SignalR makes it easy to build multi-user, real-time web applications with ASP.NET. SignalR establishes a persistent bidirectional connection which enables the server to call client methods or send messages to the client. This article gives a brief overview of SignalR. A commented link list helps getting started. This article is also a landing page for SignalR.

(The article isn't a stub any longer, but it still needs some polishing.)

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Overview

Concept

SignalR provides a bidirectional communication channel between (web) clients and a server side (web) application. SignalR is not limited to web application. A client can be every application or web page which uses JavaScript or .NET Framework 4.5 (or 4 with some restrictions) - like Windows Phone apps, SilverLight or Console applications. A SignalR server application has to be build with .NET Framework 4.5 (or 4 with some restrictions).

SignalR's communication channel is

Bidirectional
Client and server can exchange messages or invoke remote methods. I.e. the communication is not limited to the request-response cycle.
Persistent
The connection is established once and used until the application terminates. Persistent connections can reduce the overhead per call significantly.

SignalR supports four communication technologies on the transport layer. Client and Server negotiate the best communication technology – depending on their capabilities – when they establish their connection. SignalR uses the fallback strategy shown in figure 1.

Figure 1: Fallback strategy

SignalR supports these communication technologies:

WebSockets
WebSockets are the best match. WebSockets (and also Server Sent Events) are standardized by the W3C as a part of HTML 5. The .NET Framework supports WebSockets since version 4.5.
Server Sent Events
Server sent events offer a server-to-client push notification using DOM events.
Forever Frames
<<explain>>
Long Polling
<<explain>>.

SignalR has a layered communication architecture on client and server side as shown in figure 2:

Figure 2: Communication Layers

The (internal) Transport Layer handles the different communication technologies. The Persistent Connection API is independent of the underlying technology. It offers a message based API, which allows exchanging data between client and server (and vice versa). The Hub API is the next abstraction level. It enables client and server to invoke remote methods (instead of exchanging mere data).

SignalR exposes two communication levels possible:

Persistent Connection API - low level, message-oriented communication
Hub API - allows direct call of client and server methods

The Hub API offers the most convenient and preferred communication API. The Persistent Connection API might only be interesting, if you need to integrate with an existing message based server application or if you need for some reason detailed control on the message level.

Practical Scenarios

Chat application
Multiuser online game
Application Monitoring
Collaborative Work

Framework

SignalR was first implemented by Damian Edwards and … . It is available as open source.

The SignalR Framework consists of a set of .NET packages and one JavaScript package. The packages are offered via Nuget. Table 1 shows all packages and the (sub) packages they include.

Category	Package Microsoft.AspNet.	Description	S a m p l e	S i g n a l R	J s	C l i e n t	S y s t e m W e b	O w i n	R e d i s	S e r v i c e B u s	C o r e
Category	Package Microsoft.AspNet.	Description	Included packages
Sample	SignalR.Sample	A stock ticker sample	x	x	x		x	x			x
Meta Package	SignalR	Meta package for web apps on IIS & ASP.NET		x	x		x	x			x
Clients	SignalR.Js	JavaScript client API			x
Clients	SignalR.Client	.NET client API				x
Host	SignalR.SystemWeb	Meta package for ASP.NET applications					x	x			x
Host	SignalR.Owin	OWIN host for SignalR						x			x
Scaleout Buses	SignalR.Redis	Redis scaleout for SignalR							x		x
Scaleout Buses	SignalR.ServiceBus	Service bus scaleout for SignalR								x	x
Core	SignalR.Core	Server side component									x

Table 1: SignalR packages

Typical usage scenarios are:

Microsoft.AspNet.SignalR
is used by Web applications. It's a meta package which contains the most relevant packages for the server side.It also includes the Java Script client API, which is used by the delivered web pages.
SignalR.Client
is used by CLR-based client applications like Windows Phone, WPF or console apps.

System Requirements

<<to do>>

Evaluation

Pros

bidirectional communication for (web) clients
automatically selects the best available communication mechanism
it’s not restricted to web clients

Cons

Not a reliable communication
Don’t use it if you need a guaranteed delivery of your messages. Bank Transaction are a negative example.
It uses dynamics. Some syntactical errors can only be detected at runtime. However, only a few lines of code are effected. So, it’s not a real problem detected all errors related to the usage of dynamics.

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Link Collection

Videos

SignalR and WebSockets (6 min)
Scott Hanselman explains the key concepts of SignalR and WebSockets. An echo application and a stock ticker application are used as demos.
Introduction to SignalR - Creating a Cross-Platform game (Screencast on Youtube by Filip Ekberg (MVP), 76 min)
Starts with an introduction of SignalR and explains how to creates a cross-platform game for different browsers and even Windows 8 Store applications.

Overview Articles

Asynchronous scalable web applications with real-time persistent long-running connections with SignalR (Blog post, Scott Hanselman, August 2011)
Scott starts with the fact "Polling is a common example of hammering a screw." and give a brief introduction to SignalR. He explains the difference between low level connections and high level hub connections.

Official SignalR pages

SignalR (official web site)
The official web site of the SignalR project. It also links to the SignalR documentation on GitHub.
SignalR Documentation (GitHub)
<<add summary>>
SignalR Continuous Integration
<<add summary>>
SignalR (asp.net)
<<add summary>>
SignalR MSDN Library
<<add summary>>
Source Code on Github
<<add summary>>
Nightly Builds
<<add summary>>

Tutorials, Samples, Demos

Tutorial: Getting Started with SignalR
<<add summary>>
Tutorial: Getting Started with SignalR and MVC 4
<<add summary>>
Tutorial: Server Broadcast with ASP.NET SignalR
<<add summary>>
Build a Progress Bar with SignalR (MSDN Magazine, Dino Esposito, March 2012 Issue)
This MSDN article starts with the popular chat sample. In addition it explains how progress bar can be implemented using SignalR. The code sample can be downloaded.
Implementing Pub/Sub Applications with SignalR (MSDN Magazine, Bruno Terkaly, April 2012 Issue)
The article starts with a short examination of the polling problem and how it is solved using SignalR. The popular "Move Shape" is used as a demo application. He uses the popular chat sample and explains the 12 core lines of the client and server part.
High-Frequency Realtime with SignalR
<<add summary>>
SignalR Move Shape Demo
<<add summary>>
SignalR Simple Users On Line Counter
<<add summary>>
SignalR team on Jabbr (Forum)
<<add summary>>

Applications Using SignalR

Jabbr
A support forum for SignalR which is implemented using SignalR.
ShooterR Game
A multi-user online game with spaceships. You can either play the game or download the source.

Misc

Automatic camel casing of properties with SignalR hubs (MSDN Blog, Stuart Leeks, September 2012)
Stuart writes: "By default, SignalR preserves the case of property names when sending objects from the server to the client. My preference is for PascalCase on the server and camelCase on the client, and in this post I’ll show how you can achieve that."
Running SignalR on Mono (TechNet article by Filip Ekberg)
The article explains the preparation of the SignalR libraries for Mono: compile SignalR dev branch and upload it to the server running Mono and Apache. A small sample application is used to the the installation.

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