This one is not part of the study-notes series. It sits in the same category, which makes it the post that proves category pages list series and non-series articles together.
The two numbers
Recovery time objective is how long you can be down. Recovery point objective is how much data you can afford to lose. Every DR pattern is a point on the curve between those two numbers and what you are willing to pay.
Backup and restore
Cheapest, slowest. Snapshots go to S3 or a cross-Region copy, and recovery means provisioning infrastructure from scratch. RTO in hours, RPO in hours.
Pilot light
A minimal core of the system runs continuously in the recovery Region, usually the database replica and nothing else. Compute is provisioned on failover. RTO in tens of minutes.
Warm standby
A scaled-down but functional copy of the whole system runs in the second Region. Failover is a traffic switch followed by a scale-up. RTO in minutes.
Multi-site active-active
Both Regions serve production traffic. RTO approaches zero and so does RPO, and the cost is a full second environment plus the work of keeping data consistent across it.
Picking one
The pattern is chosen by the business, not by the platform team. Ask what an hour of downtime costs. If nobody can answer, that is the finding to report before you build anything.