100% Fault Tolerance in us-west-2 Region - Answer Explanation

Answer - D and E.

Since we need 6 instances running at all times, only D and E fulfil this option.

The AWS documentation mentions the following on Availability zones.

When you launch an instance, you can select an Availability Zone or let us choose one for you.

If you distribute your instances across multiple Availability Zones and one instance fails, you can design your application so that an instance in another Availability Zone can handle requests.

For more information on Regions and AZ's please visit the URL:

To ensure that the application has 100% fault tolerance in the event of a single Availability Zone (AZ) failure, it is necessary to spread the instances across multiple AZs, so that the application can continue running even if one AZ becomes unavailable.

Option A: us-west-2a with 2 instances, us-west-2b with 2 instances, us-west-2c with 2 instances This configuration places 2 instances in each AZ, which provides fault tolerance in case any one AZ fails. However, it only provides the minimum required number of instances, which may not be sufficient to handle increased traffic or failover scenarios. Therefore, this option is not optimal.

Option B: us-west-2a with 3 instances, us-west-2b with 3 instances, us-west-2c with 0 instances This configuration places all 6 instances in two AZs, which means that the application would be impacted if one of these AZs fails. Therefore, this option does not provide 100% fault tolerance.

Option C: us-west-2a with 4 instances, us-west-2b with 2 instances, us-west-2c with 2 instances This configuration places 4 instances in one AZ and 2 instances each in two other AZs. This provides fault tolerance in case one of the two AZs with 2 instances fails, but the application would still be impacted if the AZ with 4 instances fails. Therefore, this option does not provide 100% fault tolerance.

Option D: us-west-2a with 6 instances, us-west-2b with 6 instances, us-west-2c with 0 instances This configuration places all 12 instances in two AZs, which means that the application would be impacted if one of these AZs fails. Therefore, this option does not provide 100% fault tolerance.

Option E: us-west-2a with 3 instances, us-west-2b with 3 instances, us-west-2c with 3 instances. This configuration places 3 instances in each AZ, which provides 100% fault tolerance in case any one AZ fails. Additionally, it provides sufficient capacity to handle increased traffic or failover scenarios. Therefore, this option is the optimal choice for this scenario.

Therefore, the correct answers are A and E.