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    <h2>Update</h2> <p>As I continue to recieve upvotes on this, I think it is reasonable to remember that this answer is 4 years old. Web has grown in a really fast pace, so please be mindful about this answer.</p> <hr> <p>I had the same issue recently and researched about the subject.</p> <p>The solution given is called long polling, and to correctly use it you must be sure that your AJAX request has a "large" timeout and to always make this request after the current ends (timeout, error or success).</p> <h2>Long Polling - Client</h2> <p>Here, to keep code short, I will use jQuery: </p> <pre><code>function pollTask() { $.ajax({ url: '/api/Polling', async: true, // by default, it's async, but... dataType: 'json', // or the dataType you are working with timeout: 10000, // IMPORTANT! this is a 10 seconds timeout cache: false }).done(function (eventList) { // Handle your data here var data; for (var eventName in eventList) { data = eventList[eventName]; dispatcher.handle(eventName, data); // handle the `eventName` with `data` } }).always(pollTask); } </code></pre> <p>It is important to remember that (from <a href="https://api.jquery.com/jquery.ajax/" rel="nofollow noreferrer">jQuery docs</a>): </p> <blockquote> <p>In jQuery 1.4.x and below, the XMLHttpRequest object will be in an invalid state if the request times out; accessing any object members may throw an exception. In Firefox 3.0+ only, script and JSONP requests cannot be cancelled by a timeout; the script will run even if it arrives after the timeout period.</p> </blockquote> <h2>Long Polling - Server</h2> <p>It is not in any specific language, but it would be something like this:</p> <pre><code>function handleRequest () { while (!anythingHappened() || hasTimedOut()) { sleep(2); } return events(); } </code></pre> <p>Here, <code>hasTimedOut</code> will make sure your code does not wait forever, and <code>anythingHappened</code>, will check if any event happend. The <code>sleep</code> is for releasing your thread to do other stuff while nothing happens. The <code>events</code> will return a dictionary of events (or any other data structure you may prefer) in JSON format (or any other you prefer).</p> <p>It surely solves the problem, but, if you are concerned about scalability and perfomance as I was when researching, you might consider another solution I found.</p> <h2>Solution</h2> <p>Use sockets! </p> <p>On client side, to avoid any compatibility issues, use <a href="https://socket.io" rel="nofollow noreferrer">socket.io</a>. It tries to use socket directly, and have fallbacks to other solutions when sockets are not available.</p> <p>On server side, create a server using NodeJS (example <a href="http://www.stoimen.com/blog/2010/12/02/diving-into-node-js-a-long-polling-example/" rel="nofollow noreferrer">here</a>). The client will subscribe to this channel (observer) created with the server. Whenever a notification has to be sent, it is published in this channel and the subscriptor (client) gets notified.</p> <p>If you don't like this solution, try APE (<a href="http://www.ape-project.org/" rel="nofollow noreferrer">Ajax Push Engine</a>).</p> <p>Hope I helped.</p>
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    1. COdo you think 1 is a replacement for the other or is there a need for both technologies on the same project?
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    2. COIf you mean APE and NodeJS, you can choose one of them. if you mean periodic AJAX requests and the one i suggested, my solution may fallback to the ajax one when lacks socket support (refer to socket.io docs). In both cases, you need only one solution.
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