Improve Auto Scaling Responsiveness for Traffic Spikes
A development team deploys a web application on Amazon EC2 instances behind an Application Load Balancer. The application experiences intermittent 503 errors. A Solutions Architect notices that the errors coincide with high CPU utilization on the EC2 instances. What is the MOST effective way to improve the application's availability?
⚠ Common exam trap
SAP-C02 often tests whether candidates choose reactive fixes (bigger instances, faster health checks) instead of the elastic, root-cause solution — auto scaling based on the actual bottleneck metric.
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
✓
Configure an Auto Scaling group for the EC2 instances with a scaling policy based on average CPU utilization.
The 503 errors correlate with high CPU on the EC2 instances, meaning the instances are overwhelmed and the ALB is returning 503s when targets fail health checks or cannot respond. An Auto Scaling group with a CPU-based target tracking policy automatically adds instances when average CPU rises, distributing load and restoring healthy responses. This directly addresses the root cause rather than masking symptoms.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the idle timeout setting on the Application Load Balancer.
Why it's wrong here
Idle timeout governs how long the ALB holds an idle connection before closing it; it does not address CPU saturation causing 503s. It is tempting because timeout misconfiguration genuinely produces 503 errors, and raising it would be correct if backends were slow but not CPU-bound.
- ✗
Decrease the health check interval on the Application Load Balancer.
Why it's wrong here
Shortening the health check interval only detects unhealthy targets sooner; it cannot relieve the CPU saturation causing the 503s, and may increase load. Health check tuning suits scenarios where slow or failed targets linger in rotation, not capacity exhaustion, which requires scaling out or offloading work.
- ✓
Configure an Auto Scaling group for the EC2 instances with a scaling policy based on average CPU utilization.
Why this is correct
An Auto Scaling group with a target-tracking or step policy on average CPU utilisation adds EC2 capacity before sustained high CPU causes 503s, then removes it afterwards. This directly addresses the CPU-driven availability constraint with less operational effort than manual intervention.
- ✗
Use larger EC2 instance types to handle the load.
Why it's wrong here
Larger instances raise per-instance CPU capacity but leave the Auto Scaling group's desired capacity and scaling thresholds unchanged, so overload recurs once traffic grows. Vertical scaling suits steady, predictable load; the intermittent 503s during CPU spikes call for horizontal scaling across more instances.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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