7.1 WebSocket

• Analyze the requirements for a chatting application of the n-1 model
• Design and implement a chatting application of the n-1 model

• What is WebSocket?
• What does 'bidirectional communication' mean?
• When do you use?
• What port number is used?

• What is 'full-duplex' communications?
• What is the relationship between HTTP and WebSocket?
• What port number is used?
• Can WebSocket server use the port number 80?
• Can WebSocket server be integrated with a web server?
• What is a benefit to use WebSocket instead of general TCP connections?

• Here is an example.

  let ws;
  function connect() {
      if ("WebSocket" in window) {
          ws = new WebSocket("ws://....");  // not HTTP
  
          // register event listeners on the web socket object
          ws.onopen = function() {  // 'open' event listener; when the channel is open
              .... = 'Connected';
          };
          ws.onmessage = function (eobj) {  // when a message came from the server
              .... = eobj.data;
          };
          ws.onclose = function() {  // when the channel is closed from the server
              .... = 'Closed';
          };
          ws.onerror = function(eobj) {  // when there is an error
              .... = eobj.message;
          };
      }
  }
          
  function echo() {
      ws.send(....);  // send a message to the server
  }
          
  function end() {
      ws.close();  // close the channel
  }
  

• List the four events associated with WebSocket object.
  open, close, error, message
• List the two methods to send a message and close the connection.
  .send(), .close()
• if ("WebSocket" in window) ???

• Can you understand the URL in WebSocket()?
  The URL should start with ws, not http.
• Can you make multiple connections to the same WebSocket server?
  You can open another window with the same URL.

• socket.io
• nodejs-websocket
• ws: a Node.js WebSocket library
• I tried to use socket.io and nodejs-websocket modules for a simple WebSocket integrated with TRUWSJS and RESTful MongoDB service server. None of them worked well. ws is the natural choice for this class.

• A WebSocket server program integrated with your Node web server - A WebSocket server program uses the same port number that the web server uses. Do you think a WebSocket server program can be integrated with a HTTPS server too?
• A standalone WebSocket server program - A WebSocket server program uses a different port number that the web server does not use.

// Example of standalone WebSocket echo server with the port number 8888
const WebSocketServer = require("ws").Server;
const ws_server = new WebSocketServer({port: 8888});  // Use your port number
    
ws_server.on("connection", function(ws_client, request) {  // ws_client is an WebSocket object, i.e., a socket for a client.
                                                           // what if there is a connection request from another clent?
    // Event listeners on the client web socket object
    ws_client.on("message", function(msg) {  // msg is a Blob object.
        msg = String(msg);
        ws_client.send(msg);
        if (msg == "end")  // 'end' is an application level message.
            ws_client.close();
    });
    ws_client.on("close", function(code) {
        console.log(code);
        ws_client.close();
    });
    ws_client.on("error", function(err) {
        console.log(err);
        ws_client.close();
    });
});

// WebSocket server integrated with a web serverd that uses the port number 8888

// HTTP server:

const HTTP_PORT_NO = 8888;  // Use your port number
const http = require("http");
    
const http_server = http.createServer(function(request, response) {
    ...
});
http_server.listen(HTTP_PORT_NO);

// WebSocket server:

const WebSocketServer = require("ws").Server; // WebSocket server integrated with a web server
const ws_server = new WebSocketServer({server: http_server});  // different from the standalone version
    
ws_server.on("???", function(ws_client, request) {    // ws_client is an WebSocket object, i.e., a socket for a client.
                                                             // what if there is a connection request from another clent?
    ws_client.on("???", function(msg) {  // msg is a Blob object.
        msg = String(msg);
        if (msg == "end" || msg == "close" || msg == "quit") {
            ws_client.???();
        } else {
            ws_client.???(msg);  // echoing
        }	
    });
    ws_client.on("???", function(msg) {
        console.log('closed');
    });
});

const express = require("express");
const app = express();

const expressWs = require("express-ws");
expressWs(app);

app.listen(8888);  // It should be done after expressWs(app). Use your port number.

// Express service for a route
app.get("/", function(req, res, next){
    ...
});

// WebSocekt service for the "/echos" path
app.ws("/echos", function(wsClient, req) {
    console.log("connected");
    wsClient.on("???", function(msg) {  // msg is a string.
        wsClient.???(msg);
    });
    wsClient.???("???", function(msg) {
        console.log("closed");
    });
});

• Multiple users
• A message to a user from another user
• A message to all users from a user

• User management
  • Join
  • Unsubscribe
  • Sign in
  • Sign out
• One-to-one chatting
• Broadcasting

• 'Join' (or 'SignUp') from client to server:
  '{command: join, name: ..., password: ...}'
• 'Sign in' from client to server:
  '{command: sign_in, name: ..., password: ...}'
• 'Sign out' ???:
  '{command: sign_out, name: ...}'
• 'Okay' from server to client:
  '{command: okay, for: ...}'
• 'Not Okay' from server to client:
  '{command: notokay, for: ...}'
• 'Signed in' from server to each client so that they can see who is signed in:
  '{command: signed_in, name: ...}'
• 'Signed out' ???:
  '{command: signed_out, name: ...}'

---

• 'One-to-one' chatting from client to server:
  '{command: one_to_one, name: ..., to: ..., message: ...}'
• 'Broadcasting' ???:
  '{command: broadcasting, name: ..., message: ...}'
• 'From' from server to client:
  '{command: from, name: ..., message: ...}'
• 'Error' ???:
  '{command: error, message: ...}'

---

• What else do you need?
• Here is an example how to use the above application-level messages.

