Category Archives: SignalR

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)

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        if (env.IsDevelopment())


        app.UseSignalR(routes =>

(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)
entry point: (chat.js)
test command:
git repository:
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.


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.


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.


SignalR Core Part 2/3: ASP.Net Core Sockets

Disclaimer: SignalR Core is still in early stages and changing rapidly. All information in this post is subject to change.

To test some of the scenarios described in the first part of this mini-series I came up with an idea for a relatively simple application where users can report to the server the weather at their location and their report will be broadcast to all the connected clients. I called this application SocialWeather. The central part of the system is an ASP.Net Core application running SignalR Core server. The server can handle massages received in one of the following formats – JSON, Protobuf and pipe (the pipe format is a simple format I created where the data is separated by the pipe (|) and the message ends with the new line character (\n)). I also created 3 different clients – a JavaScript clients using the JSON format, a C# client using Protobuf and a lua client using the pipe format. The JavaScript client is part of a web page served by the same application that hosts the server. The C# client is a console application that that can send and receive Protobuf messages. The lua client is the most interesting as it runs on an ESP8266 development board with the NodeMCU firmware. The whole system is using the “socket” level of the new SignalR which is a kind of counterpart of persistent connection in the previous version of SignalR (so no hubs API here). All the clients use bare websockets to connect to the server (in other words there is no SignalR client involved).

The SocialWeather server, which includes the JavaScript client is a sample project in the SignalR repo and you can run it yourself – just clone the repo, run build.cmd/ to install the correct version of the runtime and restore packages. Then go to the samples/SocialWeather folder and start the server with dotnet run.

The C# and lua clients are in my personal repo. Running the C# client is straightforward. After you clone the repo you need to restore packages update the URL to the SignalR server and run the client with dotnet run. Each time you press Enter the client will generate a random weather report and send it to the server. If you type “!q:” the client will exit.

The lua client is meant to run on an ESP8266 compatible board with NodeMCU firmware installed. Preparing the board to run the client requires a bit of work. The first step is to set up serial communication to the board. If the module you have is equipped with a USB port (I have the Lolin v3 board which does have a USB port) it should be enough to install a VCP (Virtual COM Port) drivers (you can find the drivers here or here If your board doesn’t have a USB port, you will need to use an additional module (e.g.  Arduino)  for USB-to-serial translation (you can find tutorials about setting it up on the web).

The next step is to make sure that your board has the right firmware. It needs to be a NodeMCU firmware with net, http, wifi, and websocket support. You can request a build here and follow steps in this tutorial to re-image your board. When the board is ready you should be able to connect to it using a serial terminal (I used screen on Mac and Putty on Windows). On Windows determine the COM port number the board is using using the Device Manager. On Mac it will be one of the /dev/tty* devices. The speed needs to be set to 115200. When using Putty make sure the Flow Control setting is set to None or the communication will not be working correctly.


Once on the device you need to connect it to your wireless network by sending the following commands (you need to replace SSID with the name of the network and PWD with the password or an empty string for open networks):

wifi.sta.config(SSID, PWD)

We are now ready to run the client. If you haven’t already, clone the repo containing the client go to src/lua-client folder and update the URL to the SocialWeather SignalR server. Now you can transfer the file to the device (if you are connected to the device with the terminal you need to disconnect). The nodemcu-uploader Python script does the job.

If all stars aligned correctly you should be able now to start the client by executing:


it will print a confirmation message once it connected  successfully to the server and then will print weather reports sent by other client. You can also send your own weather report just by typing:


and pressing Enter.

The command may look a bit cryptic but is quite simple. The ws is a handle to the websocket instance created by the social-weather.lua script. send is a method of the websocket class so we literally invoke the send method of websocket interactively. The argument is a SocialWeather report in the pipe format: a pipe separated list of values – temperature, weather, time, zipe code – terminated with \n.

This is what it looks like when you run all three clients:


Let’s take a look at how things work under the hood. On the server side the central part is the SocialEndPoint class which handles the clients and processes their requests. If you look at the loop that processes requests it does not do any parsing on its own. Instead it offloads parsing to a formatter and deals with strongly typed instances. Formatter is a class that knows how to turn a message into an object of a given type and vice versa. In case of the SocialWeather application the only kind of messages sent to and from the server are weather reports so this is the only type formatters need to understand.

When sending messages to clients the process is reversed. The server gives the formatter a strongly typed object and leaves it to formatter to turn it into a valid wire format.

How does the server know which formatter to use for a given connection? All available formatters need to be registered in the DI container as well as mapped to a type they can handle and format. When establishing the connection, the client sends the format type it understands as a query string parameter. The server stores this value in the connection metadata and uses later to resolve the correct formatter for the connection.

On the client side things are even simpler. The lua client has just 30 lines of code half of which is concerned with printing weather reports in a human readable form. Because the format of the connection cannot change once the connection is established message parsing can be hardcoded. The rest is just setting up the websocket to connect to the server and react to incoming message notifications.

The C# client is equally simple. It contains two asynchronous loops – one for receiving messages (weather reports) from the server and one for sending messages to the server. Again, handling the wire format (which in this case is Protobuf) is hardcoded in the client.

This is pretty much all I have on ASP.Net Core Sockets. With a simple application we were able to validate that the new version of SignalR can handle many scenarios the old one couldn’t. We were able to connect to the server from different platforms/environments without using a dedicated SignalR client. The server was capable of handling clients that use different and custom message formats – including a binary format (Protobuf). Finally, all this could be achieved with a small amount of relatively simple code.

SignalR Core Part 1/3: Design Considerations

Disclaimer: SignalR Core is still in early stages and changing rapidly. All information in this post is subject to change.

