The answer is to increase the reserved concurrency for the Lambda function, as the most likely cause of throttling is that the reserved concurrency limit is set too low. When a Lambda function uses reserved concurrency, it guarantees a fixed number of concurrent executions, but if that limit is reached, all new invocations are throttled, causing SQS messages to remain in the queue until concurrency becomes available. This scenario tests your understanding of how Lambda reserved concurrency throttling interacts with SQS message processing, a common topic on the AWS Certified SAP on AWS Specialty PAS-C01 exam. A frequent trap is confusing reserved concurrency with provisioned concurrency—reserved concurrency caps executions, while provisioned concurrency pre-warms instances. Remember the memory tip: “Reserved restricts, so if SQS backs up, raise the cap.”
PAS-C01 Operations and Maintenance Practice Question
This PAS-C01 practice question tests your understanding of operations and maintenance. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 company uses AWS Lambda functions with reserved concurrency to process messages from an SQS queue. The operations team notices that the Lambda function sometimes throttles, causing messages to remain in the queue. What is the MOST likely cause and solution?
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
✓
The reserved concurrency is set too low; increase the reserved concurrency for the function.
Option D is correct because Lambda throttling occurs when the reserved concurrency limit is reached, preventing the function from processing new invocations. Since the function uses reserved concurrency, setting it too low restricts the number of concurrent executions, causing SQS messages to remain in the queue until concurrency becomes available. Increasing the reserved concurrency allows more parallel invocations, reducing throttling and improving message processing throughput.
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.
✗
The SQS visibility timeout is too short; increase it.
Why it's wrong here
Visibility timeout affects retries, not throttling.
✗
The Lambda function's dead-letter queue (DLQ) is not configured; set up a DLQ.
Why it's wrong here
DLQ handles failed messages, not throttling.
✗
The SQS queue's redrive policy is too aggressive; reduce the maxReceiveCount.
Why it's wrong here
Not related to throttling.
✓
The reserved concurrency is set too low; increase the reserved concurrency for the function.
Why this is correct
Reserved concurrency limits concurrent executions, causing throttling when exceeded.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse throttling with message processing failures and incorrectly focus on DLQ or visibility timeout settings, rather than recognizing that reserved concurrency directly controls the maximum number of concurrent Lambda executions and is the primary cause of throttling when set too low.
Detailed technical explanation
How to think about this question
Lambda reserved concurrency guarantees a set number of concurrent executions for a function, but if the function's reserved concurrency is lower than the number of simultaneous invocations from SQS (which can scale up to 1,000 messages per batch), throttling occurs with a 429 error. SQS integrates with Lambda via event source mappings that poll the queue and invoke the function, and when throttling happens, the unprocessed messages remain visible in the queue after the visibility timeout expires, leading to repeated polling cycles. In practice, setting reserved concurrency too low is a common misconfiguration when functions are expected to handle burst traffic from SQS, and the solution is to either increase reserved concurrency or use unreserved concurrency from the account-level limit.
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.
TExam Day Tips
→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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Operations and Maintenance — This question tests Operations and Maintenance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The reserved concurrency is set too low; increase the reserved concurrency for the function. — Option D is correct because Lambda throttling occurs when the reserved concurrency limit is reached, preventing the function from processing new invocations. Since the function uses reserved concurrency, setting it too low restricts the number of concurrent executions, causing SQS messages to remain in the queue until concurrency becomes available. Increasing the reserved concurrency allows more parallel invocations, reducing throttling and improving message processing throughput.
What should I do if I get this PAS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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.
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Question Discussion
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