Socket.io 1.x: use WebSockets only?

We are developing a web application that will run only on modern browsers (IE10+) for different reasons.

One of the features we implemented is Socket.io 1.x. However, by default the Socket.io client tries to support older browsers, so it starts a connection with long polling and then updates that to WebSockets. This is a waste of time and resources, given we know for sure the browser supports WS.

I've searched around, and I can only find this wiki page which, however, is about Socket.io 0.9.

Eventually, I found the documentation for engine.io-client (on which Socket.io-client is based on the 1.x branch). This is the code that I wrote and seems to be working. However, I would like to know if it's correct or if I'm doing something wrong:

io.connect('https://...', {
    upgrade: false,
    transports: ['websocket']
})

Weirdly, just setting the transports property to an array with websockets only wasn't enough; I also had to disable upgrade. Is this correct?

Update

I made some new discoveries.

With transports set to ['websocket'] only, it doesn't make any difference wether upgrade is enabled or not. Is that normal?

There are two types of "upgrades" happening with socket.io. First (in socket.io 1.0+), socket.io starts all connections with an http polling request and it may actually exchange some initial data with just an http request. Then, at some point after that, it will try to actually initiate a webSocket connection. the webSocket connection is done by sending a particular type of http request that specifies an upgrade: websocket header and the server can then respond appropriately whether it supports websocket or not. If the server agrees to the upgrade, then that particular http connection is "upgraded" to the webSocket protocol. At that point, the client then knows that webSocket is supported and it stops using the polling http requests, thus completing its upgrade to webSocket.

You can prevent the initial http polling entirely by doing this on the client:

var socket = io({transports: ['websocket'], upgrade: false});

This will prevent polling connections from your own cooperating clients. If you want to prevent any clients from ever using polling, then you can add this to the server:

io.set('transports', ['websocket']);

But, if you set this on the server, socket.io clients that are initially connecting with http polling will not work at all. So, this should only be matched with the right settings in the client such that the client never starts with polling.

This will tell both ends that you only want to use webSockets and socket.io will skip the extra http polling at the beginning. Fair warning, doing this requires webSocket support so this rules out compatible with older versions of IE that didn't yet support webSocket. If you want to retain compatibility, then just let socket.io do it's thing with a couple http requests initially.

Here's more info on the protocol upgrade from http to webSocket.

The webSockets protocol initiates EVERY webSocket with an HTTP connection. That's the way all webSockets work. That HTTP connection contains some headers on it that indicate that the browser would "like" to upgrade to the webSockets protocol. If the server support that protocol, then it responds telling the client that it will upgrade to the webSocket protocol and that very socket then switches from the HTTP protocol to the webSocket protocol. This is how a webSocket connection is designed to work. So, the fact that you see your webSocket connection starting with an HTTP connection is 100% normal.

You can configure socket.io to NEVER use long polling if that makes you feel better, but this will not change the fact that the webSocket connection will still start with an HTTP connection that is then upgraded to the webSocket protocol and it will not improve the efficiency of operation in modern browsers that support webSockets. It will, however make it so that your connection will not work in older browsers.

To tell Socket.IO to use WebSocket only instead of a few XHR requests first, just add this to the Node server:

io.set('transports', ['websocket']);

And on the client add this:

var socket = io({transports: ['websocket']});

This tells Socket.IO to only use WebSocket protocol and nothing else; it's cleaner, faster and uses a little less resources on the client and server sides.

Now you'll only see a single WebSocket connection in your network request list, just keep in mind IE9 and earlier can't use WebSocket.

I'm posting that answer because the accepted answer is not correct - it confuses the Socket.IO upgrade from long-polling AJAX to WebSocket with the WSS protocol "Connection: Upgrade" request. The issue is not that the WebSocket connection starts as HTTP and gets upgraded to WebSocket - how could it not? - but that Socket.IO starts with a long-polling AJAX connection even on browsers supporting WebSocket, and only upgrades it later after exchanging some traffic. It's very easy to see in the developer tools of Firefox or Chrome.

The author of the question is correct in his observations. The "upgrade" in Socket.IO doesn't refer to the HTTP to WSS protocol upgrade as is often misunderstood but to the upgrade of Socket.IO connection from long-polling AJAX connection to WebSocket. If you start with WebSocket already (which is not the default) then upgrade false has no effect because you don't need to upgrade. If you start with polling and disable upgrade then it stays that way and doesn't upgrade to WebSocket.

See answers by arnold and Nick Steele if you want to avoid starting with long-polling. I will explain what is going on in more detail.

This is what I observed in my experiments with simple WebSocket and Socket.IO apps:

WebSocket

2 requests, 1.50 KB, 0.05 s

From those 2 requests:

1. HTML page itself
2. connection upgrade to WebSocket

(The connection upgrade request is visible on the developer tools with a 101 Switching Protocols response.)

Socket.IO

6 requests, 181.56 KB, 0.25 s

From those 6 requests:

1. the HTML page itself
2. Socket.IO's JavaScript (180 kilobytes)
3. first long polling AJAX request
4. second long polling AJAX request
5. third long polling AJAX request
6. connection upgrade to WebSocket

Details

WebSocket results that I got on localhost:

Socket.IO results that I got on localhost:

Test yourself

I published the code on npm and on GitHub, you can run it yourself:

# Install:
npm i -g websocket-vs-socket.io
# Run the server:
websocket-vs-socket.io

and follow instrictions. To uninstall:

# Uninstall:
npm rm -g websocket-vs-socket.io

See this answer for more info.