I assume changes are usually small, you may want to try serving diffs? I.e. have the clients poll for the md5 of their _current_ list-version. On the server s

I assume changes are usually small, you may want to try serving diffs? I.e. have the clients poll for the md5 of their _current_ list-version. On the server store the diff that will upgrade them to the current version under that filename. If a client requests an unknown md5 (e.g. because he has no list or his list is corrupted) default him to a patch that contains the full file. This requires a little logic on both ends (diff/patch), but would probably slash your bandwidth requirements to a fraction. A little napkin math: 25 lists * 150kb * 1mio fetches = ~3.75T vs 25 lists * 1kb (patch) * 1mio fetches = 25G (0.025T)