这是使用ajax长轮询进行服务器推送的方法。浏览器发出一个ajax请求，该请求启动服务器端自轮询。Ajax请求保持打开状态，等待响应，直到文件更改为止，并且一旦获得响应，它就会发出一个新的长轮询请求。
这是jQuery和php的外观，实现了在html中实时更新div的示例，该示例显示了当前连接的客户端数量：
index.html：
<html><head><title>Comet Test</title> <script type="text/javascript" src="jquery.js"></script> <script type="text/javascript" src="longpolling.js"></script></head><body> Number of connected users: <div id="total">0</div></body></html>
longpolling.js：
$(document).ready(function() { connectToServer(1); });function connectToServer( incp ) { $.get("LongPolling.php", { inc: incp }, function(resp) { $('#total').html(resp); connectToServer(0); } );}LongPolling.php：
<?php# (over)write file with contents, locking the file while doing so.# just barf and die if there's an error.function update($file, $contents){ $f = fopen($file, 'w'); if(!$f) { echo "ERROR1"; exit; } # couldn't open file for writing. if(!flock($f, LOCK_EX)) { echo "ERROR2"; exit; } # couldn't get lock. fwrite($f, $contents); fclose($f); # this also releases the lock. return $contents;}# fetch the contents of the given file.# if the file doesn't exist, create it with contents "0"function fetch($file){ if(file_exists($file)) { if(!is_readable($file)) { echo "ERROR3"; exit; } $x = file_get_contents($file); } else { $x = 0; update($file, $x); } return $x;}$fc = 'connx.txt'; # file that stores the number of connections.if ( $_REQUEST['inc'] == 1 ) { # someone just connected. echo update($fc, fetch($fc)+1);} else { # someone is reconnecting (also happens immediately after connect). $last = filemtime($fc); do { # wait until some other instance causes $fc to change... sleep(1); clearstatcache(); # otherwise filemtime() results are cached! } while(filemtime($fc) == $last); echo fetch($fc);}?>注意：这不会跟踪断开连接，因此它更像是实时跟踪浏览量的总数。
