. These are notices for the msgpack5 library and its dependencies used in the MessagePack hub protocol implementation.

Using the SignalR JavaScript Client from JavaScript

Now, that we know a little bit about the JavaScript client let’s update our application to use it.

First, let’s copy all the files from the node_modules/@aspnet/signalr-client/dist/browser folder to a new ​scritps/signalr folder under the wwwroot. (See this step on github).

After the files are copied, let’s create the index.html file in the wwwroot folder and paste the contents of the html file from the tutorial. (See this step on github).

If you try to run the application at this point it will not work. The index.html has references to files like the jQuery library or the old SignalR client which don’t exist. Let’s fix that. Note that even though jQuery is no longer required to the new SignalR client I will continue to use it to minimize the number of changes I need to make. All in all this is not a tutorial on how to remove jQuery from your app so let’s not get sidetracked. Let’s start from sorting out the scripts situation. For jQuery, I will replace the link with the one to the jQuery CDN. For SignalR, I will replace the link to the signalR-2.2.1.min.js file with signalR-client-1.0.0-alpha1.js (feel free to use the ES5 version if you are using a browser that don’t support ES6 features) and remove the link to hubs since hub proxies are currently not supported. (See this step on github (github trick – notice that the link ends with ?w=1 – try removing it and see what happens. Very useful when reviewing some PRs)).

Now we can finally fix the code. Fortunately, this is not a lot of changes:

  • Instead of using proxies we will just create a new HubConnection
  • To register the callback for the client side broadcastMessage method we will use the on function
  • We will replace the done method used by jQuery deferreds to the then used by ES6 promises
  • We will invoke hub methods with the invoke function

(See this step on github).

That’s pretty much it. If you run the application now you should be able to send and receive messages.

Using the JavaScript Client from TypeScript

We now know how to use the new JavaScript SignalR client from JavaScript code. The SignalR client module contains also all necessary bits that make it possible to be consumed from TypeScript. To see how it works let’s take our chat application a bit further and convert it TypeScript.

First, make sure that you have a recent TypeScript compiler installed – run tsc --version from command line. If running the command fails or you have an older version installed install the latest one using this command:

npm install typescript -g

After installing or updating the typescript compiler we will initialize a new project by running

tsc --init

in the project folder. This will create a tsconfig.json file which will look like this:

{
  "compilerOptions": {
    "target": "es6",
    "module": "commonjs",
    "strict": true,
    "noImplicitAny": true
  }
}

after performing some cleanup. We will also add a new chat.ts file which we will leave empty for now. If you run the tsc command from project root you should see an almost empty chat.js file generated from your chat.ts file. (See this step on github).

Because we are using TypeScript and will bring dependencies using npm we will no longer need JavaScript files for the browser so let’s delete them. (See this step on github).

To be able to add and restore dependencies the client will need, let’s create a package.json file by executint the npm init command. We will leave default values for almost all settings except for the project name which needs to be lowercase.

PS C:\source\SignalRChat\SignalRChat> npm init
This utility will walk you through creating a package.json file.
It only covers the most common items, and tries to guess sensible defaults.
See `npm help json` for definitive documentation on these fields
and exactly what they do.

Use `npm install <pkg> --save` afterwards to install a package and
save it as a dependency in the package.json file.

Press ^C at any time to quit.
name: (SignalRChat) signalrchat
version: (1.0.0)
description:
entry point: (chat.js)
test command:
git repository:
keywords:
author:
license: (ISC)
About to write to C:\source\SignalRChat\SignalRChat\package.json:

{
"name": "signalrchat",
"version": "1.0.0",
"description": "",
"main": "chat.js",
"dependencies": {},
"devDependencies": {},
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1"
},
"author": "",
"license": "ISC"
}

Is this ok? (yes)
PS C:\source\SignalRChat\SignalRChat>

Now let’s add our dependencies – signalr-client, jquery and jquery typings (they enable using jquery from TypeScript). We will use the --save-dev option to save the dependencies as dev dependencies in the package.json file.

npm install @aspnet/signalr-client --save-dev
npm install jquery --save-dev
npm install @types/jquery --save-dev

We also need to install browserify – a tool which we will use to create the final script to be used by the browser:

npm install -g browserify

(See this step on github).

