The answer is that Lambda throttling causes the visibility timeout to expire before the function can process the messages, leading to duplicate SQS messages. When Lambda throttles a function due to hitting its reserved concurrency of 5, it cannot accept new invocations from the SQS queue, so the messages remain in-flight. Because the function typically finishes a batch in 10 seconds but the visibility timeout is 30 seconds, the timeout can expire while Lambda is still retrying the throttled invocation, making the messages visible again in the queue for other consumers to pick up. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how Lambda throttling interacts with SQS visibility timeouts—a common trap is assuming reserved concurrency or batch size directly cause duplicates, but the real culprit is the timing mismatch. Remember the memory tip: “Throttle + Timeout = Duplicate,” meaning if Lambda can’t process fast enough due to throttling, the visibility window closes and messages reappear.
DOP-C02 Monitoring and Logging Practice Question
This DOP-C02 practice question tests your understanding of monitoring and logging. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A DevOps engineer is troubleshooting an AWS Lambda function that processes messages from an Amazon SQS queue. The function is configured with a reserved concurrency of 5 and a batch size of 10. The SQS queue has a visibility timeout of 30 seconds, and the Lambda function typically completes processing each batch in 10 seconds. Recently, the engineer noticed that messages are repeatedly processed, causing duplicates. The CloudWatch Logs show that the function is experiencing throttling errors. What is the MOST likely cause of the duplicate processing?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Lambda is throttling the function, and the visibility timeout expires before the function can process the messages.
Option C is correct. When Lambda throttles, it cannot process the messages, so SQS does not delete them. The visibility timeout of 30 seconds may expire before Lambda retries, making the messages visible again. Option A is incorrect because the batch size of 10 is within the maximum of 10. Option B is incorrect because reserved concurrency does not cause duplicate processing directly. Option D is incorrect because DLQ would store messages, not cause duplicates.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Lambda is throttling the function, and the visibility timeout expires before the function can process the messages.
Why this is correct
Throttling prevents processing, and visibility timeout expires, making messages visible again.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The SQS queue's dead-letter queue (DLQ) is not configured, causing messages to be reprocessed.
Why it's wrong here
DLQ does not cause duplicate processing; it stores failed messages.
✗
The function's batch size is too large, causing timeouts.
Why it's wrong here
Batch size of 10 is within limit and function completes in 10 seconds.
✗
The function's reserved concurrency is too high, causing overloading.
Why it's wrong here
Reserved concurrency limits the function; it does not cause overload.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Identify which DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Monitoring and Logging — This question tests Monitoring and Logging — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Lambda is throttling the function, and the visibility timeout expires before the function can process the messages. — Option C is correct. When Lambda throttles, it cannot process the messages, so SQS does not delete them. The visibility timeout of 30 seconds may expire before Lambda retries, making the messages visible again. Option A is incorrect because the batch size of 10 is within the maximum of 10. Option B is incorrect because reserved concurrency does not cause duplicate processing directly. Option D is incorrect because DLQ would store messages, not cause duplicates.
What should I do if I get this DOP-C02 question wrong?
Identify which DOP-C02 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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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 DevOps engineer is troubleshooting a slow-running Lambda function. The function processes messages from an SQS queue. Which CloudWatch metric should be examined first to determine if the function is experiencing throttling?
easy
A.Invocations
B.ConcurrentExecutions
C.Duration
✓ D.Throttles
Why D: The Throttles metric directly indicates when Lambda is rejecting invocation requests due to concurrency limits being reached. Since the question asks specifically about throttling, this is the first metric to examine to confirm whether the function is being rate-limited by AWS.
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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.
Question Discussion
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