How does facebook, gmail send the real time notification?

Question

I have read some posts about this topic and the answers are comet, reverse ajax, http streaming, server push, etc.

How does incoming mail notification on Gmail works

Answer 1

One important issue with long polling is error handling. There are two types of errors:

1. The request might timeout in which case the client should reestablish the connection immediately. This is a normal event in long polling when no messages have arrived.

2. A network error or an execution error. This is an actual error which the client should gracefully accept and wait for the server to come back on-line.

The main issue is that if your error handler reestablishes the connection immediately also for a type 2 error, the clients would DOS the server.

Both answers with code sample miss this.

function longPoll() { 
        var shouldDelay = false;

        $.ajax({
            url: 'poll.php',
            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 (data, textStatus, jqXHR) {
             // do something with data...

        }).fail(function (jqXHR, textStatus, errorThrown ) {
            shouldDelay = textStatus !== "timeout";

        }).always(function() {
            // in case of network error. throttle otherwise we DOS ourselves. If it was a timeout, its normal operation. go again.
            var delay = shouldDelay ? 10000: 0;
            window.setTimeout(longPoll, delay);
        });
}
longPoll(); //fire first handler