You are working as AWS Solutions Architect for a large banking organization. The requirement is that under normal business hours, there would always be at least 24 web servers up and running in a region (example: US - West (Oregon)). It will be a three-tier architecture connecting to the databases. The solution offered should be highly available, secure, and cost-effective. It should respond to the heavy requests during peak hours and fault-tolerate up to one AZ failure.
What would be the best solution to meet this requirement?

Answer options:

A.In a given region, use ELB behind two different AZs, each AZ with minimum or desired 24 web servers hosted in a public subnet and Multi-AZ database architecture in a private subnet.
B.In a given region, use ELB behind three different AZs, each AZ having ASG, with minimum or desired 12 web servers hosted in a public subnet and Multi-AZ database architecture in a private subnet.
C.In a given region, use ELB behind two different AZs, each AZ having ASG, with minimum or desired 12 web servers hosted in a public subnet and Multi-AZ database architecture in a private subnet.
D.In a given region, use ELB behind three different AZs, each AZ having ASG, with minimum or desired 8 web servers hosted in public subnet and Multi-AZ database architecture in a different public subnet.

Correct Answer: B
Option A is incorrect. Everything looks good, but the designed architecture does not look cost-effective, as all the time 48 servers will be running. It does not have ASG to cater to additional load on servers. However, it is fault-tolerant to one AZ failure.
Besides, it is always a good practice to use multiple AZs to make the application highly available.
Option B is correct. The solution needs to be tolerant up to one AZ failure. It means there are always 36 web servers to cater to the service requests. If one AZ fails, then there will be 24 servers running all the time. In case two AZs fails, there will be 12 servers running. Also, ASG can be utilized to scale out the required number of servers.
Option C is incorrect. It will not be a suitable solution. If there is one AZ failure, the other AZ will have only 12 web servers running. The question requires at least 24 web servers are running at all times.
Option D is incorrect. Remember the design principle of keeping the databases in the private subnet. As this solution mentions placing databases in another public subnet, the data can be exposed over the internet, and hence it’s an insecure application.