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Alternate of server polling?

As we know, if running application also manage sessions in main memory then is there any way for server to send responses to all web clients/browsers for new recorded data in a database.
Remember: I have not made any request to server or polling to server for new records update..
Let server make responses without web request..
Objective :
No all web browsers making request or polling to server for every certain interval therefore reducing the performance issue with the application memory..
Am just against of making so many ajax calls from every web client..
Need your ideas from your past, if experienced similar..

read about websockets and socket.io.
basically with socket.io you have a connection open between browser (client) and server and server can send data which the client than receives as an event.
the client doesn't need to send a request to get that data, only open the web socket connection.
you can look at socket.io chat example: http://socket.io/get-started/chat/

WebSocket is the best and easy solution if you don't want to go through the hassle to learn Angular or others.
Both server-side and client-side can build WebSocket, and it acts as a bridge to transmit data back and forth.
I just created an easy solution for this.
Please check my new library wsm - WebSocket Manager, it works for both server-side and client-side.
Websocket Server can be built easily; this library includes several useful features.

How to trigger js event with Java?

I am building a java chatting web application.(Server and Client in one project)
follow is my condition
Springframework 4.2.3
JSP
Maven Project which converted from a Dynamic Web Project
Unable to use node.js
So User scenario in my head is
User enters some text and press Send
Ajax call to deliver messages to server
Server checks the users who are currently connected(from Session maybe)
Server calls other users script to append new message
I am quite confusing with step4. Is it possible that Java calls DOM event trigger?
How could a client get a new message event from server?
Thanks. :D
P.S.
These days majority of chatting servers are event-driven. Is it possible to build an event driven chatting server with Java?

Your solution is formed as if there is no WebSocket technology available to you. WebSocket is implemented to solve real time messaging issues. It pushes message to the destination.
But if to stick wih your method following is meaningful.
You need someplace to keep incoming messages such as database or session.setAttribute [bad idea]. Then use some ajax call loop on the clients machine to ask for a new incoming message from server.
Probably your server will slow down due to incoming flood of GET requests from multiple users.
to Your last question in post scriptum: yes, I use tomcat websocket api.jar in my projects. There is well written documentation on apache.org
As I mentioned, learn WebSocket if your users are not using old internet explorer browsers. There are bunch of tutorials on it...

Real Time Functionlity with php

I am working on attendance system where project management system is also handling using Javascript\Jquery at client side and PHP\MySQL at server side.
A feature in my web app is user message to admin. For this I have applied an ajax request which is made by setTimeout function (after every 15 seconds) to check that is there any new message comes in database if yes then return it to the admin.
It working fine but the drawback is (as you know) continuous request to server which really poor and bad. As I know that real time functionality can solve this problem I have checked some link websocket.io and signlR but there are applicable with Nodejs and asp.net.
So, how can I apply real time functionality with php or I get that new data have reached in to database without continues request with ajax.

There are actually some PHP tools for doing this now. For instance, check out Ratchet.
It's also possible to do bi-directional sockets by creating a simple TCP/IP server. I've done this before for bi-directional communication between a PHP server and a desktop app.
http://php.net/manual/en/sockets.examples.php

Express web sockets and a central server

I'm creating an app where the server and the clients will run on the same local network. Is it possible to use web sockets, or rather more specifically, socket.io to have one central server and many clients that are running native apps
? The way I understand socket.io to work is that the clients read the web-pages that are served from the server but what happens when your clients become tablet devices running native apps instead of web pages in a browser?
The scenario I'm working with at the minute will have one central server containing a MEAN app and the clients (iPads) will make GET requests to the data available on the server. However, I'd also like there to be real-time functionality so if someone triggers a POST request on their iPad, the server acknowledges it and displays it in the server's client-side. The iPad apps will (ideally) be running native phonegap applications rather than accessing 192.168.1.1:9000 from their browser.
Is this technically possible to connect to the socket server from the native apps or would the devices have to send POST requests to a central server that's constantly listening for new 'messages'? I'm totally new to the whole real-time stuff so I'm just trying to wrap my head around it all.
Apologies if this isn't totally clear, it's a bit hard to describe with just text but I think you get the idea?

Correct me if I am wrong.
You have multiple iPads running native app. They send a POST request to your node JS server which is running in a computer in the same local network. Whenever the server receives a request from app, you want to display that a request has been received in your computer screen.
If my assumptions about the scenario is correct, then it is fairly easy to do. Here are the steps to do it.
Create a small webpage (front end). Load socket IO in the front end page like this -
<script type="text/javascript" src="YOUR_SERVER_IP/socket.io/socket.io.js"></script>
Then connect to server using var socket = io(). This should trigger connection event in your backend.
Handle all POST request from apps normally. Nothing special. Just add a small snippet in between. socket.emit('new_request', request_data). This sends new_request event to front end.
Handle the new_request in your front end using socket.on('new_request', function(request_data) { ... });. That's it. No need to add anything to your native app for realtime update.
The second step would be a little complicated as it is necessary to make socket variable available inside all POST requests. Since you chose node.js, I don't think you need any help with that.

