dropbox - 保管箱服务器如何保持与其所有客户端应用程序的连接？

Question

Dropbox 拥有超过 3 亿用户。Dropbox 桌面应用程序需要为每次更新保持与 Dropbox 服务器的连接。

但是 Dropbox 服务器如何与所有桌面用户保持连接？

Answer 1

Dropbox 客户端保持 TCP 连接持续打开以监听服务器端通知。当它收到通知时，客户端会启动 HTTPS 对话以查看更改的内容并下载它。当客户端发生某些变化时，它还会启动 HTTPS 对话以更新服务器上的文件。

来源:http://www-net.cs.umass.edu/imc2012/papers/p481.pdf

The Dropbox client keeps continuously opened a TCP connection to a notification server (notifyX.dropbox.com), used for receiving information about changes performed else- where. In contrast to other traffic, notification connections are not encrypted. Delayed HTTP responses are used to implement a push mechanism: a notification request is sent by the local client asking for eventual changes; the server response is received periodically about 60 seconds later in case of no change; after receiving it, the client immediately sends a new request. Changes on the central storage are instead advertised as soon as they are performed.

虽然解密的 header 没有说明 Dropbox 使用什么服务器来保持这么多打开的 TCP 连接，但人们报告说能够保持超过 600k ( https://stackoverflow.com/a/9676852/15472 ) 甚至超过 1M ( http://blog.whatsapp.com/196/1-million-is-so-2011 )。有了足够的负载平衡，3 亿用户(其中​​只有一小部分同时连接并在彼此之间积极共享数据)似乎触手可及。