A few months ago, we started working on the new version of SignalR that will be part of the ASP.Net Core framework. Originally we just wanted to port existing code and iterate on it. However, we soon realized that doing so would prevent us from enabling new scenarios we wanted to support. The original version of SignalR was designed around long polling (note that back in the day support for websockets was not as common as it is today – it was not supported by many web browsers, it was not supported in .NET Framework 4, it was not (and still isn’t) supported natively on Windows 7 and Windows 2008 R2). A JSON based protocol was baked in and could not be replaced which blocked a possibility of using other (e.g. binary) formats. Starting the connection was heavy and complicated – it required sending 3 HTTP requests whose responses had to be correlated with messages sent over the newly created transport (you can find a detailed description of the protocol in SignalR on the wire – an informal description of the SignalR protocol – a post I wrote on this very subject). This basically meant that a dedicated client was required to talk to a SignalR server. In the old design the server was centered around MessageBus – all messages and actions had to go through the message bus. This made the code very complex and error prone especially in scale-out scenarios where all the servers were required to have the same data. The state (e.g. cursors/message ids, groups tokens etc.) was kept on the client which would then send it back to the server when needed (e.g. when reconnecting). The need of keeping the state up-to-date significantly increased the size of the messages exchanged between the server and the client in most of the non-trivial scenarios.

In the new version of SignalR we wanted to remove some of the limitations of the old version. First, we decided to no longer use long polling as the model transport. Rather, we started with a premise that a full duplex channel is available. While this might sound a lot of like websockets we are thinking that it will be possible to take it further in the future and support other protocols like TCP/IP. Note, it does not mean that the long polling and server sent events transports are going away. Only, that we would not drag better transports down to the standards of worse transports (e.g. websockets supported binary format but long polling (until XmlHttpRequest2) and server sent events didn’t so in the old version of SignalR there was no support for binary messages. In the new version we’d rather base64 encode messages if needed and let users use what websockets offers). Second, we did not want to bake in any specific protocol or message format. Sure, for hub invocations we will still need to be able to get the name of the hub method and the arguments but we will no longer care how this is represented on the wire. This opens the way to using custom (including binary) formats. Third, establishing the connection should be lightweight and connection negotiation can be skipped for persistent duplex transports (like websockets). If a transport is not persistent or uses separate channels for sending and receiving data connection negotiation is required – it creates a connection id which will be used to identify all the requests sent by a given client. However, if there are no multiple requests because the transport is full duplex and persistent (like in the case of websockets) the connection id is not needed – once the connection is established in the first request it is used to transfer the data in both directions. In practice, this means that you can connect to a SignalR server without a SignalR client – just by using bare websockets.

There are also a few things that we decided not to support in the new SignalR. One of the biggest ones was the ability re-establish a connection automatically if the client loses a connection to the server. While it may not be obvious, the reconnect feature has a huge impact on the design, complexity and performance of SignalR. Looking at what happens during reconnect should make it clear. When a client loses a connection, it tries to re-establish it by sending the reconnect request to the server. The reconnect request contains the id of the last message the client received and the groups token containing the information about groups the client belongs to. This means that the server needs to send the message id with each message so the client can tell the server what was the last message it received. The more topics the client is subscribed to the bigger the message id gets up to the point where the message id is much bigger than the actual message.

Now, when the server receives a reconnect request it reads the message id and tries to resend all the messages the client missed. To be able to do that the server needs to keep track of all messages sent to each client and buffer at least some recent messages so that it can resend them when needed. Indeed, the server has a buffer per connection which it uses to store recent messages. The default size of that buffer is 1000 messages which creates a lot of memory pressure. The size of the buffer can be configured to make it smaller but this will increase the probability of losing messages when a reconnect happens.

The groups token has similar issues – the more groups the client belongs to the bigger the token gets. It needs to be sent to the client each time the client joins or leaves a group so the client can send it back in case of reconnects to re-establish group membership. The size of the token limits the number of groups a client can belong to – if the groups token gets too big the reconnect attempt will fail due to the URL being bigger than the limit.

While auto-reconnect will no longer be supported in SignalR users can build their own solution to this problem. Even today people try restarting their connection if it was closed by adding a handler to the Closed event in which they start a new connection. It can be done in a similar fashion in SignalR core. It’s true that the client will no longer receive messages it missed but this could happen even in the old SignalR – if the number of message the client missed was greater than the size of the message buffer the newest messages would overwrite the oldest messages (message buffer is a ring buffer) so the client would never receive the oldest messages.

Another scenario we decided not to support in the new version of SignalR was allowing clients to jump servers (a multi-server scenarios). Before, the client could connect to any server and then reconnect or send a data to any other server in the farm. This required that all servers had all the data to be able to handle requests from any client. The way it was implemented was that when a server receive a message it would publish it to all the other SignalR servers via MessageBus. This resulted in a huge number of messages being sent between SignalR servers.

(Side note. Interestingly, the scenario of reconnecting to a different server than the one the client was originally connected to often did not work correctly due to server misconfiguration. The connection token and groups token are encrypted by the server before sending them to the client. When the server receives the connection token and/or groups token it needs to be able decrypt it. If it cannot, it rejects the request with the 400 (Bad Request) error. The server uses the machine key to encrypt/decrypt the data so, all machines in the farm must have the same machine key or the connection token (which is included in each request) encrypted on one server can’t be decrypted on another server and the request fails. What I have seen several times was that servers in the farm had different machine keys so, reconnecting to a different server did not actually work.)

