SSE echo from PHP duplicating at random intervals - php

Question

I'm trying to set up a PHP server send event, which works okay. But at random intervals it is pushing the same data repeatedly.
Here's a quick scenario to clarify what I'm describing: Let's say I insert a db record at 1:00:00. The record's data is pushed as it should. However, at 1:03:00 that record's data is pushed a second time. Then at 1:03:17, it is pushed again. And I now have 3 instances of the record displayed.
Why is this happening, and why at random intervals?
I increased php execution time, but the issue is still occurring.
In the browser console, I'm getting this error: net::ERR_INCOMPLETE_CHUNKED_ENCODING.
I have this for client side:
<script>
var source = new EventSource('pdo_updates.php');
var pdo_updates;
source.onmessage = function(e) {
pdo_updates = e.lastEventId + '' + e.data + '<br>';
document.getElementById("videoID").innerHTML += pdo_updates;
};
evtSource.close();
</script>
And this for server side:
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache, must-revalidate, post-check=0, pre-check=0');
function send_msg($id, $msg) {
echo "data: $msg" . PHP_EOL;
echo PHP_EOL;
ob_flush();
flush();
}
$last_event_id = floatval(isset($_SERVER["HTTP_LAST_EVENT_ID"]) ? $_SERVER["HTTP_LAST_EVENT_ID"] : False);
if ($last_event_id == 0) {
$last_event_id = floatval(isset($_GET["lastEventId"]) ? $_GET["lastEventId"] : False);
}
$last_id = 0;
try {
$conn = new PDO('mysql:host=localhost;dbname=my_db', $username, $password);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
while(1) {
$id = $last_event_id != False ? $last_event_id : $last_id;
$stmt = $conn->prepare("SELECT id, message FROM messages WHERE id > :id ORDER BY id DESC LIMIT 1");
$result = $stmt->execute(array('id' => $id));
$stmt->bindValue('id', $id);
if ($result) {
while($data = $stmt->fetch(PDO::FETCH_ASSOC)) {
if ($data) {
send_msg($data['id'], $data['message']);
$last_id = $data['id'];
}
}
}
sleep(1);
}
} catch(PDOException $e) {
echo 'ERROR: ' . $e->getMessage();
}

Answer 1

So, after trying just about every keepalive related header, and every apache timeout adjustment config, I ended up running a packet capture. I discovered there was a TCP reset was being triggered by the remote end. I have my site behind Cloudflare, and once I disabled Cloudflare the issue partially disappeared. The TCP session was being refreshed every 100 seconds and would cause the last message to appear again when that happened. This was responsible for the browser console error: net::ERR_INCOMPLETE_CHUNKED_ENCODING
At the same time, however, there was an issue in my submit code. But I'm not 100% sure why.
$.ajax({
type: "POST",
url: url,
data: $("#submitmessage").serialize(), // serializes the form's elements.
success: function(data)
{
$("#myvideo").val("");
}
});
I needed to clear the value in my input field after submit. I haven't had any duplicates since.