Architecture & practice·Risk, blast radius and change
one region went down and you found out which of your services quietly lived only in that region.
Failure domain
Also calledfault domain, failure boundary
The set of things that fail together when one thing fails. Drawing them is how an availability argument becomes concrete: a shared database, an availability zone, a single deployment pipeline and one overloaded team are all failure domains, and the last two do not appear on the architecture diagram. The design question is never 'can this fail' but 'when it does, what is inside the boundary with it'.