While loops for server-sent events are causing page to freeze

Question

I am currently working on a chat that uses Server-Sent Events to receive the messages. However, I am running into a problem. The server-sent event never connects and stays at pending because the page doesn't load.

For example:

<?php
    while(true) {
        echo "data: This is the message.";
        sleep(3);
        ob_flush();
        flush();
    }
?>

I expect that every 3 seconds, "data: This is the message." will be outputted. Instead, the page just doesn't load. However, I need this behavior for server-sent events. Is there a way to fix this?

Edit:

Full Code:

<?php
   session_start();

    require "connect.php";
    require "user.php";

    session_write_close();

    echo $data["number"];

    header("Content-Type: text/event-stream\n\n");
    header('Cache-Control: no-cache');

    set_time_limit(1200);

    $store = new StdClass(); // STORE LATEST MESSAGES TO COMPARE TO NEW ONES
    $ms = 200; // REFRESH TIMING (in ms)
    $go = true; // MESSAGE CHANGED

    function formateNumber ($n) {
            $areaCode = substr($n, 0, 3);
            $part1 = substr($n, 3, 3);
            $part2 = substr($n, 6, 4);
            return "($areaCode) $part1-$part2";
    }

    function shorten ($str, $mLen, $elp) {
        if (strlen($str) <= $mLen) { 
            return $str;
        } else {
            return rtrim(substr($str, 0, $mLen)) . $elp;
        }
    }

   do {
    $number = $data["number"];
        $sidebarQ = "
            SELECT * 
            FROM (
                SELECT * 
                FROM messages 
                WHERE deleted NOT LIKE '%$number%' 
                AND (
                    `from`='$number' 
                    OR 
                    `to`='$number'
                ) 
                ORDER BY `timestamp` DESC
            ) as mess 
            GROUP BY `id` 
            ORDER BY `timestamp` DESC";
        $query = $mysqli->query($sidebarQ);

        if ($query->num_rows == 0) {
            echo 'data: null' . $number;
            echo "\n\n";
        } else {

            $qr = array();
            while($row = $query->fetch_assoc()) {
                $qr[] = $row;
            }

            foreach ($qr as $c) {
                $id = $c["id"];
                if (!isset($store->{$id})) {
                    $store->{$id} = $c["messageId"];
                    $go = true;
                } else {
                    if ($store->{$id} != $c["messageId"]) {
                        $go = true;
                        $store->{$id} = $c["messageId"];
                    }
                }
            }

            if($go == true) {
                $el = $n = "";

                foreach ($qr as $rows) {
                    $to = $rows["to"];
                    $id = $rows["id"];
                    $choose = $to == $number ? $rows["from"] : $to;
                    $nameQuery = $mysqli->query("SELECT `savedname` FROM `contacts` WHERE `friend`='$choose' AND `number`='$number'");
                    $nameGet = $nameQuery->fetch_assoc();
                    $hasName = $nameQuery->num_rows == 0 ? formateNumber($choose) : $nameGet["savedname"];

                    $new = $mysqli->query("SELECT `id` FROM `messages` WHERE `to`='$number' AND `tostatus`='0' AND `id`='$id'")->num_rows;
                    if ($new > 0) {
                        $n = "<span class='new'>" . $new . "</span>";
                    }

                    $side = "<span style='color:#222'>" . ($to == $number ? "To you:" : "From you:") . "</span>";
                    $el .= "<div class='messageBox sBox" . ($nameQuery->num_rows == 0 ? " noname" : "") . "' onclick=\"GLOBAL.load($id, $choose)\" data-id='$id'><name>$hasName</name><div>$side " . shorten($rows["message"], 25, "...") . "</div>$n</div>";
                }
                echo 'data: '. $el;
                echo "\n\n";

                $go = false;
            }
        }

        echo " ";

        ob_flush();
        flush();
        sleep(2);
    } while(true);
?>

I would also like to note, that this infinite loop shouldn't be causing this to happen. This is just how SSE's are set up usually and it is even done so on the MDN website.

Answer 1

No doubt by now you have figured this out but on the offchance you have not I used code like the following on a couple of sse scripts and it worked like a charm. The code below is generic and does not feature your sql or recordset processing but the idea is sound(!?)

<?php
    set_time_limit( 0 );
    ini_set('auto_detect_line_endings', 1);
    ini_set('mysql.connect_timeout','7200');
    ini_set('max_execution_time', '0');

    date_default_timezone_set( 'Europe/London' );
    ob_end_clean();
    gc_enable();



    header('Content-Type: text/event-stream');
    header('Cache-Control: no-cache');
    header('Access-Control-Allow-Credentials: true');
    header('Access-Control-Allow-Methods: GET');
    header('Access-Control-Expose-Headers: X-Events');  




    if( !function_exists('sse_message') ){
        function sse_message( $evtname='chat', $data=null, $retry=1000 ){
            if( !is_null( $data ) ){
                echo "event:".$evtname."\r\n";
                echo "retry:".$retry."\r\n";
                echo "data:" . json_encode( $data, JSON_FORCE_OBJECT|JSON_HEX_QUOT|JSON_HEX_TAG|JSON_HEX_AMP|JSON_HEX_APOS );
                echo "\r\n\r\n";    
            }
        }
    }

    $sleep=1;
    $c=1;

   $pdo=new dbpdo();/* wrapper class for PDO that simplifies using PDO */

    while( true ){
        if( connection_status() != CONNECTION_NORMAL or connection_aborted() ) {
            break;
        }
        /* Infinite loop is running - perform actions you need */

        /* Query database */
        /*
            $sql='select * from `table`';
            $res=$pdo->query($sql);
        */

