AWS Certified Solutions Architect – Professional — Question 413

Your application is using an ELB in front of an Auto Scaling group of web/application servers deployed across two AZs and a Multi-AZ RDS Instance for data persistence.
The database CPU is often above 80% usage and 90% of I/O operations on the database are reads. To improve performance you recently added a single-node
Memcached ElastiCache Cluster to cache frequent DB query results. In the next weeks the overall workload is expected to grow by 30%.
Do you need to change anything in the architecture to maintain the high availability or the application with the anticipated additional load? Why?

Answer options

• A. Yes, you should deploy two Memcached ElastiCache Clusters in different AZs because the RDS instance will not be able to handle the load if the cache node fails.
• B. No, if the cache node fails you can always get the same data from the DB without having any availability impact.
• C. No, if the cache node fails the automated ElastiCache node recovery feature will prevent any availability impact.
• D. Yes, you should deploy the Memcached ElastiCache Cluster with two nodes in the same AZ as the RDS DB master instance to handle the load if one cache node fails.

Correct answer: A

Explanation

Because the RDS instance is already running at over 80% CPU utilization, a failure of the single-node Memcached cluster would redirect all read traffic back to the database, causing it to crash under the load. Deploying Memcached nodes across multiple Availability Zones ensures that if one cache node fails, the remaining cache capacity can still offload the database and prevent an outage. Relying on database fallback or waiting for automatic node recovery would still result in a database overload and application downtime.