Unsolicited notifications from server to client over http

I am working on a dropbox like system and I am wondering how the client gets notified when the files change on the server side. It is my impression that both dropbox and ubuntu one operate over HTTP ports and work as follows: 1. if files change on client machine, inotify detect开发者_StackOverflow社区s it and preforms a push from the client to the server. (I get this part) 2. if files change on the server a simple unsolicited notification (just a message saying "time to sync") is sent from the server to the client. Then the client initiates a sync to the server.

I dont really care which language I do this in. I am just wondering how the client gets contacted. Specifically, what if a client is behind a firewall with its own local IP addresses. How does the server locate it?

Also, what kind of messaging protocols would be used to do something like this? I was planning on doing this over HTTP or SSH, but I have no attachment do that.

I'm not sure what Dropbox is using, but it could be websockets (unlikely, it's a pretty new and not widely deployed thing) or more likely a pending Ajax request from the client to the server -- to which the server only responds when it has new stuff for the client. The latter is the common way to implement (well, OK -- "hack";-) some form of "server push" with HTTP.

It took a little research into networking to see how this would work, but it is far more trivial then I expected. I am now using standard Java sockets for this. Start up the server process which listens for a socket connection. Then start up the client which connects to the server.

Once the connection is made, messages can be sent back and fourth. This works through NAT (network address translation) which is standard method for routing packets on private networks behind a firewall.