In the new SignalR the idea is that the client sticks only to one server. In multi-server scenarios there is a client to server map stored externally which tells which client is connected to which server. When a server needs to send a message to a client it no longer needs to send the message to all other servers because the client might be connected to one of them. Rather, it checks what server the client is connected to and sends the message only to this client thus the traffic among SignalR server is greatly reduced.

The last change I want to talk about, somewhat related to the previous topic, is removing the built-in scale-out support. In the previous version of SignalR there was one official way of scaling out SignalR servers – a scale out provider would subclass the ScaleoutMessageBus and leave all the heavy lifting to SignalR. It sounds good in theory but with the time it became apparent that with regards to scale-out there is no “one size fits all” solution. Scaling out an applications turned out to be very specific to the application goals and design. As a result, many users had to implement their own solution to scaling out their applications yet still paid the cost of the built-in scale-out (even when using just one server there is an in-memory message bus all messages go through). While, scale-out support is no longer built-in the project contains a Redis based scale-out solution that can be used as-is or as a guidance to create a custom scale-out solution.

These are I think the biggest design/architecture decision we have made. I believe that they will allow to make SignalR simpler, more reliable and performant.

SignalR C++ Client

For the past several months I have been working on the SignalR C++ Client. The first, alpha 1 version has just shipped on NuGet and because there isn’t any real documentation for it at the moment I decided to write a blog post showing how to get started with it.
The SignalR C++ Client NuGet package contains Win32 and x64 bits for native desktop applications and is meant to be used with Visual Studio 2013.
Since the SignalR C++ Client ships on NuGet adding it to a project is easy. After creating a C++ project (e.g. a console application) right click on the project node in the solution explorer and select the “Manage NuGet Packages” option. In the Manage NuGet Package window make sure to include prelease packages in your search by selecting “Include Prerelease” in the dropdown and enter “SignalR C++” in the search window. Finally click the “Install” button next to the Microsoft ASP.Net SignalR C++ Client package which will install the SignalR C++ Client (and its dependency – C++ Rest SDK) into your project.
Installing SignalR C++ from NuGet
You can also install the package from the Package Manager Console – just open the package manager console (Tools → NuGet Package Manager → Package Manager Console) and type:

Install-Package Microsoft.AspNet.SignalR.Client.Cpp.v120.WinDesktop –Pre

The SignalR C++ Client relies heavily on asynchronous facilities provided by the C++ Rest SDK (codename Casablanca) which in turn extensively uses lambda functions introduced in C++ 11. Understanding both – asynchronous programming and lambda functions is crucial to being able to use the SignalR C++ Client effectively. I started a blog mini-series on asynchronous programming in C++ which I would recommend to read if you are not familiar with these concepts.
The SignalR C++ Client supports both programming models available in SignalR – Persistent Connections and Hubs. Persistent Connections is just a simple way of exchanging data between the server and the client while Hubs enable RPC-like programming where it is possible to invoke a method on the server from the client and vice versa. In this post I will show how to use the SignalR C++ Client to handle both – Persistent Connections and Hubs.
Before we can move to the client code we need to set up a server. Our server will have two endpoints – one for persistent connections and one for hubs. To make it easy we will use the chat server from the SignalR tutorial as a starting point and we will add a persistent connection endpoint to it. Just follow the steps in the tutorial to create the server. (Alternatively you can get the code from ths SignalR C++ Client GitHub repo – just clone the repo and open the samples_VS2013.sln file with Visual Studio 2013). Note that if you follow the steps you will end up installing the latest stable available NuGet packages into your project instead of the version used in the tutorial and therefore you need to update the index.html file to use the version of the jquery.signalR-x.x.x.min.js that was installed into your project instead of the one used in the tutorial – i.e. if you installed version 2.2.0 of SignalR you will need to change
<script src="Scripts/jquery.signalR-2.0.3.min.js"></script>
<script src="Scripts/jquery.signalR-2.2.0.min.js"></script>
You should also set up index.html as the start page by right-clicking on this file in the Solution Explorer and selecting “Set As Start Page”. After you complete all these steps you should be able to run the server (just press Ctrl+F5) and send and receive messages from the browser window that opens.
Now we need to add a Persistent Connection endpoint to our server. It’s quite easy – we just need to add the following class to the project:

using System.Threading.Tasks;
using Microsoft.AspNet.SignalR;

namespace SignalRServer
    public class EchoConnection : PersistentConnection
        protected override Task OnConnected(IRequest request,
            string connectionId)
            return Connection.Send(connectionId, "Welcome!");

        protected override Task OnReceived(IRequest request,
            string connectionId, string data)
            return Connection.Broadcast(data);

and configure the server to treat requests sent to the /echo path as SignalR persistent connection requests which should be handled by the EchoConnection class. Adding the following line to the Configuration method in the Startup class will do the trick:


Our server should be now ready to use so we can now start playing with the client.

