DOP-C02 Incident and Event Response Practice Question

A company runs a three-tier application on Amazon EC2 behind an Application Load Balancer. During an incident, users report intermittent HTTP 502 errors, and the engineer finds that some targets are failing health checks and being removed and re-added repeatedly. The application writes large log files to the instance store and the engineer suspects the health check configuration. Application startup takes about 90 seconds. Which configuration change should the engineer make to resolve the flapping targets?

⚠ Common exam trap

The trap here is treating flapping targets as a load-balancing distribution problem and enabling cross-zone balancing, when the real cause is health check timing relative to application startup.

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

✓

Increase the health check interval and unhealthy threshold, and use a longer health check grace period or slow start so targets are not removed during startup.

The targets flap because health checks fail during startup or heavy disk I/O, so the load balancer repeatedly removes and re-adds them, producing intermittent 502 responses. Extending the health check interval and unhealthy threshold and applying a grace period or slow start keeps targets in service until they are genuinely ready, which stops the flapping and the resulting errors.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Enable cross-zone load balancing on the Application Load Balancer to spread traffic more evenly.
Why it's wrong here
Cross-zone load balancing distributes traffic across availability zones but does not change how health checks evaluate individual targets. The flapping is caused by health check sensitivity during startup, so balancing traffic differently leaves the underlying removal and re-addition cycle intact and the 502 errors unresolved.
✗
Set the health check interval to 5 seconds and the unhealthy threshold to 1 so failures are detected faster.
Why it's wrong here
Making detection more aggressive worsens flapping because brief, transient stalls during startup or log flushing will immediately mark targets unhealthy. Faster detection without adequate grace periods removes healthy-but-busy targets, increasing 502 errors rather than resolving them, so this change addresses the wrong side of the problem.
✓
Increase the health check interval and unhealthy threshold, and use a longer health check grace period or slow start so targets are not removed during startup.
Why this is correct
Flapping occurs when targets are marked unhealthy during normal startup or heavy I/O. Lengthening the interval and unhealthy threshold tolerates transient failures, while a health check grace period or slow start keeps new or busy targets in service until they are ready, which directly stops the repeated removal and re-addition causing 502s.
✗
Change the target group to use TCP health checks instead of HTTP health checks.
Why it's wrong here
Switching to TCP health checks only verifies that the port is open and ignores application-level readiness. A target could pass TCP checks while still unable to serve HTTP requests, so 502 errors from the load balancer would persist and the flapping would not be reliably resolved by this change.

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DOP-C02 Incident and Event Response Practice Question

Why each option matters

Option-by-option breakdown

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