javascript - 长轮询挂起所有其他服务器请求

Question

我在我的网站上有一个简单的长轮询请求，以检查是否有任何新通知可供用户使用。就请求而言，一切似乎都完美无缺；只有在更新数据库(并为该特定用户创建了通知)后才会满足请求，并且之后会立即发送新请求。

问题

---

我注意到，当请求正在等待数据库的响应时(长轮询应该如此)，对服务器的所有其他请求 也会挂起 - 无论是媒体文件，< strong>AJAX 请求甚至新页面加载。这意味着对服务器的 所有请求都将挂起，直到我关闭浏览器并重新打开它。

更奇怪的是，如果我访问另一个我的 localhost网站(我的长投票是在 MAMP 虚拟主机网站上，www.example.com)，它们没问题，我仍然可以像什么都没发生一样使用它们 - 尽管事实上它们技术上在同一个服务器。

我的代码

---

这是我在客户端 ( longpoll.js ):

window._Notification = {
    listen: function(){
        /* this is not jQuery's Ajax, if you're going to comment any suggestions, 
        *  please ensure you comment based on regular XMLHttpRequest's and avoid
        *  using any suggestions that use jQuery */
        xhr({
            url: "check_notifs.php",
            dataType: "json",
            success: function(res){
                /* this will log the correct response as soon as the server is 
                *  updated */
                console.log(res);
                _Notification.listen();
            }
        });
    }, init: function(){
        this.listen();
    }
}

/* after page load */
_Notification.init();

这就是我在服务器端的内容 ( check_notifs.php ):

header("Content-type: application/json;charset=utf-8", false);

if(/* user is logged in */){
    $_CHECKED = $user->get("last_checked");

    /* update the last time they checked their notifications */
    $_TIMESTAMP = time();
    $user->update("last_checked", $_TIMESTAMP);

    /* give the server a temporary break */
    sleep(1);

    /* here I am endlessly looping until the conditions are met, sleeping every
    *  iteration to reduce server stress */
    $_PDO = new PDO('...', '...', '...');
    while(true){
        $query = $_PDO->prepare("SELECT COUNT(*) as total FROM table WHERE timestamp > :unix");
        if($query->execute([":unix" => $_CHECKED])){
            if($query->rowCount()){
                /* check if the database has updated and if it has, break out of
                *  the while loop */
                $total = $query->fetchAll(PDO::FETCH_OBJ)[0]->total;
                if($total > 0){
                    echo json_encode(["total" => $total]);
                    break;
                }
                /* if the database hasn't updated, sleep the script for one second,
                *  then check if it has updated again */
                sleep(1);
                continue;
            }
        }
    }
}   

/* for good measure */
exit;

---

我已经阅读了有关 NodeJS 和建议用于长轮询的各种其他框架，但不幸的是，它们目前对我来说遥不可及，我不得不使用 PHP 。我也环顾四周，看看 Apache 配置中是否有任何东西可以解决我的问题，但我只遇到了 How do you increase the max number of concurrent connections in Apache? ，考虑到我仍然可以使用我的其他 localhost，所提到的似乎不是问题。 同一服务器上的网站。

我真的很困惑如何解决这个问题，所以感谢所有帮助，
干杯。

Answer 1

实际发生的是 php 正在等待此脚本结束(锁定)以服务对同一文件的下一个请求。

As you can read here :

• there is some lock somewhere -- which can happen, for instance, if the two requests come from the same client, and you are using file-based sessions in PHP : while a script is being executed, the session is "locked", which means the server/client will have to wait until the first request is finished (and the file unlocked) to be able to use the file to open the session for the second user.
• the requests come from the same client AND the same browser; most browsers will queue the requests in this case, even when there is nothing server-side producing this behaviour.
• there are more than MaxClients currently active processes -- see the quote from Apache's manual just before.

实际上某处有某种锁。您需要检查发生了什么锁。也许 $_PDO 有锁，您必须在 sleep(1) 之前将其关闭以保持解锁状态，直到您发出下一个请求。

您可以尝试提高您的 MaxClients 和/或申请 this answer

Perform session_write_close() (or corresponding function in cakephp) to close the session in the begin of the ajax endpoint.