DOP-C02 Incident and Event Response Practice Question
An application running on Amazon ECS Fargate is experiencing intermittent HTTP 503 errors from the Application Load Balancer (ALB). The target group health checks are passing. Which configuration is MOST likely causing this issue?
⚠ Common exam trap
Many candidates assume 503 errors are always caused by health check failures or overload, but the question explicitly states health checks are passing, so the issue must be related to connection draining or deregistration behavior rather than target health or load balancing algorithms.
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 deregistration delay is set too short, causing connections to be closed before requests complete.
The intermittent HTTP 503 errors from the ALB, despite health checks passing, indicate that the target (ECS Fargate task) is being deregistered while still processing active requests. A deregistration delay that is too short causes the ALB to close connections to the target before the application has finished responding, resulting in 503 errors for in-flight requests. The default deregistration delay is 300 seconds; setting it too low (e.g., 5 seconds) can cause this 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 deregistration delay is set too short, causing connections to be closed before requests complete.
Why this is correct
The deregistration delay (connection draining) is the time ALB gives in-flight requests to finish after a target is deregistered. On ECS Fargate, when a task is stopped, the ALB stops routing new requests and, if the delay is too short, forcibly closes the connection before long-running requests complete, causing clients to receive 502/503 responses. For high-latency or streaming workloads, the delay must exceed the longest expected request duration.
- ✗
The ALB's slow start duration is too long, causing requests to be dropped.
Why it's wrong here
Slow start is an ALB target-group feature that ramps traffic to a newly registered target gradually, starting it with a small share and increasing over the configured duration. A long slow-start duration does not drop or reject requests; it only temporarily shifts traffic away from that new target to other healthy targets. Since the load balancer continues to route all healthy targets normally, no 503 errors are generated due to slow start alone.
- ✗
The health check interval is set too low, causing targets to be marked unhealthy prematurely.
Why it's wrong here
The health check interval is how often, in seconds, the ALB sends a health check request to each target; a lower interval simply means checks run more frequently. A target is marked unhealthy only after a consecutive number of failed checks (the unhealthy threshold) occur, so a short interval alone does not prematurely fail a healthy target. If the target continues to return 200 OK, lowering the interval does not cause 503 responses or close in-flight requests; it may even detect failures sooner.
- ✗
The ALB's circuit breaker is tripping due to high error rates.
Why it's wrong here
Application Load Balancers do not implement a circuit breaker that trips on high error rates. The ALB routes requests based solely on target health status; even if a target is reporting 5xx errors, the ALB will keep sending traffic to it unless its health checks start failing. High observed error rates are a symptom, not a trigger for stopping traffic, and if no healthy targets remain the ALB returns 503, but a circuit breaker is not the mechanism.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.