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phpmysqlajaxlong-polling

XHR long polling for chat application (instead sockets)?

I need to create a user-to-user real time chat system. I managed to create a simple AJAX/PHP/MySQL script for the chat, but one thing worries me in the PHP/MySQL part:

while(true) {
    // ...SELECT * FROM messages ORDER BY date DESC LIMIT 1...

    if($row['date'] > $_POST['last']) {
        echo json_encode($row);

        break;
    }

    sleep(1);
}

Doesn't that mean that it will SELECT the table every 1 second and wouldn't that overload the server?

I tried to use PHP sockets, but it was a nightmare. I had to buy SSL certificate, also, the server was crashing on tests with many users, so I decided to use the long pulling system for now. If I remember correctly, Facebook was using XHR long polling before switching to sockets (if they switched at all).

Solution

  • Well, your question is too generalized. While it mostly depends on the load your will receive and your server specifics, it's generally discouraged to use long polling.

    There are certain assumptions made in your question, that are not totally true at all.

    I had to buy SSL certificate, also

    Did you? As far as I know there are free certificate issuers, such as letsencrypt. The problem here is maybe you are using shared hosting with FTP only access. Consider getting a VPS from some cloud provider, e.g. DigitalOcean. You will have full-access to the virtual environment there. And most of the cheapest VPSs are on the same price you can get a shared hosting.

    Facebook was using XHR long polling before switching to sockets

    Yes, they were and some times they fall back to them too. But, first of all - they have a lot of computing power. They can afford these things. And second, the facebook web chat is not the fastest app I have ever used :)

    -

    With indexed columns and a few records on a normal MySQL single instance, you will not notice any slow downs. When the data grows and so do the users (simultaneous connections), you will gradually find out that you need to optimize things and one day you will eventually go to the WebSocket's favor.

    PHP at heart is not meant to be asynchronous. All the async things along with the whole event loop you will need to either do by yourself or compose several frameworks to come to the rescue.

    I would generally suggest a complete proprietary WebSocket implementation with asynchronous runtime. You either take a look at Ratchet for PHP or use ws for Node.js.