An Auto Scaling group serves an application with a slow, multi-minute boot process. The team configures a warm pool with instances kept in the Stopped state to cut scale-out latency. Immediately after setup, while instances sit in the warm pool but have not yet been drawn into service, how do they affect the group's reported desired capacity?
- Each pooled instance immediately counts toward the group's desired capacity, the same as an InService instance
- Pooled instances double-count, appearing in both the warm pool metrics and the desired capacity
- The warm pool cannot exist unless desired capacity is first reduced to zero
- Pooled instances sit outside the desired capacity and only begin counting toward it once they leave the pool and enter service
Why D? And why not the others?
Correct answer: D. Pooled instances sit outside the desired capacity and only begin counting toward it once they leave the pool and enter service
A warm pool sits alongside an Auto Scaling group as a separate pool of pre-initialized instances that are not counted as part of the group's desired capacity while they remain in the pool; only when a scale-out event draws an instance out of the warm pool into the group, known as a warm start, does that instance begin counting toward desired capacity. This separation is the entire point of the feature: it lets instances finish a lengthy boot process ahead of time so the group can absorb demand quickly, without inflating the counted, InService fleet beforehand. Instances are never counted twice; the warm pool and the group's InService fleet are tracked as distinct populations precisely so cost and capacity accounting stay separate until a warm start occurs. There is also no requirement to reduce desired capacity to zero before creating a warm pool — its default size is simply calculated from the difference between the group's maximum capacity and its current desired capacity, so both coexist normally from the start.
Source: AWS documentation: Amazon EC2 Auto Scaling — warm pools (core concepts)