scaling

Ask whether Multi-AZ means a DB instance or a DB cluster. One has a non-readable standby; the other has two readers. That distinction decides whether the proposal can relieve the primary.

A busy date key can throttle while table capacity remains available. Check the actual key distribution and throttling reason before adding capacity or shards.

Where linear models break: compounding growth, step functions, peak days and saturation cliffs. Forecast per constrained resource, backtest, subtract lead time to get the real order-by date.

Two controllers with different opinions and no shared hysteresis. Flat traffic with an oscillating node count means the loop is driven by something other than demand, and the candidates are few.

Stateless does not mean there is no state. It means the state is not in the instance, which is what lets you kill any instance at any time. Every scaling and deployment property people want depends on that one thing.

The default answer is 'horizontal' because interviews reward it. The correct answer names the constraint that makes distribution expensive and knows when one big machine is the engineering choice.