长轮询 - jquery + php

Question

I want to long poll a script on my server from within a phonegap app, to check for things like service messages, offers etc.

I'm using this technique in the js:

(function poll(){
$.ajax({
    url: "/php/notify.php",
    success: function(results){
        //do stuff here
    },
    dataType: 'json',
    complete: poll, 
    timeout: 30000,
});
})();

which will start a new poll every 5 minutes (will be stopping the polling when the app is 'paused' to avoid extra load)

I am not sure how to set up the php though? I can set it up so it doesnt return anything and just loops trough the script, but how to make it return a response as soon as i decide i want to send a message to the app? my php code so far is:

<?php

include 'message.php';

$counter = 1;

while($counter > 0){
//if the data variable exists (from the included file) then send the message back to the app
if($message != ''){
    // Break out of while loop if we have data
    break;
}
}

//if we get here weve broken out the while loop, so we have a message, but make sure
if($message != ''){
// Send data back
    print(json_encode($message));
}

?>

message.php contains a $message variable (array), which normally is blank however would contain data when i want it to. The problem is, when i update the $message var in message.php, it doesnt send a response back to the app, instead it waits until it has timed out and the poll() function starts again.

so my question is, how do i set-up the php so i can update the message on my server and it be sent out instantly to anyone polling?

Answer 1

Long polling is actually very resource intensive for what it achieves

The problem you have is that it's constantly opening a connection every second, which in my opinion is highly inefficient. For your situation, there are two ways to achieve what you need; the preferred way being to use web sockets (I'll explain both):

---

Server Sent Events

To avoid your inefficient Ajax timeout code, you may want to look into Server Sent Events, an HTML5 technology designed to handle "long-polling" for you. Here's how it works:

In JS:

var source = new EventSource("/php/notify.php");
source.onmessage=function(event) {
  document.getElementById("result").innerHTML+=event.data + "<br>";
};

In PHP:

You can send notifications & messages using the SSE API interface. I don't have any code at hand, but if you want me to create an example, I'll update this answer with it

This will cause Javascript to long-poll the endpoint (your PHP file) every second, listening for updates which have been sent by the server. Somewhat inefficient, but it works

---

WebSockets

Websockets are another ballgame completely, and are really great

Long-Polling & SSE's work by constantly opening new requests to the server, "listening" for any information that is generated. The problem is that this is very resource-intensive, and consequently, quite inefficient. The way around this is to open a single sustained connection called a web socket

StackOverflow, Facebook & all the other "real-time" functionality you enjoy on these services is handled with Web Sockets, and they work in exactly the same way as SSE's -- they open a connection in Javascript & listen to any updates coming from the server

Although we've never hard-coded any websocket technology, it's by far recommended you use one of the third-party socket services (for reliability & extensibility). Our favourite is Pusher