Multi-AZ deployments, read replicas, and Aurora all get mentioned as RDS availability features. What's actually different between them, and when do you use each?

Core Summary

Multi-AZ is a synchronous standby that exists purely for failover, not for serving traffic. A read replica is asynchronous and exists to serve read traffic, with failover as a secondary, slower use. Aurora replaces both mechanisms with a shared, distributed storage layer, which is why an Aurora Replica can do double duty that a standard RDS replica can't.