Scaling: survive many requests at once

Performance makes one request faster. Scaling makes many requests survive at the same time. Different problem, different toolbox — don't reach for this one when a single endpoint is slow for a single user (that is ../performance/SKILL.md).

The whole job in one line: diagnose the bottleneck, apply the cheapest lever that moves it, re-measure under load. Repeat until the next bottleneck appears or you hit your target.

Prime directive: never add infrastructure without a measurement first. A replica, a queue, or a third app instance you bought on a hunch costs money every month and usually moves the wrong tier. Measure, then add.

Start here — order of operations

Levers are ordered by payoff per dollar. Caching is nearly free and wins biggest; replicas and autoscaling cost forever. Climb the ladder in order; stop the moment the symptom clears.