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Introduction to SignalR

What is SignalR?

ASP.NET SignalR is a library for ASP.NET developers that simplifies the process of adding real-time web functionality to applications. Real-time web functionality is the ability to have server code push content to connected clients instantly as it becomes available, rather than having the server wait for a client to request new data.
SignalR can be used to add any sort of "real-time" web functionality to your ASP.NET application. While chat is often used as an example, you can do a whole lot more. Any time a user refreshes a web page to see new data, or the page implements long polling to retrieve new data, it is a candidate for using SignalR. Examples include dashboards and monitoring applications, collaborative applications (such as simultaneous editing of documents), job progress updates, and real-time forms.
SignalR also enables completely new types of web applications that require high frequency updates from the server, for example, real-time gaming. For a great example of this, see the ShootR game.
SignalR provides a simple API for creating server-to-client remote procedure calls (RPC) that call JavaScript functions in client browsers (and other client platforms) from server-side .NET code. SignalR also includes API for connection management (for instance, connect and disconnect events), and grouping connections.
Invoking methods with SignalR
SignalR handles connection management automatically, and lets you broadcast messages to all connected clients simultaneously, like a chat room. You can also send messages to specific clients. The connection between the client and server is persistent, unlike a classic HTTP connection, which is re-established for each communication.
SignalR supports "server push" functionality, in which server code can call out to client code in the browser using Remote Procedure Calls (RPC), rather than the request-response model common on the web today.
SignalR applications can scale out to thousands of clients using Service Bus, SQL Server or Redis.
SignalR is open-source, accessible through GitHub.

SignalR and WebSocket

SignalR uses the new WebSocket transport where available, and falls back to older transports where necessary. While you could certainly write your application using WebSocket directly, using SignalR means that a lot of the extra functionality you would need to implement will already have been done for you. Most importantly, this means that you can code your application to take advantage of WebSocket without having to worry about creating a separate code path for older clients. SignalR also shields you from having to worry about updates to WebSocket, since SignalR will continue to be updated to support changes in the underlying transport, providing your application a consistent interface across versions of WebSocket.
While you could certainly create a solution using WebSocket alone, SignalR provides all of the functionality you would need to write yourself, such as fallback to other transports and revising your application for updates to WebSocket implementations.

Transports and fallbacks

SignalR is an abstraction over some of the transports that are required to do real-time work between client and server. A SignalR connection starts as HTTP, and is then promoted to a WebSocket connection if it is available. WebSocket is the ideal transport for SignalR, since it makes the most efficient use of server memory, has the lowest latency, and has the most underlying features (such as full duplex communication between client and server), but it also has the most stringent requirements: WebSocket requires the server to be using Windows Server 2012 or Windows 8, and .NET Framework 4.5. If these requirements are not met, SignalR will attempt to use other transports to make its connections.

HTML 5 transports

These transports depend on support for HTML 5. If the client browser does not support the HTML 5 standard, older transports will be used.
  • WebSocket (if the both the server and browser indicate they can support Websocket). WebSocket is the only transport that establishes a true persistent, two-way connection between client and server. However, WebSocket also has the most stringent requirements; it is fully supported only in the latest versions of Microsoft Internet Explorer, Google Chrome, and Mozilla Firefox, and only has a partial implementation in other browsers such as Opera and Safari.
  • Server Sent Events, also known as EventSource (if the browser supports Server Sent Events, which is basically all browsers except Internet Explorer.)

Comet transports

The following transports are based on the Comet web application model, in which a browser or other client maintains a long-held HTTP request, which the server can use to push data to the client without the client specifically requesting it.
  • Forever Frame (for Internet Explorer only). Forever Frame creates a hidden IFrame which makes a request to an endpoint on the server that does not complete. The server then continually sends script to the client which is immediately executed, providing a one-way realtime connection from server to client. The connection from client to server uses a separate connection from the server to client connection, and like a standard HTML request, a new connection is created for each piece of data that needs to be sent.
  • Ajax long polling. Long polling does not create a persistent connection, but instead polls the server with a request that stays open until the server responds, at which point the connection closes, and a new connection is requested immediately. This may introduce some latency while the connection resets.

For more information on what transports are supported under which configurations, see Supported Platforms.
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