Optimized variable storage in the cloud

I need a very fast and efficient way to store a variable in the cloud. This is because I'm making a Dropbox-like application that shares files and folders, and I need to make sure before uploading changed files that no other changes were made. This needs to be done in a fast way, since no one would want to sit at their screen for a minute, waiting to see if their file was uploaded, and some of the people using the application have bad internet connection.

So, is there any optimized way to store variables in the cloud? :)

You would need some kind of web service running on an ip address you own so that external remote users can place information at that location.

That would be the standard way of having a central data store that a variety of remote users need to share.