We can now start working on the code. First, we need to import the dependencies we are going to use. We can do that by adding the following two lines at the top of our chat.ts file:

import * as signalR from "@aspnet/signalr-client"
import * as $ from "jquery

Now we can move the script from our .html file to the .ts file. If you do that and play a little bit with the code you will notice that intellisense now tells you about class members and function parameters and if you press F12 (in Visual Studio Code) it will take you to the function header. Another thing, you will see is an error on line 5.  This TypeScript telling you that there is a type mismatch for the parameter passed to the jQuery val() function – the prompt() function can return null which is not a valid input for the val() function.

VSCodeSignalR

In our case we know that prompt will return string so we will just cast the result to string to suppress the error.

Since we moved the function to the .ts file we can now remove all the JavaScript code from our index.html file. We can also remove all the tags since we no longer depend on them to bring dependencies (we also already deleted the scripts). (See this step on github).

Let’s compile our chat.ts file now by running tsc command. If you look at the generated chat.js file you will notice that it looks pretty much the same as the source chat.ts file with some additional lines at the top. You will also notice that it does not have the required dependencies (i.e. signalr-client and jquery). This is where browserify comes into play. We will use browserify to generate the final version of the file with all the dependencies. Let’s run the following command (you may need to create the wwwroot/scripts folder if one does not exist) from the project folder:

browserify .\chat.js -o .\wwwroot\scripts\chat.js

Take a look at the chat.js file that was created by browserify and now you will see that the file is much bigger and contains all the required dependencies. If we include this file in our index.html with the tag, start the application and open in the browser you will see that it works and you can send and receive messages. (See this step on github). We could even automate build steps (e.g. with gulp) but it’s out of scope for this post.

Summary

In this post, we looked at using the new SignalR JavaScript client in web applications. We learned how to use the client from both JavaScript and TypeScript. We tried to port an application using the previous version of SignalR to see how hard it is. In the next part, we will take a look at using the client in NodeJS applications.

Entity Framework 6 Easter of Love

While Entity Framework Core along with ASP.NET Core get all the hype today, Entity Framework 6 is still the workhorse of many applications running every day which won’t be converted to the Core world anytime soon, if at all. Because of this I decided to spend some time to give my EF extensions a small refresh to adapt to the changing landscape.

Github

Some of my extensions were hosted on Codeplex. I do most of the work on Github these days and Github is nowadays a de facto standard for open source projects. Codeplex not only looks dated but is also missing a lot of features Github has (searching the code on Github is far from perfect but Codeplex does not offer it at all). All in all this turned out to be the right decision given that it was recently announced that Codeplex is being shutdown. Anyways, here is where my projects previously hosted on Codeplex found their new homes:

Updating projects

I developed most of my EF extensions before Visual Studio 2015 was released. I found that opening them in Visual Studio 2015 was not a good experience – Visual Studio would update project/solution files automatically leaving unwanted changes. Therefore, I updated solution files to the version compatible with Visual Studio 2015. I also moved to a newer version of XUnit which does not require installing an XUnit runner extension in Visual Studio to enable running tests. Even though the solution files are marked as Visual Studio 2015 compatible they can be opened just fine with Visual Studio 2017 which shipped in the meantime.

New versions

This is probably the most exciting: I released new versions of a few of my extensions.

2nd Level Cache for Entity Framework

2nd Level Cache (a.k.a. EFCache) 1.1.0 contains only one new feature. This feature will, however, make everyone’s life easier. Until now the default caching policy cached results for all queries. In the vast majority of cases this behavior is not desired (or plainly incorrect) so you had to create your own policy to limit caching only to results from selected tables. In EFCache 1.1.0 you can specify store entity sets (i.e. which correspond to tables in the database) for which the results should be cached when creating the default caching policy. As a result you no longer have to create your own policy if you want to control simple caching. This change is not breaking.

