Voice of a Developer: Javascript WebSocket - Part 27

JavaScript is a language of the Web. This series of articles will talk about my observations learned during my decade of software development experience with JavaScript.

Before moving further let us look at the previous articles of the series:

In this article, we will understand WebSocket, which is a new revolution in client server communication. Modern browsers support this protocol. The server shall also support WebSocket thus a handshake between client & server is possible.

WebSocket

It is based on ws schema and establishes full duplex connection between client & server. At client-end, it is a browser and it can work with any server capable of running WebSocket protocol.

Advantages of WebSocket

  • Faster than TCP

    Quick response from server where you are using HTTP connection to connect. TCP connection lifecycle is SYN, SYN/ACK, ACK and then send a GET request. TCP header contains 10 mandatory fields and an optional field. TCP has close ends at both sides – Refer useful diagram from Wiki.

    diagram

    However, WebSocket simply receive the response with a small header.

  • Better than other methods like polling, AJAX

    For duplex communication, developers used technique like polling to constantly synchronize with server. This technique was never effective for duplex communication. Hence, WebSocket came into picture.

  • Easy to use

    It is easy to use at both ends. At server, it is independent of platform too.

Disadvantages of WebSocket

  • Open connection

    The disadvantage is that it keeps the connection open on the server for the duration of the time user is interacting with the page. Therefore, it consumes more resources on the server.

  • Single channel communication

    If your app does not need too much duplex communication with the server then you can think of using other approaches like long polling, AJAX over WebSocket.

WebSocket server

I suggest to use /download any server supporting WebSocket. In IIS, you can also enable WebSocket via adding Application Development features. For testing, I want to use WebSocketd server like UNIX.

Steps to run server:

  • Download WebSocketd from URL.
  • Extract zip file in a folder like C:\websocket
  • The server is platform independent and can run any program
  • Create a server program in C# and build to generate myProgram.exe,
    1. using System;  
    2. using System.Threading;  
    3.   
    4. //myProgram.cs  
    5. class Counter  
    6. {  
    7.     static void Main()  
    8.     {  
    9.         for (int i = 1; i <= 10; i++)  
    10.         {  
    11.             Console.WriteLine(i);  
    12.             Thread.Sleep(500);  
    13.         }  
    14.     }  
    15. }  
  • You can copy myProgram.exe into c:\websocket for convenience
  • Open command shell and goto c:\websocket and run below command
  • websocketd.exe -- port 80 myProgram.exe

cmd

It gave above error because port 80 is already in use. We can validate by using netstat command.

cmd

So I will use port 8082 to run WebSocket server.

websocketd.exe -- port 8082 myProgram.exe

cmd

WebSocket client object

With this API, you can send and receive messages to a server without having to poll the server.

Receiving messages

After connection is established, we can receive messages using onmessage event.

  1. var ws = new WebSocket('ws://localhost:8082/');  
  2. // this will establish connection with server at port 8082  
  3. //notice ws: this is new URL schema for WebSocket connection  
  4. ws.onmessage = function(event) {  
  5. console.log('Count is: ' + event.data);  
  6. }; // this will receive output from server in event.data  
output

Sending messages

After connection is established via onopen event, we can send messages using send method.
  1. ws.onopen = function () {  
  2. console.log('OnOpen');  
  3. ws.send('my message');  
  4. }  
Log errors

Use onerror event to trap error generated from WebSocket,
  1. ws.onerror = function (error) {  
  2. console.log('WebSocket Error ' + error);  
  3. };  
ReadyState

If you remember, we have ready state in XHR API. Similary, WebSocket also maintains connection state. It starts from 0 to 3, 
  • 0 | Connecting – the connection is not yet open
  • 1 | Open – the connection is open & ready to communicate
  • 2 | Closing – the connection is closing
  • 3 | Closed – the connection is closed

Close

To close an open connection use close method, like ws.close();

Summary

The web is full of technologies and the intent is to make the web fast & better than before. Our quest shall be to give users good experiences and good performance applications. Please share your feedback / comments.