AJAX code analysis

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AJAX code analysis

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#1 by (6 votes)
#2 by (2 votes)

1

I found this code, which is like a Long Polling , and wanted to know if I might have problems using it. And I also wanted to know the cons of this code if I use it.

pagina.html = > page that the automatic update occurs

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="iso-8859-1">
    <title>Untitled Document</title>
    <script src="ajax.js" language="JavaScript" type="text/javascript"></script>
    <script type="text/javascript">
    obj_online = new montaXMLHTTP();
    function Online(){      
        obj_online.open("GET","ultimasmensagens.php",true); // Na pagina ultimassenhas esta a programação que lista as informações do BD
        obj_online.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        obj_online.onreadystatechange = function(){
            if(obj_online.readyState == 4){
                document.getElementById("online").innerHTML = obj_online.responseText;
                clearTimeout(re);
                setTimeout("Online()",5000);
            }
        }
        obj_online.send(null);
        var re = setTimeout("reenvia()",10000);
    }
    </script>
</head>
<body onLoad="setTimeout('Online()',2000);">
    <div id="online">
    </div>
</body>
</html>

ajax.js

function montaXMLHTTP(){
    try{
        myObj = new XMLHttpRequest()
    }catch(e){
        myObj = new ActiveXObject("Microsoft.XMLHTTP"); 
    }
    return myObj;
}

ultimasmensagens.php = > page that lists the messages

<!DOCTYPE HTML>
<html>
<head>
    <meta charset="iso-8859-1">
    <title>Untitled Document</title>
</head>
<body>
<? 
include 'config.php'; // conexao com o bd
$mensagens = mysql_query("SELECT * FROM mensagens order by idmensagem DESC limit 5") 
    or die (mysql_error());
while($minhalista = mysql_fetch_array($mensagens)){ ?>
    <?=$minhalista['titulo']?><br />
<? } ?>
</body>
</html>

php mysql ajax

asked by anonymous 25.03.2014 / 17:57

2 answers

2

The system is legal, Lucas. But to become long polling, you need to add the "long". Something like this:

include 'config.php'; // conexao com o bd
$count = 0;
// 30 tentativas ou 30 segundos até encontrar a primeira atualização.
while ($count < 30) {
    // Adicione um campo em WHERE para saber se a mensagem é nova. O ideal é uma data
    $mensagens = mysql_query("SELECT * FROM mensagens WHERE creation > '$ultimaAtualizacao' order by idmensagem DESC limit 5")  or die (mysql_error());
    while($minhalista = mysql_fetch_array($mensagens)){
        echo $minhalista['titulo'];
        break;
    }

    // Não sobrecarregue seu banco. Espere 1 segundo antes de uma nova tentativa.
    sleep(1);
    $count++;
}

There are many pros and cons in this type of polling. But I find it far better than requesting every 3 seconds, as is common in many chats out there.

The use of the websocket for is still unstable and does not work in all browsers. I still prefer the long polling along with nodejs

25.03.2014 / 19:22

How to multiply the value of 3 inputs using pure javascript? Pick up the most recent month's records

score 6 · Accepted Answer

This is not Long Polling and I do not recommend using the script by the following:

A connection to the server is established every 5 seconds, even without any changes to the server.

If this were a chat, in practice it would work like this:

I write the message and send it to you that went to the kitchen and can not respond;

Every 5 seconds the script makes a request to the server to see if you have answered my message;

You took in the kitchen about 10 minutes, so there are 12 requests per minute, that is, the script made 120 requests to the server without any need.

You replied but wrote about 3 messages within 5 seconds (fast right)

Suddenly I get 3 messages at once ...

Now look at the problems:

120 requests were made and consumed server resources without any need because there was no response from you.

Messages were pending because I only get one update every 5 seconds.

Multiple connections at the same time making the same request will overwhelm the server, and may even "knock" it out of memory to process all requests.

Long polling would be different, in practice it would look like this:

I write the message and send it to you that went to the kitchen and can not respond;

The link is open waiting for a response from you and there are no more requests until the response arrives;

You took in the kitchen for about 10 minutes, but since there are only requests if there is an answer, only 1 request occurred.

You replied but wrote about 3 messages within 5 seconds (fast right)

I get one response at a time, because when you pressed to send the first message, there was a change on the server and the connection that was pending returned a response, closed the connection, and then reset it again.

See, only 1 request was made to the server, which consumed much less resources.

This is a practical explanation, search for PHP + LONG POLLING, there are many examples.