Using Persistent Connections

Our EchoConnection sends the "Welcome!" string to the client when it connects and then broadcasts messages it receives from the client to all connected clients. On the client side, after the client connects successfully, we will wait for the user to enter a string which will be sent to the server. We also print any message the client receives from the server. To be notified about messages we need to register a callback which will be invoked for whenever a message is received. Finally, if the user types “:q” (this is the command you want to remember when you try to git commit on a new box but forgot to configure the text editor git should use) the client will close the connection and exit. The code that does all of it is shown below (again, you can get the code from the SignalR-Client-Cpp repo – it is in the PersistentConnectionSample.cpp file)

void send_message(signalr::connection &connection,
                  const utility::string_t& message)
        // fire and forget but we need to observe exceptions
        .then([](pplx::task<void> send_task)
        catch (const std::exception &e)
            ucout << U("Error while sending data: ") << e.what();

int main()
    signalr::connection connection{ U("http://localhost:34281/echo") };
    connection.set_message_received([](const utility::string_t& m)
        ucout << U("Message received:") << m 
              << std::endl << U("Enter message: ");

        // fine to capture by reference - we are blocking 
        // so it is guaranteed to be valid
            for (;;)
                utility::string_t message;
                std::getline(ucin, message);

                if (message == U(":q"))

                send_message(connection, message);

            return connection.stop();
        .then([](pplx::task<void> stop_task)
                ucout << U("connection stopped successfully") << std::endl;
            catch (const std::exception &e)
                ucout << U("exception when starting or closing connection: ") 
                      << e.what() << std::endl;

    return 0;

You may want to try more than one instance of the client to see that all clients receive messages broadcast by the server.

The code is intuitively simple but there are some interesting nuances so let’s take a closer look at it. In the main function, as explained above, we create a connection and we use the set_message_received function to set a handler that will be called whenever we receive a message from the server. Then we start the connection using the connection.start() function. If the connection started successfully we run a loop that reads messages from the console. Note that we know that connection started successfully because if connection.start() threw an exception this continuation would not run at all because it is a value based continuation (you can find more on how exceptions in C++ async work in my blog post on this very subject). Whenever a user enters a message we send the message to the server using the connection.send() function. Sending messages happens in the fire-and-forget manner but we still need to handle exceptions to prevent from crashes caused by unobserved exceptions. When the user enters “:q” we break the loop and move on to the next continuation which stops the connection. This continuation is interesting because it actually can be invoked in one more case. Note that this is a task based continuation so it will be invoked always – even if a previous task threw. As a result this continuation is also an exception handler for the task that starts the connection. Moving back to stopping the connection – stopping a connection can potentially throw so again we need to handle the exception to prevent from crashes.
There are two more important things. One is that tasks are executed asynchronously so we have to block the main thread to prevent the program from exiting and terminating all the threads (see another blog post of mine for more details). In our case we can just use the task::get() function – it is sufficient, simple and works. The second important thing is related to how we capture the connection variable in one of the continuations. We capture it by reference. We can do that because we block the main thread and therefore we ensure that the reference we captured will be always valid. In general case however capturing local variables by reference will lead to undefined behavior and crashes if the function that started a task exited before the task completes (or is even started) since the variable will go out of scope and the captured reference will no longer be valid. Blocking a thread to wait for the task works but usually is not the best way to solve the problem. If you cannot ensure that the reference will be valid when a task runs you should consider capturing variables by value or, if it is not possible (like in the case of the connection and hub_connection instances) capture a std::shared_ptr (or std::weak_ptr). Regardless of how you capture your variables (maybe except for primitive values captured by value) you need to make sure you work with them in a thread safe way because you never know what thread a task is going to run on.

Using Hubs

The sample for hub connections shows how to connect and communicate with the SignalR sample chat server. The code looks like this (you can also find it on GitHub in the HubConnectionSample.cpp):

void send_message(signalr::hub_proxy proxy, const utility::string_t& name,
                  const utility::string_t& message)
    web::json::value args{};
    args[0] = web::json::value::string(name);
    args[1] = web::json::value(message);

    proxy.invoke<void>(U("send"), args)
        // fire and forget but we need to observe exceptions
        .then([](pplx::task<void> invoke_task)
        catch (const std::exception &e)
            ucout << U("Error while sending data: ") << e.what();

void chat(const utility::string_t& name)
    signalr::hub_connection connection{U("http://localhost:34281")};
    auto proxy = connection.create_hub_proxy(U("ChatHub"));
    proxy.on(U("broadcastMessage"), [](const web::json::value& m)
        ucout << std::endl << << U(" wrote:") 
              << << std::endl << U("Enter your message: ");

        .then([proxy, name]()
            ucout << U("Enter your message:");
            for (;;)
                utility::string_t message;
                std::getline(ucin, message);

                if (message == U(":q"))

                send_message(proxy, name, message);
        // fine to capture by reference - we are blocking
        // so it is guaranteed to be valid
            return connection.stop();
        .then([](pplx::task<void> stop_task)
                ucout << U("connection stopped successfully") << std::endl;
            catch (const std::exception &e)
                ucout << U("exception when starting or stopping connection: ")
                      << e.what() << std::endl;

Let’s quickly go through the code. First we create a hub_connection instance which we then use to create a ChatHub proxy. Then we use the on function to set up a handler which will be invoked each time the server invokes the broadcastMessage client method. Note that the callback takes a json::value as a parameter which is an array containing parameters for the client method. If you ever used the SignalR .NET client then you probably notice it is different from what you are used to. The SignalR .NET Client does not expose JSON directly but uses reflection to convert the JSON array to typed values which are then passed to the On method as parameters. Since there isn’t reach reflection in C++ it is the responsibility of the user to interpret the parameters the SignalR C++ Client passes to the lambda in the on function. Once the hub proxy is set up we can start the connection and wait for the user to enter a message which we will then send to the server by invoking the server side send hub method. Server side hub methods are invoked with the hub_proxy::invoke function. This function has two flavors. One for methods that don’t return values: hub_proxy::invoke<void>(...), and one used to invoke non-void hub methods: hub_proxy::invoke<json::value>(...). If a server side hub method returns a value it will be returned to the user as a json:value and, again, it is up to the user to make sense out of it. There are also convenience overloads of hub_proxy::invoke function you can use to invoke a parameterless hub method which don’t take the arguments parameter. They will save you a couple lines of code required to create an empty JSON array.
Long running server side methods can notify the client about their progress. If the client wants to receive these notifications it can provide a callback that should be invoked each time a progress message is received. The callback is passed as the last parameter of the hub_proxy::invoke functions and defaults to a lambda expression with an empty body. The sample chat server does not have any method sending progress messages but if you are interested you can check SignalR end to end tests that tests this scenario.
That’s mostly it. The remaining code is just closing the connection and handling exceptions is done the same way as for Persistent Connection.
One important thing worth noting is how we capture the hub_proxy instance in the lambda – we do it by value. hub_proxy type has semantics of std::shared_ptr where all copies point to a single implementation instance which won’t be deleted as long as there is at least one instance that has a reference to it. As a result if you capture a hub_proxy instance by value in a lambda it will be valid even if the original variable is not around anymore. (Not that this matters in this particular example since we are blocking the thread anyways so even if we captured the hub_proxy instance by reference everything would work since the original variable does not go out of scope until the connection is closed).