  Client-A (Dave)                                     Server                                                     Client-B (John)
          ----------------Connection-------------------->  
                                                      ws_client object created
          ---->'{command: sign_in, name: Dave, ???}'---->  
          <----'{command: okay|notokay, for: sign_in}'--- 
                                                      keep 'Dave' and ws_client socket object in an object
          <----'{command: signed_in, name: John}'--------
          <----'{command: ???, name: Tom}'---------------
                                                         ----------->'{command: signed_in, name: Dave}'------------->
          ---->'{command: one_to_one, name: Dave, to: John, message: Hello}'
                                                         ----------->'{command: from, name: Dave, message: Hello}'-->
          ---->'{command: ???, name: ???, to: John, ???: Bye}'
                                                         ----------->'{command: ???, name: Dave, message: Bye}'----->
          ---->'{command: sign_out, name: Dave}'-------->  
                                                      Delete 'Dave' and ws_client socket object from the object
                                                         ----------->'{command: ???, name: Dave}'------------------->
          <---------------Disconnection----------------->                                                        
  

• Any good idea?
• You may review JSON and AJAX.
• Trial 5. Let's change the echo client program in Trial 1 so that the program can send the JSON strings of the above commands, such as Join, SignIn, SignOut, and OneToOne. You can read the command of JSON string type from the <input>, convert it to an object, convert it back to a JSON string, and send it. Test it with your echo server, or ws://198.162.21.132:8080/~mlee/comp4620/Software/TRUWSJS/rev.4.fork/test_ws_echo_server.wsjs
  <button id='tr5-connect'>Connect</button><br> <input id='tr5-message'> <button id='tr5-send'>Send</button> <button id='tr5-colse'>Close</button> <div id='tr5-result-pane'>The message from the server here.</div> <script> // Click event listeners on the above buttons for "Connect", "Send", and "Close". // For example, for "Connect", the function, connect(), should be invoked in the event listener. ???? var ws; function connect() { if ("WebSocket" in window) { ws = new ???("ws://...."); // register event listeners on the web socket object ws.??? = function() { // 'open' event listener; when the channel is open .... = 'Connected'; }; ws.??? = function (eobj) { // when a message came from the server .... = eobj.data; }; ws.onclose = function() { // when the channel is closed .... = 'Closed'; }; ws.onerror = function() { // when there is an error .... }; } } function echo() { ... ws.send(....); // send a message to the server } function end() { ws.close(); // close the channel } </script> Submit

  
  
  

• You would better to use a database.
• Let's use MongoDB. Do you remember how MongoDB is different from MySQL?

• Let's make the user interface simple.
• What do you need?
  • Menu for signin, signout, join, and unsubscribe, with name and password inputs
  • Textarea for sending messages
  • Two div elements for received messages and the list of signed users
• Can you make the above user interface quickly?

• singed_in
• signed_out
• from
• error
• Here is an example.

  let ws = new WebSocket("ws://198.162.21.132:8888/");  // should be ws:
  
  ws.onopen = function() {  // Web Socket is connected, send messages using send()
      ...
  };
  
  ws.onmessage = function (evt) { 
      let msgfromserver = JSON.parse(evt.data);
      switch(msgfromserver.command) {
          case 'signed_in':
              ...
          case 'signed_out':
              ...
          case 'from':
              $('#output').append(msgfromserver.name + ': ' + msgfromserver.message + '<br>');
              ...
          case 'error':
              ...
      }
  };
  
  ws.onclose = function() {  // websocket is closed.
      ...
  };
  
  ws.onerror = function(evt) { 
      ...
  };
  
  $('#one_to_one').click(function() {   
      let msgtoserver = {};
      msgtoserver.command = 'one_to_one';
      msgtoserver.name = myname;
      msgtoserver.to = $('#receiver').text();
      msgtoserver.message = $('#message').text();
      ws.send(JSON.stringify(msgtoserver));
  });
  ...
  

• Here is the echo-server example that we studied before.

  const WebSocketServer = require('ws').Server,
      ws_server = new WebSocketServer({port: 8888});
      
  ws_server.on('connection', function(ws_client) {  // ws_client is an WebSocket object, i.e., a socket for clients.
      ws_client.on('message', function(msgfromclient) {
          ws_client.send(msgfromclient);
          if (msgfromclient == 'end')
              ws_client.close();
      });
      ws_client.on('close', function(code) {
          ws_client.close();
      });
      ws_client.on('error', function(err) {
          ws_client.close();
      });
  });
  

• Which event should be used for the exchange of user messages betweent the client and server?

• join
• sign_in
  • The client sockets should be saved with the corresponding usernames.
• sign_out
• one_to_one
• The client socket for the receipient should be found.
• broadcasting
• Here is an example.

  const WebSocketServer = require('ws').Server,
      ws_server = new WebSocketServer({port: 8888});
  
  let ws_clients = {};  // {userName:ws_client, ...}, where userNames are unique
      
  ws_server.on('connection', function(ws_client) {  // ws_client is an WebSocket object, i.e., a socket for clients.
      ws_client.on('message', function(message) {
          msgfromclient = JSON.parse(message);
          switch(msgfromclient.command) {
              case 'join':
                  ...
                  break;
              case 'sign_in':
                  ...
                  ws_clients[msgfromclient.name] = ws_client;
                  ...
                  break;
              case 'sign_out':
                  ...
                  ??? ws_clients[msgfromclient.name];  // delete the corresponding ws_client
                  ...
                 break;
              case 'one_to_one':
                  // find the ws_client for msgfromclient.to, and send msgfromclient.message to the ws_client
                  ws_clients[???].send(???);
                  break;
              case 'broadcasting':
                  ...
                  break;
              ...
          }
      });
      ws_client.on('close', function(code) {
          ...  // broadcasting it
          ...  // find the socket from ws_clients and delete it
      });
      ws_client.on('error', function(err) {
          ...  // broadcasting it
          ...  // find the socket from ws_clients and delete it
      });
  });