通过Python通过PHP将实时传感器数据作为服务器发送事件（SSE）？

Question

I'm in the process of updating a web app presenting real-time sensor data, which in it's current first iteration does this by continuous AJAX polling. However, in order to make this more like a 'true real-time' app, I would like it to be event-based.

I've been reading up on event-based techniques, and based on the fact that the real-time communication only has to go one way (server - > client), I have chosen to go with Server-Sent Events (SSE) for now instead of something like websockets. As described here on the Mozilla Docs , this is easily implemented on the server side with something like (a little simplified):

<?php
// SSEscript.php
date_default_timezone_set("America/New_York");
header("Content-Type: text/event-stream\n\n");

while (1) {
   if ($new_data_available) {
      echo "data:". $data;
   } 

   sleep($short_time_to_spare_cpu);
}
?>

and on the client side with:

<script>
var evtSource = new EventSource("SSEscript.php");

evtSource.onmessage = function(e) {
   var data = e.data;
   // Do something with data object
} 
</script>

All the above works fine for me. However, the sensor data is initially retrieved by a Python script running continuously on the server, so how do I transfer the sensor data from the Python script to the PHP script IMMEDIATELY when it is retrieved, so that an event can be generated and sent?

Can I do something like depicted below? :

At the same time all new data is stored in a MySQL db, so I could of course make the PHP script query the db really often for new entries, but there has to be a smarter way. So can I make 2.1 and 2.2 in the image happen at the same time?

All the answers I could find in here describes how data can be transferred by making the PHP execute the Python script, but that is not what I want as this has to run whether or not a user asks for data.

Is a kind of socket the way to go, and if so, can you point me in the direction of how to do so? I hope you can help me out!

Answer 1

I use redis to do the signalling between python and redis. After python do all the stuffs, rpush a token (or the latest data) into a redis queue. In the PHP I use a while(true) loop to hold the request and redis lpop(queue,timeout) to wait for the token, and send data out, something like this:

<?php
    require __DIR__.'/predis-1.0/autoload.php';
    header("Content-Type: text/event-stream");
    header("Cache-Control: no-cache");
    header("Connection: keep-alive");

    $lastId = isset($_SERVER["HTTP_LAST_EVENT_ID"]) ? 
              $_SERVER["HTTP_LAST_EVENT_ID"] : null;
    if (isset($lastId) && !empty($lastId) && is_numeric($lastId)) {
        $lastId = intval($lastId);
        $lastId++;
    }

    $index = isset($_GET['index']) ? $_GET['index'] : null;

    echo "retry: 2000\n";

    $client = new Predis\Client();
    while (true) {
        $data = $client->blpop('queue',5);
        if ($data) {
            error_log("$index : " . strlen($data));
            sendMessage($lastId, $data);
            $lastId++;
        }
    }

    function sendMessage($id, $data) {
        echo "id: $id\n";
        echo "data: $data\n\n";
        ob_flush();
        flush();
    }