Swoole从入门到入土(18)——WebSocket服务器[心跳ping]

        由于 WebSocket 是长连接，如果一定时间内没有通讯，连接可能会断开。这时候需要心跳机制，WebSocket 协议包含了 Ping 和 Pong 两个帧，可以定时发送 Ping 帧来保持长连接。

1、心跳原理图：

 2、websocket协议控制帧描述

Control frames are identified by opcodes where the most significant bit of the opcode is 1.

Currently defined opcodes for control frames include 0x8 (Close), 0x9 (Ping), and 0xA (Pong). Opcodes 0xB-0xF are reserved for further control frames yet to be defined.

Control frames are used to communicate state about the WebSocket.

Control frames can be interjected in the middle of a fragmented message.

All control frames MUST have a payload length of 125 bytes or less and MUST NOT be fragmented.

从原文可知，Ping的协议头是0x9，Pong的协议头是0xA，控制帧最大载荷为125bytes且不能拆分

3、代码示例：

1) 发送ping帧

use Swoole\WebSocket\Frame;
use Swoole\WebSocket\Server;

$server = new Server('127.0.0.1', 9501);
$server->on('message', function (Server $server, Frame $frame) {
    $pingFrame = new Frame;
    $pingFrame->opcode = WEBSOCKET_OPCODE_PING;
    $server->push($frame->fd, $pingFrame);
});
$server->start();

2) 响应ping帧

$server->on('message', function (Swoole\WebSocket\Server $server, $frame) {
    if ($frame->opcode == 0x09) {
        echo "Ping frame received: Code {$frame->opcode}\n";
        // 回复 Pong 帧
        $pongFrame = new Swoole\WebSocket\Frame;
        $pongFrame->opcode = WEBSOCKET_OPCODE_PONG;
        $server->push($frame->fd, $pongFrame);
    } else {
        echo "Message received: {$frame->data}\n";
    }
});

4、关于ping与pong的结论

· 心跳包中可能会携带数据
· 当收到Ping帧的时候需要立即返回一个Pong帧
· 在连接建立之后，随时都可以发送Ping帧
· 心跳是用来测试链接是否存在和对方是否在线
· 在响应Ping帧的的Pong帧中，必须携和被响应的Ping帧中相同的数据

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