Saintmode lets you deal with a backend that is failing in random ways
for specific requests. It maintains a blacklist per backend, marking the
backend as sick for specific objects. When the number of objects marked
as sick for a backend reaches a set threshold, the backend is considered
sick for all requests. Each blacklisted object carries a TTL, which
denotes the time it will stay blacklisted.

WWW: https://github.com/varnish/libvmod-saintmode