What is SignalR?

 ASP.NET SignalR is an asynchronous signaling library for ASP.NET developers’ for creating 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. This new Microsoft API is very much helpful for replacing the earlier web applications where user has to refresh to see new data or a page uses "long polling" to retrieve new data. Examples for the application that you can consider for working with SignalR are 
• Real-time gaming
• Chat Application
• Dashboards
• Monitoring applications 
• Collaborative applications
• Real time forums
• Job updates
• Trading
• Traffic updates
etc….

Why SignalR and How it works?
1. SignalR handles connection management automatically, and lets you broadcast messages to all connected clients simultaneously
2. The connection between the client and server is persistent, unlike a classic HTTP connection, which is re-established for each communication
3. 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.
Web Sockets is a next-generation bidirectional communication technology for web applications which operates over a single socket and is exposed via a JavaScript interface in HTML 5 compliant browsers. SignalR uses WebSockets when it available, a connection for SignalR starts as HTTP, and is then promoted to a WebSocket connection if it is available.
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.
If HTML5 support is there then SignalR uses Server Sent Events/EventSource (if the browser supports Server Sent Events (supported in majority browsers except Internet Explorer.)
Other Transport modes supported by SignalR are called Comet transports based on the Comet web application model.
In Comet Web application model 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.
In this Model Forever Frame (for IE) and Ajax Long polling are supported.
• Forever Frame provides creates one-way realtime connection from server to client .It uses hidden IFrame which makes a request to an endpoint on the server and then continually sends script to the client which is immediately executed. 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.
• 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.

SignalR Pictorial representation of what has been discussed till now 



SignalR API
SignalR package is available by NuGet: Following are component in the package.
• SignalR – A meta package that brings in SignalR.Server and SignalR.Js 
• SignalR.Server – Server side components needed to build SignalR endpoints 
• SignalR.Js – Javascript client for SignalR 
• SignalR.Client - .Net client for SignalR 
• SignalR.Ninject – Ninject dependency resolver for SignalR
The SignalR API contains two models for communicating between clients and servers: 
• Persistent Connections - A Connection represents a simple endpoint for sending single-recipient, grouped, or broadcast messages
• Hubs - A Hub is a more high-level pipeline built upon the Connection API that allows your client and server to call methods on each other directly.

More reading:
• http://www.asp.net/signalr/overview/signalr-20
• http://signalr.net/
• https://github.com/SignalR/SignalR/wiki