PAS-C01 Technology Practice Question
A company runs a critical web application on EC2 instances behind an Application Load Balancer. The application uses Auto Scaling and a MySQL RDS Multi-AZ DB instance. Users report intermittent connection timeouts, and CloudWatch metrics show CPUUtilization spikes to 90% on the DB instance. The application queries are read-heavy with occasional writes. What is the MOST cost-effective design change to improve performance?
⚠ Common exam trap
Test-takers frequently choose ElastiCache (Option A) thinking caching always reduces DB load, but for a read-heavy workload with occasional writes, an RDS read replica is more cost-effective and directly addresses the CPU spikes from read queries without the overhead of cache invalidation and additional infrastructure.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add an RDS read replica in the same region and configure the application to route read queries to the replica.
The most cost-effective design change because adding an RDS read replica offloads read-heavy queries from the primary DB instance, reducing CPU utilization without requiring a larger instance. Since the workload is read-heavy with occasional writes, a read replica handles SELECT traffic while the Multi-AZ primary manages writes, improving performance at a lower cost than scaling up.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Implement ElastiCache Redis cluster to cache query results and reduce DB load.
Why it's wrong here
Caching adds complexity and cost; for this scenario, a read replica directly addresses the DB CPU spike.
- ✗
Enable RDS Performance Insights and create a CloudWatch alarm to reboot the DB instance on high CPU.
Why it's wrong here
Rebooting does not resolve the root cause of high CPU and causes downtime.
- ✓
Add an RDS read replica in the same region and configure the application to route read queries to the replica.
Why this is correct
A read replica offloads read traffic, reducing CPU on the primary, and is cost-effective for read-heavy workloads.
- ✗
Switch the DB instance to a larger instance type to handle the load.
Why it's wrong here
Scaling up increases cost; the workload is read-heavy, so a read replica is more cost-effective.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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