        /* Process recordset from db */
        /*
        $payload=array();
        foreach( $res as $rs ){
            $payload[]=array('message'=>$rs->message);  
        }
        */

        /* prepare sse message */
        sse_message( 'chat', array('field'=>'blah blah blah','id'=>'XYZ','payload'=>$payload ) );

        /* Send output */
        if( @ob_get_level() > 0 ) for( $i=0; $i < @ob_get_level(); $i++ ) @ob_flush();
        @flush();

        /* wait */
        sleep( $sleep );
        $c++;

        if( $c % 1000 == 0 ){/* I used this whilst streaming twitter data to try to reduce memory leaks */
            gc_collect_cycles();
            $c=1;   
        }
    }



    if( @ob_get_level() > 0 ) {
        for( $i=0; $i < @ob_get_level(); $i++ ) @ob_flush();
        @ob_end_clean();
    }
?>

Answer 2

While this is not a direct answer as to the problem, try using this method to find the error.. Your not getting errors, but this should help you find them maybe?

Basically you want to have a simple PHP script which includes your main script, but this page enables errors... Example below..

index.php / Simple Error Includer

<?php
    ini_set('display_errors',1);
    ini_set('display_startup_errors',1);
    error_reporting(-1);
    require "other.php";
?> 

other.php / You Main Script

<?php
    ini_set('display_errors',1);
    ini_set('display_startup_errors',1);
    error_reporting(-1);
 weqwe qweqeq
 qweqweqweqwe

?> 

If you create a setup like this, if you view index.php you will see the following error Parse error: syntax error, unexpected 'qweqeq' (T_STRING) in /var/www/html/syntax_errors/other.php on line 5 because it does not have an invalid syntax on the main page and allows any includes to be error checked..

But if you where to view other.php, you would simply get a white / blank page because its unable to validate the whole page/script.

I use this method in my projects, that way regardless of what i do in other.php or any linked php pages, i will see an error report for them.

Please understand the code before commenting to say this disables error control means you did not bother to RTM

Fill the buffer

Another issue in the past that i remember was filling the buffer before it would output to the browser. So try something like this before your loop.

echo str_repeat("\n",4096);  // Exceed the required browser threshold 
for($i=0;$i<70;$i++)     {
    echo "something as normal";
    flush();
    sleep(1);
}

Examples at http://www.sitepoint.com/php-streaming-output-buffering-explained/

Answer 3

It seems like the sleep function is interfering with the output. Putting the sleep function AFTERWARDS did work:

<?php
while(true) {
    echo "data: This is the message.";
    ob_flush();
    flush();
    sleep(3);
    }

As other people suggest, I would encourage to use AJAX instead of an infinite loop, but that was not your question.

Answer 4

The following code works fine here, also using Mayhem his str_repeat function to add 4k of data (that is usually the minimum for a tcp packet to be flushed by php)

echo str_repeat(' ', 4096);
while(true)
{
    echo "data: This is the message.";
    flush();
    sleep(3);
}

Answer 5

One thing I have noticed here is sleep() function in combination with ob_start() and - THERE IS NO - ob_start() anywhere in the full code example, yet there is flush() and ob_flush() ..

What are you flushing anyway? And why not simply ob_end_flush() ?

The thing is that sleep() than echo(), than sleep() again, than echo() again, etc, etc.. has no effect when output buffering is turned on. Sleep function works as expected when output buffering is not in play - in between. In fact, it might *(and it will) produce quite unexpected results, and those results won't be the one we want to see.

Answer 6

Instead of using loop try this code given below which is working(tested myself) fine as per your requirement

echo "data: This is the message.";
$url1="<your-page-name>.php";
header("Refresh: 5; URL=$url1");

what this will do is it will call itself every 5 seconds (in your case set it to 3 instead of 5) and echo the output.

Answer 7

I had the same issue and finally found the easy and quick solution on kevin choppin's blog:

Session Locks
First and foremost, if you're using sessions for whatever reason you will need to make them read-only on the stream. If they're writable, this will lock them everywhere else, so any page loads will hang while the server waits for them to become writable again. This is easily fixed by calling; session_write_close();

Answer 8

I am going to take a chance and state the obvious,

you could query the server every 3 seconds, and let the client do the waiting...

This could be done easily with javascript

for example, try this code and name if file.php

<?php
$action='';
if (array_key_exists('action',$_GET))
{$action=$_GET['action'];}
if ($action=='poll')
{
 echo "this message will be sent every 3 sec";
}
else
{
?><HTML><HEAD>
<SCRIPT SRC="http://code.jquery.com/jquery-2.1.3.min.js"></SCRIPT>
<SCRIPT>
function doPoll()
{
    $('#response').append($.get("file.php?action=poll"));
    setTimeout(doPoll, 3000);
}
doPoll();
</SCRIPT>
</HEAD><BODY><DIV id="response"></DIV></BODY></HTML><?php
}

Answer 9

Could it be as simple as the script timing out?

Eventually PHP scripts self terminate if they run for too long. The solution for when you don't want this to happen is to keep resetting the time out.

So a simple addition might be all you need:

<?php
    while(true) {
        echo "data: This is the message.";
        set_time_limit(30);
        sleep(3);
        ob_flush();
        flush();
    }
?>

Of course, that might not be it but my gut instinct is that this is the problem.

• http://php.net/manual/en/function.set-time-limit.php

UPDATE: I noticed in the comments that you are using some free hosting. If they are running PHP in safe mode then you cannot reset your timeout.

Answer 10

I suggest using if() statement instead of using while. And in your case your condition is always true, hence it is in infinite loop.