Doing the right things

There are a few things you need to be aware of when working with the SignalR C++ client.

Prepare the connection before starting

SignalR C++ Client uses callbacks to communicate with the user’s code. However you can only set these callbacks when the connection is in the disconnected state. Otherwise an exception will be thrown. Similarly, when using hubs you have to create hub proxies before you start the connection.

Process messages fast or asynchronously

The callbacks invoked when a message is received (connection::set_message_received, hub_proxy::on) are invoked synchronously from the thread that receives messages. This is to ensure that the callbacks are invoked in the same order as the order the messages were received. The drawback is that the new messages won’t be received until the callback completes processing the current message. Therefore you need to process messages as quickly as possible or, if you don’t care about order, process messages asynchronously.

Handle exceptions

Any kind of network communication is susceptible to errors. Intermittent connection losses and timeouts may and will happen and they will result in exceptions. These exceptions have to be handled otherwise your app will crash due to an unobserved exception. To make exception handling easier the SignalR C++ Client follows a pattern where functions returning pplx::task<T> will not throw exceptions in case of errors but will instead return a faulted task. This saves the user from having to have an exception handler in addition to handling exceptions in a task based continuation. (You can read more about handling exceptions when using tasks here.

Capture connection and hub_proxy instances correctly

Copy constructors and copy assignment operators of the connection and hub_connection classes are intentionally deleted. This was done to prevent from capturing connection and hub_connection instances by value in lambda expressions (I also don’t think there is a clear answer as to what the operation of copying a connection should do). The issue with capturing connection and hub_connection instances by value is that because these classes use a std::shared_ptr pointing to the actual implementation it is possible to create a cycle which would prevent from destroying the connection instance (i.e. the destructor wouldn’t run). The cycle would result not only in a memory leak but would also keep the connection running after the variable went out of scope if the connection was not explicitly stopped – the destructor is responsible for stopping connections that were not stopped explicitly. The cycle would be created if a connection/hub_connection instance was captured by value in callbacks that are passed back to and stored in the connection or hub_connection instances i.e. callbacks passed to connection::set_message_received, set_reconnecting, set_reconnected, set_disconnected functions on both connection and hub_connection.
While deleting copy constructors and copy assignment operators on connection and hub_connection classes makes it more difficult to create a cycle it does not make it impossible – you could still inadvertently create a cycle if you capture a std::shared_ptr<connection> or std::shared_ptr<hub_connection> by value. So, what to do? The safest way is to create a weak pointer (std::weak_ptr) to the connection and capture this pointer. Then in the callback you will need call std::weak_ptr::lock() function to obtain a shared pointer to the connection. You need to check the return value of the std::weak_ptr::lock() function – it will return the nullptr if the instance it points to was destroyed (in which case you probably will want just to exit the callback). You could also try capturing the connection instance by reference by you will have to ensure that the reference is always valid when the callback is executed which sometimes might be hard to do.
Note that capturing hub_proxy instances by value is fine. Hub_proxy is linked back to the connection using a weak pointer so capturing hub_proxy instances by value won’t create cycles. If you, however, try invoking a function on a hub_proxy instance that outlived its connection an exception will be thrown.

Stop connection explicitly

While the connection is being stopped when the instance goes out of scope it is recommended to explicitly stop connections. Stopping the connection explicitly has a few advantages:

  • Throwing an exception from the destructor in C++ results in undefined behavior. As a result the connection class destructor (or to be more accurate the connection_impl dtor) catches and swallows all the exceptions. When stopping connections explicitly all the exceptions are passed back to the user (in form of a faulted task) giving the user a chance to handle them the way they see it fit
  • Stop is an asynchronous operation but the destructor isn’t. Therefore when the connection is running when the destructor is called the destructor blocks and waits until the stop operation completes. Since the destructor runs in the current thread you might experience “unexpected” delays – “unexpected” because the delay will happen even though you didn’t invoke any function explicitly. Rather the destructor is just called automatically for you when the variable leaves the scope (or when you call delete on dynamically allocated instances). These delays could be especially annoying if the current thread the destructor is running in happens to be the UI thread
  • Relying on the destructor stopping the connection may result in the connection not being stopped at all. Internally the connection is using std::shared_ptr pointing to the actual implementation which will be destroyed only when no one is referencing it. In case of a bug where the connection implementation instance stores a callback that captures its connection instance a cycle is created and the reference count will never reach 0. In this situation the destructor will never be called which means that not only would memory be leaked but also that the connection would never be stopped


The SignalR C++ Client is able to log its activities. You can control what activities are being logged and how they are logged. To control what’s being logged pass a trace_level to the connection or hub_connection constructor. The default setting is to log all activities. To control how the activities are logged you need to create a class derived from the log_writer class and pass a std::shared_ptr pointing to your writer in the when creating a connection/hub_connection instance. Your implementation has to ensure that logging is thread safe as the SignalR C++ Client does not synchronize logging in any way. If you don’t provide you own log_writer the SignalR C++ Client will use the default implementation that uses the OutputDebugString function to log entries. This is especially useful when debugging the client with Visual Studio since entries logged this way will appear in the Visual Studio Output window.


