An application consists of a fleet of EC2 Instances. These Instances are launched in the Oregon (us-west-2) region which consists of 3 availability zones. This application needs 6 Instances running at all times. As an architect, you need to distribute the instances so that the application could still maintain its capacity if any one of the availability zone goes down. Also, you need to ensure that the cost is kept to a minimum? Which of the following configurations would you consider?

Answer options:

A.6 Instances running in us-west-2a, 6 Instances running in us-west-2b, 6 Instances running in us-west-2c
B.3 Instances running in us-west-2a, 3 Instances running in us-west-2c
C.6 Instances running in us-west-2a, 3 Instances running in us-west-2b, 3 Instances running in us-west-2c
D.3 Instances running in us-west-2a, 3 Instances running in us-west-2b, 3 Instances running in us-west-2c

Answer – D
So now, let’s look at
Option A.If any availability zone goes down, we will have a total of 12 instances running. This is an additional 6 over the requirement of the question and will result in a higher cost.
So now, let’s look at Option
B.If the availability zone us-west-2a goes down, then you will have only 3 instances running. Because the other 3 instances are running in the us-east-2c region.
So now, let’s look at Option
C.If either us-west-2b or us-west-2c availability zone goes down, we will have a total of 9 instances running. This is an additional 3 over the requirement of the question and will result in a higher cost.
So now, let’s look at Option
D.If either us-east-2a or us-west-2b or us-west-2c availability zone goes down, there will be a total of 6 instances running, which is what we need.
For more information on Regions and Availability zones, please refer to the below URL-
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-regions-availability-zones.html