Three replicas of one service landed on the same node and one reboot killed them all. How do affinity and anti-affinity actually work?

Hard versus soft rules, topology keys, weights that lose to everything else, and the IgnoredDuringExecution half nobody remembers. Placement shapes blast radius only when paired with disruption budgets.

Grounded in researched DevOps, SRE and platform engineering interview loops, written to a senior-engineer editorial bar, and never padded to hit a word count.