While the SignalR C++ Client is fully functional it contains a couple of limitations. Currently the client supports only the webSockets transport. It also does not support detecting stale connections using the heartbeat mechanism. (The way heartbeat works in other clients is that the server sends keep alive messages every few seconds and if the client misses a few of these keep alive messages it will consider the connection to be stale/dead and will try restarting the connection. The SignalR C++ Client currently ignores keep alive messages). Finally, the SignalR C++ Client does not support sending or receiving State information.


This is only an alpha 1 release. I expect there will be some bugs that have not been caught so far and fixing them should be a priority. Another interesting exercise is to try to make the SignalR C++ Client work on other platforms – specifically on Linux and Mac OS. It should be possible because the SignalR C++ Client is built on top of C++ REST SDK which is cross platform. Finally – depending on the feedback – adding new transports (at least the longPolling transport) may be something worth looking at.

SignalR on the Wire – an informal description of the SignalR protocol

I have seen the question asking about a description of the SignalR protocol come up quite a lot. Heck, when I started looking at SignalR I too was looking for something like this. Now, almost a year later, after I architecturally redesigned the SignalR C# client and wrote from scratch the SignalR C++ Client I think I can describe the protocol quite accurately. So, here we go.
In my view the protocol used by SignalR consists of two parts. The first part is related to connection management i.e. how the connection is started, stopped, reconnected etc. This part contains some quite complicated bits (especially around starting the connection) and it is mostly interesting to people who want to write their own client (which, I believe, is a minority). The second part which, I think, the vast majority of users is actually interested in is what are all these “H”s, “A”s, “I”s etc. SignalR is putting on the wire and writing to logs. I will start from the first part and then will describe the second part.
Disclaimer: In some cases I will be talking about differences among the clients. I have only worked with the SignalR .NET client, the SignalR C++ Client and the SignalR JavaScript Client (“worked” in this case is an overstatement – I just fixed a few bugs and looked at the code several times). I am aware of other SignalR clients like the Java or Objective-C one but I have not tried them nor looked at the code and I don’t know what they do, how they do it and how much they conform to the description below.

Connection Management
SignalR manages the connection by using the HTTP(S) protocol. Actions are initiated by the client which sends HTTP requests that contain the requested action and a sub-set of common parameters. The requests can be sent using the GET or (when using protocol version 1.5) POST method. Not all the requests require all the parameters. Here are the parameters used in SignalR requests with their descriptions:

  • transport – the name of the transport being used. Valid values: webSockets, longPolling, serverSentEvents, foreverFrame
  • clientProtocol – the version of protocol used by the client. The most recent version is 1.5 however it is only used by the JavaScript client since the change that mandated bumping the version of the protocol to 1.5 is only relevant for this client. The .NET and C++ clients currently use version 1.4. Note that the server is designed to support down-level clients (i.e. clients using previous versions of the protocol) and the current (2.2.0) version supports protocol versions from 1.2 to 1.5
  • connectionToken – a string that identifies the sender. It is returned in the response to the negotiate request. See this document for more details on connection token.
  • connectionData – a url-encoded JSon array containing a list of hubs the client is subscribing to. For instance if the client is subscribing to two hubs – “my_hub”, “your_hub” the array to be sent looks like this: [{"Name":"my_hub"},{"Name":"your_hub"}] and after url-encoding it becomes:
  • messageId – the id of the last received message. Used for reconnecting and – when using the longPolling transport – in poll requests
  • groupsToken – a token describing what groups the connection belongs to. Used for reconnecting
  • queryString – an arbitrary query string provided by the user; appended to all requests

Starting the Connection
Starting the connection is the most complicated task related to connection management performed by a SignalR client. It requires sending three requests to the server – negotiate, connect and start. The whole sequence looks as follows:

  • the client sends the negotiate request. The response to the negotiate request contains a number of client configuration settings
  • the client starts the transport by sending the connect request. The connect request has to complete within the timeout returned by the server in the response to the negotiate request. The response to the connect request (a.k.a. init message) is sent on the newly started transport (i.e. if you use webSockets transport it will be sent on the newly opened websocket, if you use serverSentEvents it will be sent on the newly opened event stream if you use longPolling it will be sent as a response to the connect/poll request)
  • once the init message has been received the client sends the start request. The server confirms it received the start request by responding with the {Response: Started} payload

You can also find some details about the start sequence here.

Connection Management Requests
Here is a list of requests the client sends to start, stop and reconnect the connection.

