DOP-C02 Incident and Event Response Practice Question
A company uses AWS Lambda functions to process incoming events from Amazon S3. The operations team notices that some events are not being processed, and there is no error in the Lambda function logs. What is the most likely cause?
⚠ Common exam trap
Test-takers frequently assume missing events are due to permission or timeout errors, but the absence of any error logs points to throttling, where the function is never invoked and thus no logs are generated.
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 Lambda function has reserved concurrency set to a low value, causing throttling.
When a Lambda function has reserved concurrency set to a low value, it limits the number of concurrent executions allowed for that function. If incoming S3 events exceed this limit, the Lambda service throttles the invocations, causing some events to be silently dropped without generating errors in the function logs because the function never actually runs. This matches the symptom of missing events with no error logs.
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 Lambda function has reserved concurrency set to a low value, causing throttling.
Why this is correct
Reserved concurrency caps the number of concurrent executions for the function. When all slots are busy, Lambda immediately rejects new invocations with a Throttle error. Because the code never starts, no START/END/REPORT lines are emitted, so CloudWatch Logs for the function remain empty. The S3 event notification will retry for a few hours, but each throttled attempt leaves no log.
- ✗
The S3 event notification is configured to send to an SNS topic that is not subscribed to the Lambda function.
Why it's wrong here
If S3 is configured to publish to an SNS topic that has no Lambda subscription, the notification is undeliverable. SNS will log a delivery failure and may retry, but Lambda is never called, so there are no Lambda CloudWatch Logs. Rather than a silent absence of logs, you would typically see SNS delivery status failures, possibly in CloudTrail, which makes this inconsistent with the observed symptom.
- ✗
The S3 bucket policy does not allow the Lambda function to be invoked.
Why it's wrong here
An S3 bucket policy that lacks lambda:InvokeFunction permission would cause the invocation request to fail during authorization. Lambda's API would return AccessDeniedException before the function is executed, so no function logs appear. However, this failure is recorded in AWS CloudTrail as an API call from S3 to Lambda, giving a clear audit trail — the absence of any CloudTrail error would rule this out.
- ✗
The Lambda function has a timeout that is too short.
Why it's wrong here
A function timeout only occurs after the handler has been invoked and is running past its time limit. Lambda stops the execution and writes an execution report to CloudWatch Logs containing 'Task timed out after X seconds' plus the billed duration. That means you would see log events, not an absence of logs, so this does not explain a completely silent CloudWatch Logs group.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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JA
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.