After last springs Ubuntu.com DDoS I was inspired to build a new apt-cacher with the primary goals of: Operating when upstream is down and being more robust and reliable than the existing apt-cachers.

It survives upstream outages by taking snapshots of the state when the upstream is healthy, and only adopting new snapshots when they are verified healthy. It can track what packages you typically request, and when the upstream updates it will proactively request any of those that are updated; new snapshots will delay adoption until your "hot" packages are in cache.