» negotiate – negotiate connection parameters
Required parameters: clientProtocol, connectionData (when using hubs)
Optional parameters: queryString
Sample request:


Sample response:


Url – path to the SignalR endpoint. Currently not used by the client.
ConnectionToken – connection token assigned by the server. See this article for more details. This value needs to be sent in each subsequent request as the value of the connectionToken parameter
ConnectionId – the id of the connection
KeepAliveTimeout – the amount of time in seconds the client should wait before attempting to reconnect if it has not received a keep alive message. If the server is configured to not send keep alive messages this value is null.
DisconnectTimeout – the amount of time within which the client should try to reconnect if the connection goes away.
TryWebSockets – whether the server supports websockets
ProtocolVersion – the version of the protocol used for communication
TransportConnectTimeout – the maximum amount of time the client should try to connect to the server using a given transport

» connect – starts a transport
Required parameters: transport, clientProtocol, connectionToken, connectionData (when using hubs)
Optional parameters: queryString
Sample request:


Sample response (a.k.a. init message):


The connect request starts a transport. If you are using the webSockets transport the client will use the ws:// or wss:// scheme to open a websocket. If you are using the serverSentEvents transport the client will open an event stream. For the longPolling transport the connect request is treated by the server as the first poll request. The response to the connect request is sent using the newly opened channel and is a JSon object containing the property "S" set to 1 (a.k.a. init messge). The server however does not guarantee this message to be the first message sent to the client (e.g. there can be a broadcast in progress which will be sent to the client before the server sends the init message. This is interesting in case of the longPolling transport because the response to the connect request will close the pending connect request even though it is not the init message. The init message will in that case be sent as a response to a subsequent poll request).

» start – informs the server that transport started successfully
Required parameters: transport, clientProtocol, connectionToken, connectionData (when using hubs)
Optional parameters: queryString
Sample request:


Sample response:


start request was added in the version 1.4 of the protocol to make some scenarios work reliably on the server side. Adding this request to the start sequence made things complicated on the client since though since there is quite a few things that can go wrong after the client received the init message but before it received a response to the start message (like the connection is lost and the client starts reconnecting, the user stops the connection etc.).

» reconnect – sent to the server when the connection is lost and the client is reconnecting
Required parameters: transport, clientProtocol, connectionToken, connectionData (when using hubs), messageId, groupsToken (if the connection belongs to a group)
Optional parameters: queryString
Sample request:


Sample response: N/A
Similarly to the connect request the reconnect request starts (re-starts) the transport. For the longPolling transport from the client perspective it is just yet another form of poll, for the serverSentEvents transport a new event stream will opened, for the webSockets transport it will open a new websocket. The messageId tells the server what was the last message the client received and the groupsToken tells the server what groups the client belonged to before reconnecting.

» abort – stops the connection
Required parameters: transport, clientProtocol, connectionToken, connectionData (when using hubs)
Optional parameters: queryString
Sample request:


Sample response: empty
Remarks: The JavaScript and C++ clients send abort request in a fire and forget manner and ignore all the errors. The .NET client blocks until response is received or a timeout occurs, what apart from taking more time, causes some issues (like this bug).

» ping – pings the server
Required parameters: none
Optional parameters: queryString
Sample request:


Sample response:

{ "Response": "pong" }

Remarks: The ping request is not really a “connection management request”. The sole purpose of this request is to keep the ASP.NET session alive. It is only sent by the the JavaScript client.

SignalR Messages
Before we can take a look at the messages SignalR puts on the wire we need to discuss how different transports send and receive messages. The webSockets transport is quite simple since it is creating a full-duplex communication channel used to send data from the server to the client and from the client to the server. Once the channel is setup there are no further HTTP requests until the client is stopped (the abort request) or the connection was lost and the client tries to re-establish the connection (the reconnect request). The serverSentEvents transport creates an event stream that is used to receive messages from the server. If the client wants to send a message to the server it creates a send HTTP POST request and sends the data in the request body. The longPolling transport creates a long running HTTP request which the server will respond to if it has a message for the client. If the server does not send any data within a configured timeout (calculated as the sum of the ConnectionTimeout received in the response to the negotiate request + 10 seconds – which by default is 120 seconds) the current poll request will be closed and the client will start a new poll request (this is to prevent proxies from closing the long running request which would result in unnecessary reconnects). Sending messages works in the same way as for the serverSentEvents transport – a send HTTP request containing the message in the request body is sent to the server. Here are the descriptions of the send and poll requests.

» send – sends data to the server. Used by the serverSentEvents and longPolling transports
Required parameters: transport, clientProtocol, connectionToken, connectionData (when using hubs), data (sent in the request body)
Optional parameters: queryString
Sample request:


Data send int the request body (url encoded, see the description below) :


Sample response (see the description below):

{ "I" : 0 }

» poll – starts a (potentially) long running polling request that the server will use to send data to the client. Used only by the longPolling transport
Required parameters: transport, clientProtocol, connectionToken, connectionData (when using hubs), messageId (the JavaScript client sends messageId in the request body)
Optional parameters: queryString
Sample request:


Sample response (see the description below):

    {"H":"ChatHub","M":"broadcastMessage","A":["client","test msg1"]},
    {"H":"ChatHub","M":"broadcastMessage","A":["client","test msg2"]},

Persistent Connection Messages

The protocol used for persistent connection is quite simple. Messages sent to the server are just raw strings. There isn’t any specific format they have to be in. The C# client has a convenience Send() method that takes an object that is supposed to be sent to the server but all this method does is just converting the object to JSon and invoke the Send() overload that takes string. Messages sent to the client are more structured. They are JSon strings with a number of properties. Depending on the purpose of the message different properties can be present in the payload or the message may have no properties (KeepAlive messages). The properties you can find in the message are as follows:

C – message id, present for all non-KeepAlive messages

M – an array containing actual data.


S – indicates that the transport was initialized (a.k.a. init message)


G – groups token – an encrypted string representing group membership


T – if the value is 1 the client should transition into the reconnecting state and try to reconnect to the server (i.e. send the reconnect request). The server is sending a message with this property set to 1 if it is being shut down or restarted. Applies to the longPolling transport only.

L – the delay between re-establishing poll connections. Applies to the longPolling transport only. Used only by the JavaScript client. Configurable on the server by setting the IConfigurationManager.LongPollDelay property.