Store Functions for Entity Framework

I received a couple of community Pull Requests which are worth sharing so yesterday I published on NuGet the new new version of the Store Functions for Entity Framework (1.1.0) containing these contributions. pogi-b added support for Built-in functions so you can now map built-in store functions (e.g. FORMAT or MAP) and use them in your queries. PaulVrugt added ability to discover function stubs marked as private. The first change is not breaking. If you happened to have private function stubs that were not discovered before (a.k.a. dead code) they will be discovered now as a result of the second change.

EF6 CodeFirst View Generation T4 Template for C#

Visual Studio 2017 now requires extensions to use VSIX v3 format. The EF6 CodeFirst View Generation T4 Template for C# extension used format v1 and could not be installed in Visual Studio 2017. I updated the VSIX format to v3 and dropped support for Visual Studio 2010 and 2012.

Note: I have not updated other view generation templates for EF4/EF5 to work with Visual Studio 2017. If you need them to work with VS 2017 let me know and I will update.

Happy Easter!

 

Running ASP.NET Core Applications with IIS and Antares (Azure Websites)

I have seen a few articles (including official docs on http://docs.asp.net) about publishing and running  ASP.NET Core applications in IIS (or Azure/Antares). Unfortunately, I was not satisfied by either of them. Yes, they showed steps you need to follow to make things work. Yes, they touched on some aspects of how things work. No, they did not explain what’s really happening, why it’s happening and how the blocks fit together. Hence this post – something I would like to read if I wanted to run my ASP.NET Core application using IIS or Azure.

Before we can get into details we need to understand how things work at a high level. The most important thing is that ASP.NET Core applications are no longer tightly coupled to IIS as it was with previous versions. Rather, IIS is acting now merely as a reverse proxy and the application itself runs as a separate process using the Kestrel HTTP server. Decoupling ASP.NET from IIS was necessary to enable running ASP.NET Core applications on other platforms. It also makes development easier because it allows to avoid the overhead of IIS/IIS Express during development by making it possible to run your application directly using Kestrel. Note that in production environment it is recommended to always run Kestrel behind a reverse proxy like IIS or  NGINX.

Going back to IIS HTTP requests are handled as follows:

  1. IIS receives a request
  2. IIS (ASP.NET Core Module) starts the ASP.NET Core application in a separate process (if the application is not already running) – i.e. the application no longer runs as w3wp.exe but as dotnet.exe or myapp.exe
  3. IIS forwards the  request to the application
  4. Application processes the request and send the response to IIS
  5. IIS forwards the response to the client

Out of these 5 steps, step two is the most interesting, the most complicated and the most fragile. There is a few pieces that need to be aligned to successfully start an ASP.NET Core application from IIS: ASP.NET Core Module, web.config file and application configuration. Let’s take a look at them one by one and discuss their role.

ASP.NET Core Module (sometimes abbreviated to ANCM) is a native IIS module that starts the application and implements reverse proxy functionality. It is installed as part of ASP.NET Core tooling for Visual Studio or can be installed separately – (the Windows (Server Hosting) package from https://www.microsoft.com/net/download.

web.config – tells IIS to use ASP.NET Core Module to process requests. It also tells ASP.NET Core Module what process (application) to start. Note, that you don’t use web.config to configure your application – it is only used by IIS. Here is how a typical web.confing of an ASP.NET Core application looks like:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.webServer>
    <handlers>
      <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified"/>
    </handlers>
    <aspNetCore processPath="dotnet" arguments=".\HelloWorld.dll" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" />
  </system.webServer>
</configuration>

Application configuration – a typical Main method of an ASP.NET Core application looks like this:

var host = new WebHostBuilder()
            .UseKestrel()
            .UseContentRoot(Directory.GetCurrentDirectory())
            .UseIISIntegration()
            .UseStartup<Startup>()
            .Build();

from the perspective of running the application using IIS the lines that are important are UseKestrel and UseIISIntegration.  UseKestrel configures Kestrel as the application web server. This is important from IIS perspective since you can’t use WebListener and IIS together at the moment (https://github.com/aspnet/IISIntegration/issues/8). UseIISIntegration does a bit of magic to fulfill ASP.NET Core Module expectations and registers the IISMiddleware.

