DOP-C02 Incident and Event Response Practice Question
An application running on Amazon EC2 instances behind an Application Load Balancer (ALB) is experiencing intermittent 503 errors. The target group health checks are failing. The DevOps engineer checks the instance logs and finds that the application is running but taking longer than 30 seconds to respond. What is the MOST likely cause?
⚠ Common exam trap
DOP-C02 often tests the misconception that 503 errors are always due to security groups or instance failures, overlooking health check timeout misconfigurations.
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
✓
The health check timeout is set too low, causing the ALB to mark instances unhealthy.
The most likely cause of intermittent 503 errors and failing health checks is that the health check timeout is set too low. The application takes longer than 30 seconds to respond, but if the health check timeout is set to a value less than the application response time, the ALB will mark the instance as unhealthy, leading to 503 errors. Adjusting the health check timeout to accommodate the application's response time would resolve the issue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The Auto Scaling group's scaling policy is too aggressive, causing frequent instance replacements.
Why it's wrong here
An aggressive Auto Scaling policy is not a cause of Application Load Balancer 503 responses. When instances are replaced, the Auto Scaling group launches new instances and waits for them to pass health checks before terminating the old ones, assuming a healthy instance is still in service. Scaling policies do not directly affect the ALB's ability to route traffic, and unless the group is simultaneously terminating all healthy instances (which would be an unrelated configuration), the replacement process does not trigger 503s.
- ✗
The security group for the ALB does not allow inbound traffic from the internet.
Why it's wrong here
If the ALB's security group did not allow inbound traffic from the internet, all requests would fail consistently rather than intermittently. The ALB would not accept connections on the listener, so clients would receive connection timeouts or 504 errors, not a 503 Service Unavailable response. An intermittent 503 pattern indicates the ALB is accepting traffic but lacks a healthy target to forward requests to.
- ✓
The health check timeout is set too low, causing the ALB to mark instances unhealthy.
Why this is correct
A health check timeout set too low can cause the ALB to mark otherwise functional instances as unhealthy when the application's response time occasionally exceeds the timeout. The ALB health check settings include an interval, timeout, and unhealthy threshold; if a slow application misses the timeout a few consecutive times, the target is deregistered and the ALB returns 503 Service Unavailable when no healthy targets remain. This matches the symptom of intermittent errors under load, as the application may respond normally at times but exceed the timeout during traffic spikes.
- ✗
The EC2 instances are running out of memory and the application is crashing.
Why it's wrong here
EC2 memory exhaustion typically causes the operating system to kill processes via the OOM killer, which would make the application unresponsive and fail health checks continuously, not intermittently. Furthermore, the scenario states that the application is still responding, albeit slowly, so a crashed or killed process would not be serving requests at all. Memory pressure would likely manifest as degraded performance or 502/504 errors, not the 503 Service Unavailable that the ALB returns when it has no healthy targets.
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Same concept, more angles
1 more way this is tested on DOP-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company runs a web application on Amazon EC2 instances behind an Application Load Balancer (ALB). The application experiences intermittent 503 errors. The engineer suspects the ALB is returning these errors because the target instances are unhealthy. Which metric should the engineer monitor to confirm this suspicion?
hard- A.RequestCount
- B.UnhealthyHostCount
- ✓ C.HealthyHostCount
- D.TargetResponseTime
Why C: The ALB publishes 'HealthyHostCount' metric showing the number of healthy targets. When this count drops to zero, the ALB cannot forward requests and returns 503 errors. Option A (RequestCount) is incorrect because it measures total requests, not health. Option B (UnhealthyHostCount) is a valid metric but does not directly confirm the suspicion that targets are unhealthy; a decreasing HealthyHostCount is more direct. Option D (TargetResponseTime) measures latency, not health status.
JA
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
This DOP-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DOP-C02 exam.