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    <p><em>In the examples below the client is the browser and the server is the webserver hosting the website.</em></p> <p>Before you can understand these technologies, you have to understand <em>classic</em> HTTP web traffic first.</p> <h2>Regular HTTP:</h2> <ol> <li>A client requests a webpage from a server.</li> <li>The server calculates the response</li> <li>The server sends the response to the client. </li> </ol> <p><img src="https://i.stack.imgur.com/TK1ZG.png" alt="HTTP"></p> <h2>Ajax Polling:</h2> <ol> <li>A client requests a webpage from a server using regular HTTP (see HTTP above).</li> <li>The client receives the requested webpage and executes the JavaScript on the page which requests a file from the server at regular intervals (e.g. 0.5 seconds).</li> <li>The server calculates each response and sends it back, just like normal HTTP traffic.</li> </ol> <p><img src="https://i.stack.imgur.com/qlMEU.png" alt="Ajax Polling"></p> <h2>Ajax Long-Polling:</h2> <ol> <li>A client requests a webpage from a server using regular HTTP (see HTTP above).</li> <li>The client receives the requested webpage and executes the JavaScript on the page which requests a file from the server.</li> <li>The server does not immediately respond with the requested information but waits until there's <strong>new</strong> information available.</li> <li>When there's new information available, the server responds with the new information.</li> <li>The client receives the new information and immediately sends another request to the server, re-starting the process. </li> </ol> <p><img src="https://i.stack.imgur.com/zLnOU.png" alt="Ajax Long-Polling"></p> <h2>HTML5 Server Sent Events (SSE) / EventSource:</h2> <ol> <li>A client requests a webpage from a server using regular HTTP (see HTTP above).</li> <li>The client receives the requested webpage and executes the JavaScript on the page which opens a connection to the server.</li> <li><p>The server sends an event to the client when there's new information available. </p> <ul> <li>Real-time traffic from server to client, mostly that's what you'll need</li> <li>You'll want to use a server that has an event loop</li> <li>Not possible to connect with a server from another domain</li> <li>If you want to read more, I found these very useful: <a href="https://developer.mozilla.org/en-US/docs/Server-sent_events/Using_server-sent_events" rel="noreferrer">(article)</a>, <a href="http://html5doctor.com/server-sent-events/#api" rel="noreferrer">(article)</a>, <a href="http://www.html5rocks.com/en/tutorials/eventsource/basics/" rel="noreferrer">(article)</a>, <a href="http://jaxenter.com/tutorial-jsf-2-and-html5-server-sent-events-42932.html" rel="noreferrer">(tutorial)</a>.</li> </ul></li> </ol> <p><img src="https://i.stack.imgur.com/ziR5h.png" alt="HTML5 SSE"></p> <h2>HTML5 Websockets:</h2> <ol> <li>A client requests a webpage from a server using regular http (see HTTP above).</li> <li>The client receives the requested webpage and executes the JavaScript on the page which opens a connection with the server.</li> <li><p>The server and the client can now send each other messages when new data (on either side) is available.</p> <ul> <li>Real-time traffic from the server to the client <strong>and</strong> from the client to the server</li> <li>You'll want to use a server that has an event loop</li> <li>With WebSockets it is possible to connect with a server from another domain.</li> <li>It is also possible to use a third party hosted websocket server, for example <a href="http://pusher.com/" rel="noreferrer">Pusher</a> or <a href="http://www.leggetter.co.uk/real-time-web-technologies-guide" rel="noreferrer">others</a>. This way you'll only have to implement the client side, which is very easy!</li> <li>If you want to read more, I found these very useful: (<a href="http://www.developerfusion.com/article/143158/an-introduction-to-websockets/" rel="noreferrer">article</a>), <a href="https://developer.mozilla.org/en-US/docs/WebSockets/Writing_WebSocket_client_applications" rel="noreferrer">(article)</a> (<a href="http://net.tutsplus.com/tutorials/javascript-ajax/start-using-html5-websockets-today/" rel="noreferrer">tutorial</a>).</li> </ul></li> </ol> <p><img src="https://i.stack.imgur.com/CgDlc.png" alt="HTML5 WebSockets"></p> <h2>Comet:</h2> <p>Comet is a collection of techniques prior to HTML5 which use streaming and long-polling to achieve real time applications. Read more on <a href="http://en.wikipedia.org/wiki/Comet_%28programming%29" rel="noreferrer">wikipedia</a> or <a href="http://www.ibm.com/developerworks/web/library/wa-reverseajax1/index.html" rel="noreferrer">this</a> article.</p> <hr> <blockquote> <p>Now, which one of them should I use for a realtime app (that I need to code). I have been hearing a lot about websockets (with socket.io [a node.js library]) but why not PHP ?</p> </blockquote> <p>You can use PHP with WebSockets, check out <a href="http://socketo.me/" rel="noreferrer">Ratchet</a>. </p>
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    1. POWhat are Long-Polling, Websockets, Server-Sent Events (SSE) and Comet?
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    1. COThis is awesome! I am reading up on SSE and found this article, it's very nice - like I've now compared stuff, can you also include SSE here so we can also cross-check it's difference with Websocket?
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    2. CO@Tieme Oh was that it? I thought SSE meant Server-Sent Events. Anyway, thanks, I see it now.
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    3. COQ: in php let's say you were using websocket would every client connected to my server using ws: would have one thread allocated to him/her and its size would be ~2mb as is the case with normal requests? how would that differ in nodejs? How many concurrent clients can nodejs handle and when it breaks what happens?
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