KeepAlive messages
KeepAlive messages are empty object JSon strings (i.e. {}) and can be used by SignalR clients to detect network problems. SignalR server will send keep alive messages at the configured time interval. If the client has not received any message (including a keep alive message) from the server within a certain period of time it will try to restart the connection. Note that not all the clients currently support restarting connection based on network activity (most notably it is not supported by the SignalR C++ Client). Sending keep alive messages by the server can be turned off by setting the KeepAlive server configuration property to null.

Hubs Messages

Hubs API makes it possible to invoke server methods from the client and client methods from the server. The protocol used for persistent connection is not rich enough to allow expressing RPC (remote procedure call) semantics. It does not mean however that the protocol used for hub connections is completely different from the protocol used for persistent connections. Rather, the protocol used for hub connections is mostly an extension of the protocol for persistent connections.
When a client invokes a server method it no longer sends a free-flow string as it was for persistent connections. Instead it sends a JSon string containing all necessary information needed to invoke the method. Here is a sample message a client would send to invoke a server method:

{"H":"chathub","M":"Send","A":["JS Client","Test message"],"I":0,
  "S":{"customProperty" : "abc"}}

The payload has the following properties:
I – invocation identifier – allows to match up responses with requests
H – the name of the hub
M – the name of the method
A – arguments (an array, can be empty if the method does not have any parameters)
S – state – a dictionary containing additional custom data (optional, currently not supported by the C++ client)

The message sent from the server to the client can be one of the following:

  • a result of a server method call
  • an invocation of a client method
  • a progress message

Server Side Hub Method Invocation Result

When a server method is invoked the server returns a confirmation that the invocation has completed by sending the invocation id to the client and – if the method returned a value – the return value, or – if invoking the method failed – the error. There are two kinds of errors – general errors and a hub errors. In case of a general error the response contains only an error message and the error is turned by the client into a generic exception – the .NET client throws an InvalidOperationException, the C++ client throws a std::runtime_error and the JavaScript client creates an Error with the Exception as the source. Hub errors contain a boolean property set to true to indicate that they are hub errors and they may contain some additional error data. Hub errors are turned into a HubException by the .NET Client, a signalr::hub_exception by the C++ client and the JavaScript client creates an Error with source set to HubException. Here are sample results of a server method call:


A server void method whose invocation identifier was "0" completed successfully.

"{"I":"0", "R":42}

A server method returning a number whose invocation identifier was "0" completed successfully and returned the value 42.

{"I":"0", "E":"Error occurred"}

A server method whose invocation identifier was "0" failed with the error "Error occurred"

{"I":"0","E":"Hub error occurred", "H":true, "D":{"ErrorNumber":42}}

A server method whose invocation identifier was "0" failed with the hub error "Hub error occurred" and sent some additional error data.

Here is the full list of properties that can be present in the result of server method invocation:

I – invocation Id (always present)
R – the value returned by the server method (present if the method is not void)
E – error message
Htrue if this is a hub error
D – an object containing additional error data (can only be present for hub errors)
T – stack trace (if detailed error reporting (i.e. the HubConfiguration.EnableDetailedErrors property) is turned on on the server). Note that none of the clients currently propagate the stack trace to the user but if tracing is turned on it will be logged with the message
S – state – a dictionary containing additional custom data (optional, currently not supported by the C++ client)

Client Side Hub Method Invocation

To invoke a client method the server extends the protocol used for persistent connections. The difference is that instead of sending a free flow text in the message portion of the message the server sends a JSon string that contains all the details needed to invoke the method (like the hub and method names and arguments). Here is an example of a message sent by the server to invoke a hub method on the client:

{"C":"d- F430FB19", "M":[{"H":"my_hub", "M":"broadcast", "A":["Hi!", 1]}] }

As you can see the “envelope” in form of message id or message property is the same as for persistent connections. The interesting part from the hub point of view is the value of the M property:

{"H":"my_hub", "M":"broadcast", "A":["Hi!", 1]}

This structure is quite similar to what the client is using to invoke a server hub method (except there is no invocation id since the server does not expect any response to this message).
H – the name of the hub
M – the name of the hub method
A – arguments (an array, can be empty if the method does not have any parameters)
S – state – a dictionary containing additional custom data (optional, currently not supported (ignored) by the C++ client)

Progress Message

The last kind of message sent from the server to the client is a progress message. When a server method is a long running method the server can send the information about the progress of execution of the method to the client. Similarly to the client method invocation the progress information is embedded in the message portion of a persistent connection message. The entire message looks like this:

{"C":"d-5E80A020-A,1|B,0|C,15|D,0", M:[{I:"P|1", "P":{"I":"0", "D":1}}] }

but the progress message itself looks like this:

{I:"P|1", "P":{"I":"0", "D":1}}

The structure containing information about progress contains two properties:
I – kind of an invocation id but prepended with "P|". Used only by older clients.
P – an object containing actual information about progress

The object containing “real” progress information has the following properties:
I – invocation id that tells which invocation this progress message applies to
D – progress data returned by the method

Note that there might be multiple progress messages sent to the client before the server sends the actual result of the invoked method.

Recent Protocol Revisions

  • 1.4 – introduction of the start request
  • 1.5 – requests can now be sent using the POST method. This helps avoid a memory leak when using the longPolling transport in Chrome and IE browsers (bug 2953). Only used by the JS client when with the longPolling transport. Note that the only properties the server checks the request body for are the groupsToken and the messageId

That’s pretty much it. The SignalR protocol is not very complex but the little caveats and exceptions may make the implementation a bit troublesome.