Websockets are an extension to the HTTP specification that support full duplex, aka two-way, session-based communication between client and server. A standard HTTP exchange between client and server is initiated by a client request. The server, on receiving the request, sends back a response, then closes the connection between the two. The server can’t initiate an exchange, and communication can’t go both directions at once. Websockets expand on HTTP by allowing for both of these features.
A websocket connection works as follows: the client request via HTTP or HTTPS. The server responds with an upgrade message and sends a key for encryption. The client responds, the server opens an encrypted socket between the two. Either side can send a message at any time, and either side can close the connection. The protocol supports a
message event, for when a new message arrives. and a
close event, for when the remote host closes the connection.
WebSockets bear some resemblance to MQTT, a message-based protocol for communication between low-power networked devices. Here is a comparison between the two.
These examples use two different server-side websocket libraries, ws and express-ws. The latter is just like the former, but integrates better with express.js, so you can just declare the websocket connection as a route.
These clients are duplicated in the
public directory of each of the servers
To get the servers below working, you’ll need node.js installed. Once you have that, download the repository, then, on the command line, change directories into the directory of each server example. Once you’re there, install the libraries using npm, the node package manager, like so:
$ npm install
This will download the necessary libraries. Then run the server like so:
The two servers
WsServerWithExpress are both HTTP servers that also support webSockets. When you’re running them, open a browser and enter
http://localhost:8080/ in the address bar. The server script will serve you the
index.html page in the
public folder, which is a browser-based client of the server. You can also load the jsClient example by opening the
index.html page from that example in a browser. It will connect to the server running on your terminal window as well.
To stop any node.js script, type control-C in the terminal window.
The p5jsClient example is written as a companion to the
ExpressWsServer and the ArduinoWebSocketClient example. Normally, the Arduino example will send and receive messages from websocket.org on port 80 (see line 22-23 of the example), but if you fill in the IP address of your computer and change the port to 8080, it will connect to your local server. Likewise, the p5jsClient will connect to localhost:8080, your local server. When the Arduino client sends a new message, the p5jsClient will display it.
WsServerExample does not have a HTTP server included in it, it is just a websocket server. You can run it the same way as the others, but only the Arduino example of an HTML file opened from the local filesystem can connect to it. The server will not serve an HTML page itself.