javascript - Facebook如何知道某人是否在线

Question

我正在使用 Node.js（Express 框架）和 socket.io 做一个实时网站，在某些时候它必须检测是否有人“在线”（基本上是如果有人打开了网页）。

我认为 Facebook 聊天就是一个群组示例。但它背后的工程逻辑是什么？

非常感谢！

Answer 1

这张来自 Facebook 的幻灯片（http://www.erlang-factory.com/upload/presentations/31/EugeneLetuchy-ErlangatFacebook.pdf）详细解释了它的消息系统。系统有comet(channel)集群，保持与浏览器的TCP连接，并定期将连接的用户列表推送到Presence Server。因此，如果用户在线或不在线，网页 (PHP) 可以查询 Presence Server。

Answer 2

Not sure exactly how Facebook does this but if you serve a page to the user that has javascript in it, which makes asynchronous requests at a specific interval say every minute. Then when you receive a request the user is marked online with a timeout slightly greater then the ajax interval. If the time-out expires without another ajax request the user is marked offline.

You could possibly hook into the page load and page close events to only send requests when a user loads the page or leaves it as well. This would require less requests.

EDIT

I see your using socket io I'm not super familiar with this but I imagine you could use the socket connection / disconnection events provided.

Answer 3

客户端定期向服务器发送保持活动消息，在客户端 JS 中实现。

如果服务器在某个超时时间内没有收到keep-alive，它可以将用户标记为离线。当 websocket 显式关闭时，它也可以这样做。