When you learn Disaster Recovery, you memorize four terms: Backup & Restore, Pilot Light, Warm Standby, and Multi-Site Active-Active. But these four aren't different technologies — they're four points marked on one continuous spectrum. At one end of the spectrum is "keep almost nothing running in advance and build it all after failure"; at the other end is "a second environment runs full-stack even in normal times." As you climb the stages, RTO and RPO shrink but cost rises steeply