With the information above we can now drill into how IIS starts ASP.NET Core applications. When IIS  receives a request for an ASP.NET Core application it passes the request to the ASP.NET Core Module. IIS was configured to do this by the following entry in the the web.config file:

<handlers>
  <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified"/>
</handlers>

Upon receiving the request the ASP.NET Core Module will attempt to start the application if it is not already running. The name of the process to start and its arguments are specified in web.config  as the processPath and arguments attributes on the aspNetCore element. ASP.NET Core Module also sets a few environment variables for the application process – ASPNETCORE_PORT , ASPNETCORE_APPL_PATH and ASPNETCORE_TOKEN. Here is where the UseIISIntegration magic happens. When the application starts, the code inside UseIISIntegration method tries to read these environment variables and if they are not empty they will be used to configure the url/port the application will listen on. (If the above environment variables are not set UseIISIntegration won’t try to configure anything so that you can use your own settings when running the application directly (i.e. without IIS)). One important detail to pay attention to is where you put the call to UseIISIntegration when configuring your application with WebHostBuilder. You need to make sure that you don’t try to set server urls after you called UseIISIntegration otherwise the url set by UseIISIntegration will get overwritten and your application will be listening on a different port that Asp.NET Core Module expects. As a result things will not work.

ASPNETCORE_TOKEN is a pairing token. IIS middleware added to the pipeline by UseIISIntegration will check each request if it contains this value and will reject requests that don’t. This is to prevent from accepting requests that did not come from IIS.

These are the fundamental blocks needed to run your ASP.NET Core application with IIS and on Azure (Antares). You can now go and set things up as described in some tutorials and actually understand what you are doing and why.

There is, however, a  second level of confusion which happens when you start using Visual Studio and you see additional magic. The first thing that is confusing is web.config. You create a new ASP.NET Core application, open web.config and you see this line instead of what I showed above:

<aspNetCore processPath="%LAUNCHER_PATH%" arguments="%LAUNCHER_ARGS%" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" forwardWindowsAuthToken="false"/>

You start wondering what are these %LAUNCHER_*% environment variables, who is supposed to set them and how IIS (or whoever) knows what values to put there. Honestly, I don’t exactly know how these environment variables work but I treat them as placeholders that Visual Studio replaces when you start your application with F5/Ctrl+F5. When you publish your application to run in a production environment you can’t have these placeholders in web.config – no one knows about them and no one is going to replace them or set values (I also don’t think you can just set environment variables with these names and they will be picked up automatically – this is why I call these strings placeholders – they look as if they were environment variables but I don’t think they behave as environment variables). So how does this work then? If you look at your project.json file you will see a "scripts" section looking more or less like this:

"scripts":
{
"postpublish":
"dotnet publish-iis --publish-folder %publish:OutputPath% --framework %publish:FullTargetFramework%"
}

It just tells dotnet to run the publish-iis tool after the application is published. What is publish-iis (or a better question would be “what publish-iis isn’t”)? publish-iis isn’t… doing much. There are a lot of misconceptions about the publish-iis tool but it actually is a very simple tool. It goes to the folder where the application was published (not your project folder) and checks if it contains a web.config file. If it doesn’t it will create one. If it does it will check what kind of application you have (i.e. whether it is targeting full CLR or Core CLR and – for Core CLR – whether it is a portable or standalone application) and will set the values of the processPath and arguments attributes removing %LAUNCHER_PATH% and %LAUNCHER_ARGS% placeholders on the way. Note that publish-iis is not a Visual Studio tool. It’s independent and whether you are using Visual Studio or you publish your application from command line using dotnet publish it will work as long as it is configured as a postpublish script in your project.json. That’s pretty much what publish-iis is.

