You've implemented EC2 Auto Scaling for the web layer behind an ELB. User traffic is quite unpredictable and changes rapidly so you rely on auto scaling to provide the best user experience and performance. Weekly you receive a report from a third-party web analytics company that shows how your website is performing from the perspective of a customer. Unfortunately, during moments of high demand, customer response is still slower than your target.
What might be some causes of this and possible corrective actions but yet remain as cost-efficient as possible? Select 2.
A. The default cool down time for scaling is too long. Create a scaling-specific cool down that overrides the default time.
B. Your Internet Gateway has become saturated. Create a new VPC, ELB and auto-scaling group with its own Internet Gateway and route traffic equally to either ELB.
C. You don't have enough instances active during peak load times. Implement a scheduled scaling and increase the number of minimum instances during peak times.
D. Customer demand is not stable enough to properly trigger scaling thresholds of your existing scaling policies. Implement a step scaling policy configured around CPU utilization.
E. Your launch configuration is setup to launch undersized instances. Edit your current launch configuration to launch larger instances to better absorb the load.
F. You need more dynamic web capacity than your current configuration. Create a new launch configuration to launch smaller instances and implement a step scaling policy with a shorter cool down period.
A D
技巧：排除明顯錯誤選項，在沒有明顯錯誤的選項中選擇最合理的選項。
A. 正確。使用簡單擴展策略的情況下，如果冷卻時間過長是有可能導致擴展和縮減策略響應時間變得過長，從而導致不能及時提供合適數量的實例，進而無法有效地吸收突增的用戶流量。
B. 不正確。明顯錯誤選項。因為無論 Internet Gateway 還是 ELB 都是 AWS 完全托管的服務, AWS 保證它們具有高度可用性和自動擴展性，不會出現工作負載超過負荷的情況。
C. 不正確。明顯錯誤選項。因為有可預測的周期性的工作負載才適合采用計劃擴展策略的方式。
D. 正確。步進擴展策略是推薦的解決方案。
E. 不正確。明顯錯誤選項。因為啟動配置（launch configuration）一旦創建成功就無法進行任何的修改。
F. 不正確。明顯錯誤選項。因為步進擴展策略（step scaling policy）中不存在冷卻時間（Cool Down time），步進擴展策略采用的實例預熱的方式來調整激活下一個擴展策略是否執行的合適時機。