Downtime is always bad, and you want to avoid it. But sometimes you can get away with it. This is one of the magic things about content caching and being able to serve stale content. You can avoid the perception of downtime (to some extent) and give your website the appearance of being up and running at full speed, even when your backend is down.
How to configure saint mode and grace in Varnish 4.1
A very important feature of Varnish is the ability to deal with misbehaving servers. In this blog I will explain how to properly configure saint mode and grace.
Saint mode and grace have been present since Varnish 2.1, but to make this blog post easier I will cover the differences between Varnish 3 (EOL for real!) and the current stable Varnish 4.1.