How does dropbox server keep connection alive with all its client app?

Question

Dropbox has more than 300M user.Dropbox desktop application need to keep connection alive with dropbox server for every updates.

But how does dropbox server keep connection alive with all its desktop user?

Answer 1

The dropbox client keeps a TCP connection constantly open to listen for server-side notifications. When it receives a notification, the client initiates an HTTPS conversation to see what changed and download it. When something changes on the client side, it also initiates an HTTPS conversation to update the files on the server.

Source: 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.

While the decrypted headers give no indication of what servers Dropbox uses to keep so many open TCP connections, people report being able to keep over 600k (https://stackoverflow.com/a/9676852/15472) or even over 1M (http://blog.whatsapp.com/196/1-million-is-so-2011). With enough load-balancing, 300M users, of which only a fraction of which are connected simultaneously and actively share data within each other, certainly seems within reach.

Answer 2

I doubt that all 300M users are connected at the same time... And by the amount of storage they provide, they will have enough servers to handle the needed amount of connections, maybe 1% of their user count at a time.

If you like to investigate yourself, you could use tools like TCPView (part of Sysinternals Suite) to check which connections are opened by the application, or Wireshark to check the transferred data.
I assume that you mean 'update' of storage content; that could also happen on fixed intervals by opening a connection, getting the files list and closing the connection afterwards. In this case the connection would be used for a few seconds in an interval of e.g. 5 minutes. This would again reduce the number of needed simultaneous connections by factor ~100.