add article for websocket (#19913)

* add article for websocket

An article on WebSocket implementation using ws module in node.js.

* Update index.md

guide/english/nodejs/websocket/index.md

@@ -0,0 +1,73 @@
+---
+title: WebSocket
+---
+
+
+# WebSocket
+[WebSocket](https://www.websocket.org) is a computer communications protocol, providing full-duplex communication channels over a single TCP connection. The 
+WebSocket protocol enables interaction between a web client (such as a browser) and a web server with lower overheads, 
+facilitating real-time data transfer from and to the server. This is made possible by providing a standardized way for the 
+server to send content to the client without being first requested by the client, and allowing messages to be passed back and 
+forth while keeping the connection open. In this way, a two-way ongoing conversation can take place between the client and the 
+server.
+
+In [HTTP](https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol), server can only respond to a request of client, it cannot send a response without client request. Although, WebSocket 
+and HTTP are different but WebSocket has been designed such that they both are compatible.<sup>1</sup>
+
+
+## WS
+We will be using the [`ws`](https://www.npmjs.com/package/ws) module to implement webSocket in node.js.
+
+### Installing
+`npm install ws -g`
+
+### Usage
+#### Server
+Lets create a server that sends a message to client when a connection is established, and logs message sent by the client.
+```javascript
+var WebSocketServer = require('ws').Server;
+var wss = new WebSocketServer({port: 8080});
+
+wss.on('connection', function(ws) {
+    ws.on('message', function(message) {
+        console.log('Message received: %s', message);
+    });
+    ws.send('Hello from server');
+});
+```
+`new WebSocketServer({port: 8080})` starts a server on port no. 8080. Then the lines following are the callback functions. 
+`function(ws)` is called once a connection is established with a client. `function(message)` is called when a client sends a 
+message to the server.
+Save the code in a file named `ws_server.js`.
+
+#### Client
+Use the code below to connect to the server
+```javascript
+var WebSocket = require('ws');
+var ws = new WebSocket('ws://localhost:8080/websockets/');
+
+ws.on('open', function() {
+    ws.send('Msg from client');
+});
+ws.on('message', function(data, flags) {
+    console.log('Msg received in client: %s ', data);
+});
+```
+`new WebSocket('ws://localhost:8080/websockets/')` establishes a connection with the server on port 8080, that we have created 
+above. `ws.on('open', function() ` sends message to server when connection is established. The other function logs the message 
+sent by the server.
+Save the code in a file as `ws_client.js`.
+
+Now lets run our code. Go to the directory in which you saved your file and run the following commands
+
+**Server:**
+`$ node ws_server.js`
+
+**Client:**
+`$ node ws_client.js`
+
+From the output you can see that the server starts and is ready to accept connection. When the client starts it first establishes a connection with the server, then sends a message and listens for an incoming message from server. Once the connection is established between server and client, `ws.send()` is fired and messages are exchanged between both of them, and due to the asynchronous nature of `node.js`, both of them at the same time, are listening also.
+ 
+ ## Reference
+ - [Wiki page](https://en.wikipedia.org/wiki/WebSocket)
+ - [Linode guide](https://www.linode.com/docs/development/introduction-to-websockets/)