I'm looking to try and set up a FTP server which will serve a file structure and files which don't exists on the server (the file structure information is stored on cassandra, and the files are on s3). The file structure and files will also be unique to the user, so different logins will generate the file structure for that particular user, but I guess I can adress that later.

firstly is this possible?

I imagine there will be some methods for listing folders, and listing files that I can override and in there I can make a request to get the list and return it.

I found this example of a ftp server but I can't seem to find out how I can override or change FTPRealm to alter how it lists files/folders. 

Any advice or pointers to some ftp tutorials would be hugely appreciated,

Many thanks!

Paul