which architecture provides high availability?

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 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 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 one RDS (Relational Database
Service) Instance deployed with read replicas in the two other AZs.

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 m each AZ inside an Auto Scaling Group behind an ELS 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 a Multi-AZ RDS (Relational
Database services) deployment.

Explanation:
Amazon RDS Multi-AZ Deployments
Amazon RDS Multi-AZ deployments provide enhanced availability and durability for Database (DB) Instances,
making them a natural fit for production database workloads. When you provision a Multi-AZ DB Instance,
Amazon RDS automatically creates a primary DB Instance and synchronously replicates the data to a standby
instance in a different Availability Zone (AZ). Each AZ runs on its own physically distinct, independent
infrastructure, and is engineered to be highly reliable. In case of an infrastructure failure (for example, instance
hardware failure, storage failure, or network disruption), Amazon RDS performs an automatic failover to the
standby, so that you can resume database operations as soon as the failover is complete. Since the endpointfor your DB Instance remains the same after a failover, your application can resume database operation without
the need for manual administrative intervention.
Enhanced Durability
Multi-AZ deployments for the MySQL, Oracle, and PostgreSQL engines utilize synchronous physical replication
to keep data on the standby up-to-date with the primary. Multi-AZ deployments for the SQL Server engine use
synchronous logical replication to achieve the same result, employing SQL Server-native Mirroring technology.
Both approaches safeguard your data in the event of a DB Instance failure or loss of an Availability Zone.
If a storage volume on your primary fails in a Multi-AZ deployment, Amazon RDS automatically initiates a
failover to the up-to-date standby. Compare this to a Single-AZ deployment: in case of a Single-AZ database
failure, a user-initiated point-in-time-restore operation will be required. This operation can take several hours to
complete, and any data updates that occurred after the latest restorable time (typically within the last five
minutes) will not be available.
Amazon Aurora employs a highly durable, SSD-backed virtualized storage layer purpose-built for database
workloads. Amazon Aurora automatically replicates your volume six ways, across three Availability Zones.
Amazon Aurora storage is fault-tolerant, transparently handling the loss of up to two copies of data without
affecting database write availability and up to three copies without affecting read availability. Amazon Aurora
storage is also self-healing. Data blocks and disks are continuously scanned for errors and replaced
automatically.
Increased Availability
You also benefit from enhanced database availability when running Multi-AZ deployments. If an Availability
Zone failure or DB Instance failure occurs, your availability impact is limited to the time automatic failover takes
to complete: typically under one minute for Amazon Aurora and one to two minutes for other database engines
(see the RDS FAQ for details).
The availability benefits of Multi-AZ deployments also extend to planned maintenance and backups. In the case
of system upgrades like OS patching or DB Instance scaling, these operations are applied first on the standby,
prior to the automatic failover. As a result, your availability impact is, again, only the time required for automatic
failover to complete.
Unlike Single-AZ deployments, I/O activity is not suspended on your primary during backup for Multi-AZ
deployments for the MySQL, Oracle, and PostgreSQL engines, because the backup is taken from the standby.
However, note that you may still experience elevated latencies for a few minutes during backups for Multi-AZ
deployments.
On instance failure in Amazon Aurora deployments, Amazon RDS uses RDS Multi-AZ technology to automate
failover to one of up to 15 Amazon Aurora Replicas you have created in any of three Availability Zones. If no
Amazon Aurora Replicas have been provisioned, in the case of a failure, Amazon RDS will attempt to create a
new Amazon Aurora DB instance for you automatically.