Troubleshooting

Since there are a few pieces that need to be aligned to run ASP.NET Core application with IIS things tend to go wrong. If you don’t know how these pieces are supposed to work together (i.e. you did not read the first part of this post) you can search the internet, try random hints from stackoverflow and pray and most likely you still won’t be able to make your application work. But now you know know how things are supposed to work so you can be much more effective in troubleshooting problems. I will only focus on the infamous 502.3 Bad Gateway error as this is the most common one. I read about other failures (https://docs.asp.net/en/latest/publishing/iis.html) but so far have not seen any of them. So, if your application does not work with IIS what to do?

  • Make sure ASP.NET Core Module is installed. It will be installed on your dev box because it is installed with Visual Studio Web Tooling but it may not be on the server you are deploying your application to
  • Try running your published application without IIS – in the command prompt go to the folder where the application was published to and run dotnet {myapp}.dll (a portable Core CLR app) or {myapp}.exe (a standalone application or an application targeting full CLR)
  • If above works – make sure dotnet.exe is on the global %PATH%. It might be on the %PATH% for you but IIS is running using a different account which may not have path to dotnet.exe set. Add the path to the folder dotnet.exe lives in (typically C:\Program Files\dotnet) to the global %PATH% environment variable. I had to do iisreset.exe to make the change effective in IIS
  • Check web.config file of the published application – verify it has actual values and not %LAUNCHER_PATH%/%LAUNCHER_ARGS% placeholders. Make sure the processPath  corresponds to the application type (i.e. you can’t start a portable application with {myapp}.exe because there is only {myapp}.dll in the folder)
  • Check event log – ASP.NET Core Module writes to event log and you can find some useful (and some bogus) entries in the event log
  • Turn on logging – you probably noticed stdoutLogFile and stdoutLogEnabled attributes on the aspNetCore element. They are very helpful to diagnose issues – especially issues related to application start up. If you set stdoutLogEnabled to true ASP.NET Core Module will write all the output written by the application to the console to the stdoutLogFile file. Note that the folder configured in the stdoutLogFile attribute must exist, otherwise (at least at the moment) the file won’t be created. In case of Azure/Antares the path should look like:  \\?\%home%\LogFiles\stdout  (the \\?\%home%\LogFiles folder always exists). In fact if you publish your application to Azure/Antares using Visual Studio tooling it will make publish-iis set stdoutLogPath to point to the folder above and you can turn on logging by merely setting stdoutLogEnabled to true.
    Note that std out logging should not be used as a poor man’s file logging. It’s very useful to diagnose startup issues since they often happen before loggers are created and/or configured but if you want to log to a file configure your application to use a real logging and logging framework. (There is also a plan to create a simple file logger https://github.com/aspnet/Logging/issues/441)

app_offline

The last thing to mention is app_offline.htm. app_offline.htm is a feature of ASP.NET Core Module where ASP.NET Core Module will monitor your application directory and if it notices the app_offline.htm file (note – at the moment the file must not be empty: https://github.com/aspnet/IISIntegration/issues/174) it will stop your application and will respond to requests with the contents of the app_offline.htm file. This makes application deployment much easier since you no longer need to deal with the problem of locked files that are loaded into the process of a running application. Once you remove the app_offline.htm file from the folder ASP.NET Core Module will start your application on the first request. app_offline.htm is used by the deployment tool (WebDeploy) that ships with Visual Studio (note that in preview1 this tool uses app_offline.htm only when deploying to Azure/Antares but it is supposed to be fixed so that app_offline.htm is used by default when deploying to file system (think: IIS)).

So, these are the basics of running ASP.NET Core application with IIS. The topic is much broader but the details described in this post will hopefully make working with IIS  in the ASP.NET Core world less painful and help those who need to transition from previous versions of ASP.NET.