DOP-C02 Incident and Event Response Practice Question
An application runs on EC2 instances behind an ALB. Users report intermittent 503 errors. The engineer checks ALB metrics and sees 'SurgeQueueLength' increasing periodically. What is the most likely cause?
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 application instances are not able to process requests quickly enough, causing the request queue to back up.
A high SurgeQueueLength indicates that the ALB is receiving more requests than the target instances can process, causing the request queue to back up. When the queue exceeds the limit (1024 requests), the ALB returns 503 errors. Option B is incorrect because health check failures typically cause the ALB to stop routing traffic to unhealthy instances, reducing the queue rather than increasing it. Option C is incorrect because an expired SSL certificate would cause TLS handshake failures, not 503 errors. Option D is incorrect because a misconfigured security group would block traffic entirely, resulting in connection timeouts or 503s, but it would not cause the SurgeQueueLength to increase.
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 application instances are not able to process requests quickly enough, causing the request queue to back up.
Why this is correct
When the application instances cannot process requests fast enough, their internal request queue gradually fills up. The ALB forwards new connections to these instances, but if the backend fails to accept or complete the connection within the configured timeout, the load balancer returns HTTP 503 Service Unavailable. This is a performance/capacity bottleneck, not a network or configuration fault.
- ✗
The target group health checks are failing, causing the ALB to route traffic to unhealthy instances.
Why it's wrong here
If target group health checks begin failing, the ALB automatically stops routing traffic to those unhealthy instances and deregisters them from rotation. Therefore, failing health checks reduce the pool of available targets, but they do not cause the ALB to route traffic to unhealthy instances — that behavior is the opposite of what actually happens. If all instances become unhealthy, the ALB would return 503 because there are no healthy targets, but the queue buildup described in the scenario is not caused by health check failures.
- ✗
The SSL certificate on the ALB has expired.
Why it's wrong here
An expired SSL certificate on the ALB causes TLS handshake failures before any HTTP request is received. The client will see a certificate validation error and the connection is aborted at the TLS layer, so no HTTP status code such as 503 is ever produced. Even if the certificate is misconfigured, the ALB cannot return an HTTP response because the transport security negotiation has already failed.
- ✗
The ALB security group is blocking traffic from the clients.
Why it's wrong here
If the ALB's security group blocks incoming traffic from clients, the packets are dropped at the network layer. The client's connection attempts will time out or receive a TCP reset, but the ALB never sees the request and therefore cannot generate an HTTP 503 response. A security group misconfiguration would manifest as a connectivity issue, not an application-layer availability error.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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