Tmds.SockJS is an ASP.NET Core implementation of the SockJS protocol. The library maintains the standard ASP.NET WebSocket interface even when websockets are emulated.

AppVeyor:

From: https://github.com/sockjs/sockjs-client

SockJS is a browser JavaScript library that provides a WebSocket-like object. SockJS gives you a coherent, cross-browser, Javascript API which creates a low latency, full duplex, cross-domain communication channel between the browser and the web server.

Tmds.SockJS enables SockJS on ASP.NET Core. It can be installed added as a middleware and requires no change to the WebSocket implementation. For example, if the server provides an standard websocket endpoint at '/websocket', a SockJS endpoint can be added at '/sockjs' with this one-liner:

app.UseSockJS("/sockjs", new SockJSOptions() { RewritePath = "/websocket" });

This source package contains a number of WebSocket extension methods. It can be used independent of Tmds.SockJS.

These are the most interesting methods:

Task SendAsync(string)
Task<string> ReceiveTextAsync() // returns 'null' when peer closed the WebSocket

Task SendCloseAsync()

This example implements an 'echo' websocket service. The SockJS endpoint for the service is at the '/echo' path.

In project.json add Tmds.SockJS and Tmds.WebSockets.Sources to the dependencies:

	"dependencies": {
		"Tmds.SockJS": "1.0.0",
		"Tmds.WebSockets.Sources": "1.0.0",
	},

Inside the Startup.cs Configure-method we setup the SockJS end-point and implement the echo service:

public void Configure(IApplicationBuilder app)
{
	// from Microsoft.AspNet.WebSockets.Server
	app.UseWebSockets();
	
	// add a SockJS end point to the website
	app.UseSockJS("/echo");

	// routing logic
	app.Use(async (context, next) =>
	{
		if (context.Request.Path == "/echo")
		{
			// use the standard WebSocket API to handle websocket connections
			var ws = await context.WebSockets.AcceptWebSocketAsync();
			while (true)
			{
				string received = await ws.ReceiveTextAsync();
				if (received == null)
				{
					break;
				}
				await ws.SendAsync(received);
			}
		}
		else
		{
			// not for us to handle
			await next();
		}
	});

	// other application logic
	app.Run(async (context) =>
	{
		await context.Response.WriteAsync("Hello World!");
	});
}

The tables below show what browsers are supported using sockjs-client and Tmds.SockJS. Strikethrough is used to indicate the technique is not supported, alltough sockjs-client supports it. Techniques in the Streaming and Polling column are implemented in Tmds.SockJS. Techniques in the Websockets column are implemented in Microsoft.AspNet.WebSockets.

The SockJS protocol does not allow to emulate all features of standard websockets (RFC6455):

• Splitting a message into several sends (WebSocket.SendAsync: endOfMessage)
• Sending binary messages (WebSocket.SendAsync: messageType)
• Receive close parameters (Tmds.SockJS will provide a close message when the session timed out)
• Select a Subprotocol (WebSocket.Protocol equals string.Empty)

ASP.NET SignalR (http://signalr.net/) includes a mechanism for websocket emulation just like SockJS. SignalR builds on top of that to provide a bi-directional remote procedure call (RPC) channel between the client and the server. Both the cliend and server use the SignalR API. If you don't need control over the WebSocket subprotocol: use SignalR instead of Tmds.SockJS.