You are working as an AWS consultant for a start-up company. They have developed a web application, that requires a lot of memory, for their employees to share files with external vendors securely. They created an AutoScaling group for the web servers that require two m4.large EC2 instances running at all times, scaling up to a maximum of twelve instances. Post-deployment of the application, a huge rise in cost was observed. Due to a limited budget, the CTO has requested your advice to optimize the usage of instances in the Auto Scaling groups. What do you suggest for reducing the costs and minimizing the risk of adverse impact on the performance?

Answer options:

A.Create an Auto Scaling group with t2. micro On-Demand instances.
B.Create an Auto Scaling group with a mix of On-Demand & Spot Instance. Select the On-Demand base as zero. Above On-Demand base, select 100% of On-Demand instance & 0% of Spot Instance.
C.Create an Auto Scaling group with all Spot Instance.
D.Create an Auto Scaling group with a mix of On-Demand & Spot Instance. Select the On-Demand base as 2. Above On-Demand base, select 20% of On-Demand instance & 80% of Spot Instance.

Correct Answer – D
Auto Scaling group supports a mix of On-Demand & Spot instances, which helps design a cost-optimized solution without impacting the performance. You can choose the percentage of On-Demand & Spot instances based on the application requirements. With Option D, the Auto Scaling group will have 2 instances initially as the On-Demand instances. In contrast, the remaining instances will be launched in a 20 % On-Demand instance & 80% Spot Instance ratio. 
No matter, if 80% of spot instances get terminated. The required 20% on-demand will be available with 2 on-demand instances always running.
Option A is incorrect. With t2. Micro, there would be a cost reduction, but it will impact the performance of the application. The question requires that the performance is not impacted, so changing the instance type is not suitable.
Option B is incorrect as there would not be any cost reduction with all On-Demand instances.
Option C is incorrect. Although this will reduce cost, all spot instances in an auto-scaling group may cause inconsistencies in the application & lead to poor performance.
For more information on Auto Scaling with multiple Instance types & purchase options, refer to the following URLs:
https://aws.amazon.com/blogs/aws/new-ec2-auto-scaling-groups-with-multiple-instance-types-purchase-options/
https://docs.aws.amazon.com/autoscaling/ec2/userguide/AutoScalingGroup.html#asg-purchase-options