An Auto Scaling group is meant to spread its instances across three Availability Zones for fault tolerance, but after a recent zone impairment and recovery the group ended up with most instances concentrated in one Availability Zone. The team wants the group to keep capacity spread roughly evenly across its Availability Zones on its own, so the loss of one zone removes only about a third of capacity. Which property of the Auto Scaling group provides this behaviour?
- AThe group's cooldown period, which after each scaling activity holds the new distribution in place long enough for the zones to settle into an even spread over time.
- BAvailability Zone balancing built into the group, which launches new instances in the zones with the fewest instances and rebalances to restore an even spread across the configured zones. Correct
- CThe group's health check grace period, which once lengthened lets instances start in under-used Availability Zones before they are evaluated and counted toward the spread.
- DThe group's termination policy set to OldestInstance, which by always removing the oldest instances gradually shifts the population toward a balanced distribution across the zones.
Why A is wrong: A cooldown only spaces out scaling actions; it has no role in balancing instances across zones, so a skewed distribution would simply persist.
Why B is correct: Auto Scaling inherently favours the least-populated zone on launch and rebalances across zones, so the group restores an even spread and one zone loss removes only its share of capacity.
Why C is wrong: The grace period delays health evaluation after launch and says nothing about which zone an instance lands in, so it cannot even out a zone skew.
Why D is wrong: A termination policy decides which instance to remove on scale-in, not how to distribute launches, so choosing OldestInstance does not drive an even cross-zone spread.