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Your company runs a customer facing event registration site. This site is built with a 3-tier
architecture with web and application tier servers and a MySQL database. The application
requires 6 web tier servers and 6 application tier servers for normal operation, but can run on a
minimum of 65% server capacity and a single MySQL database.
When deploying this application in a region with three availability zones (AZs), which architecture
provides high availability?

A.
A web tier deployed across 2 AZs with 3 EC2 (Elastic Compute Cloud) instances in each A2
inside an Auto Scaling Group behind an ELB (elastic load balancer), and an application tier
deployed across 2 AZs with 3 EC2 instances In each AZ inside an Auto Scaling Group behind an
ELB, and one RDS (Relational Database Service) instance deployed with read replicas in the
other AZ.

B.
A web tier deployed across 3 AZs with 2 EC2 (Elastic Compute Cloud) instances in each AZ
inside an Auto Scaling Group behind an ELB (elastic load balancer), and an application tier
deployed across 3 AZs with 2 EC2 instances In each AZ inside an Auto Scaling Group behind an
ELB, and a Multi-AZ RDS (Relational Database Service) deployment.

C.
A web tier deployed across 2 AZs with 3 EC2 (Elastic Compute Cloud) instances in each AZ
inside an Auto Scaling Group behind an ELB (elastic load balancer), and an application tier
deployed across 2 AZs with 3 EC2 instances in each AZ inside an Auto Scaling Group behind an
ELB, and a Multi-AZ RDS (Relational Database Service) deployment

D.
A web tier deployed across 3 AZs with 2 EC2 (Elastic Compute Cloud) instances in each AZ
inside an Auto Scaling Group behind an ELB (elastic load balancer), and an application tier
deployed across 3 AZs with 2 EC2 instances in each AZ inside an Auto Scaling Group behind an
ELB, and one RDS (Relational Database Service) instance deployed with read replicas in the two
other AZs.

Explanation:
http://highscalability.com/blog/2016/1/11/a-beginners-guide-to-scaling-to-11-million-users-onamazons.html
https://www.airpair.com/aws/posts/building-a-scalable-web-app-on-amazon-web-services-p1

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