The Ubuntu DDoS last month inspired to me make a better apt cache service. It's looking like I'll be cutting a 1.0 release later this week (after extensive testing in my environment).

https://github.com/linsomniac/apt-cacher-ultra

The primary features I'm focusing on are: It can serve packages if the upstream is unavailable or corrupt, it is reliable.

It snapshots and verifies the cache, and then only updates the snapshot when: a new metadata is available, it has downloaded updated packages that you commonly request, all the metadata checks out.

It's been running in my environment with ~200 clients, ~50 of them get reinstalled every day and then do a full set of package updates and installs. Been working great, even when I shut down Internet access while doing it.