Not totally clear on your project, but I'll try to give you some pointers.
An effective way to send data between native apps and a server is using a REST server. REST is based on HTTP requests and allows you to modify data on the server, which can connect to your database. The data returned is typically either JSON or XML formatted. See here for a brief intro: http://www.infoq.com/articles/rest-introduction
Android/iOS/etc have built in APIs for making HTTP requests. Your native app would send a request to the server, parse the response, and update your native UI accordingly. The same server can be used from a website using jQuery ajax HTTP requests.
Express.js is more suited to serving web pages and includes things like templating. Look into "restify" (see here: mcavage.me/node-restify/) if you just want to have a REST server that handles requests. Both run on top of node.js (nodejs.org).
As far as real-time communication, if you're developing for iOS look into APNS (Apple Push Notification Service). Apple maintains a persistent connection, and by going through their servers you can easily send messages to your app. The equivalent of this on Android is GCM (Google Cloud Messaging).
You can also do sockets directly if that's easier for you. Be careful with maintaining an open socket on a mobile device though, it can be a huge battery drain. Here's a library for connecting ObjC to Socket.IO using websockets, it may be useful for you: https://github.com/pkyeck/socket.IO-objc
Hope that helps!

To answer your question, it is definitely possible. Socket.io would serve as the central server that can essentially emit messages to all of the client. You can also make Socket.io listen for the messages from any of the clients and serve the emitted message to the rest of the clients.
Here's an example of how socket.io can be used. Simply clone, npm install, and run using 'node app.js'

All you have to do is to provide a valid server address when you connect your socket from the iPad clients:
var socket = io.connect( 'http://my.external.nodejs.server' );
Let us know if you need help with actual sending/receiving of socket events.

It is possible to connect to Websockets from your apps.
If you are using PhoneGap then you need a pluging that gives support to websockets in your app (the client) and then use websocket like normal way using Javascript see this.
If your app is native iOS look into this it could help you.

The primary use of the Sockets in your case is to be a bidirectional "pipe" between an app and server. There is no need of server sending the whole web-page to the native app. All what you need is to send some data from server to the client(app) in response to POST (or GET) request and then using this data on client side to update client's UI in real-time. If you are going to use moderate amount of devices (say tens of them), you may have connected all of them to the server permanently keeping individual socket connection open for every individual link server-to-app. Thus you may deliver data and update client's state in real time.
In fact web browsers also employ sockets to communicate to web servers. However as in general case there is no control on amount of concurrent clients in Internet, for the sake of limited networking resources conservation, servers do not keep sockets open for a long time, closing it just after the web-page was sent to client (or timeout has expired). That's how HTTP protocol works on the low level. The server waiting for the HTTP clients (browsers) by listening the 80 port, responding them by sending the whole web page content, then closing the connection and keep waiting for another requests on the same port.
In your case it's basically a good idea to use socket.io as it's a uniform implementation of sockets (ok WebSockets) on both client and server side. The good starting point is here

Realtime web programming: how does it work?

As a web developer, I have developed a chat service and some other real-time collaborative services with the help of third-party services like Redis or Pusher. They provide simple API's that I can use publish/subscribe model to achieve bidirectional communication over the webserver. I want to now implement a simple push notification without the use of any third-party services, but I am not quite sure how to achieve this. The situation is as below:
Backend is in Python (Django)
A user receives a private message from another user.
The recipient should be notified without having to refresh the web browser.
My questions:
From this post, Django seems a bad option to achieve such functionality while Node.js is a good one. Is it true? If it is, why is that?
Is it possible to open a websockets from client to the server, to listen to certain changes to a specific model? (ex. when there's a new message in Message model, update the DOM)
I appreciate any help a lot!

WebSockets are exactly what you want here. They are however, a (comparatively) recent addition to browsers and as a result, support is not ubiquitous. WebSocket Support
The websocket model allows you to connect a socket from a server to a client with a web browser, then send messages from the server and receive them asynchronously at the client and vice versa.
Because Node.js is Javascript and the chances are you will be writing your client in Javascript, it lends itself as a sensible choice for writing coupled components, such as in this scenario.
The most popular WebSocket library is Socket.io which was built for use with Node.js. With Socket.io, your notification model would look something like this:
Server
io.sockets.on('connection', function (socket) {
socket.emit('notification', { name: 'Notification', message: 'It\'s here');
});
Client
var notifications = document.getElementById('nlist');
socket.on('notification', function(notification) {
var div;
// make some DOM changes
document.title = notification.name;
div = document.createElement('div');
div.innerHTML = notification.message;
notifications.appendChild(div);
socket.emit('received-notification');
});
Socket.io allows you to use custom event names, as shown here, which makes designing communication APIs that little bit easier. It also has fallbacks for XHR Long Polling and Flash sockets, in the circumstance that the user does not have Web Sockets.
Node will be faster than Django in this circumstance, but you may find that your codebase is more manageable in Django if this is your first venture into Node. It can be kinda difficult to design an application with a series of callbacks like this. I haven't ever used WebSockets with Django, but my experience with Tornado and WebSockets has been poor.

If you just need unidirectional communication as pushing notifications, you'll better have a look at Server Side Events instead of Websockets.
SSE is much easier to implement, it doesn't need any special protocol (just standard HTTP) and does automatic reconnection.
Creating private messages would be done using good old XMLHttpRequests and should be filtered on client-slide.
Node.js is pretty good for serving SSE: its event loop implementation only uses one thread to manage multiple persistent connections.
Have a look at sse-pubsub which provides an easy way to send notifications.