The answer is to increase the Lambda function’s reserved concurrency to 500. Throttling errors, specifically HTTP 429 responses, occur when the number of concurrent invocations exceeds either the account-level concurrency limit or the function’s reserved concurrency setting. By raising reserved concurrency from 100 to 500, you directly eliminate the scaling bottleneck that causes Lambda throttling during traffic spikes, allowing the function to handle more invocations from the SQS queue without being rejected. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how reserved concurrency acts as a hard cap and how increasing it is the most direct fix for 429 errors, versus adjusting batch size or SQS visibility timeouts, which are common traps. A helpful memory tip: “Reserved concurrency is the throttle valve—turn it up to stop the 429 flood.”
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 runs a serverless application using AWS Lambda functions that process messages from an Amazon SQS queue. The function scales up to handle high traffic but sometimes experiences throttling errors (HTTP 429) from Lambda. The company wants to improve the resilience of the application by reducing throttling. The SQS queue is configured as a Lambda event source with a batch size of 10. The Lambda function has a reserved concurrency of 100. Which combination of actions will best reduce throttling? (Choose the single best answer.)
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Increase the Lambda function's reserved concurrency to 500.
Throttling errors (HTTP 429) occur when Lambda function invocations exceed the account-level concurrency limit or the function's reserved concurrency. By increasing the reserved concurrency from 100 to 500, the function can handle more concurrent invocations, reducing the likelihood of throttling when traffic spikes. This directly addresses the scaling bottleneck without changing the event source or message processing pattern.
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.
✗
Change the SQS queue to use a FIFO queue to guarantee exactly-once processing.
Why it's wrong here
FIFO queues have lower throughput and may increase throttling.
✗
Increase the SQS batch size to 50 to process more messages per invocation.
Why it's wrong here
Larger batch size may increase processing time and still throttle if concurrency is low.
✗
Use a dead-letter queue (DLQ) for unprocessed messages and set up a CloudWatch alarm to trigger a second Lambda function to reprocess them.
Why it's wrong here
DLQ handles failures, but doesn't reduce throttling.
✓
Increase the Lambda function's reserved concurrency to 500.
Why this is correct
Higher reserved concurrency allows more concurrent executions, reducing throttling.
Clue confirmation
The clue word "best" 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 choose a dead-letter queue or batch size change, but the core issue is insufficient concurrency allocation, which only reserved concurrency adjustment can fix.
Detailed technical explanation
How to think about this question
Lambda's reserved concurrency sets a hard limit on the number of concurrent executions for a function, independent of the account-level regional concurrency limit (default 1000). When the SQS event source mapping invokes Lambda, it pulls messages in batches; if the function's reserved concurrency is exhausted, subsequent invocations are throttled, and messages remain in the queue until the concurrency frees up. Increasing reserved concurrency allows the event source mapping to invoke more concurrent function instances, directly reducing 429 errors.
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.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Increase the Lambda function's reserved concurrency to 500. — Throttling errors (HTTP 429) occur when Lambda function invocations exceed the account-level concurrency limit or the function's reserved concurrency. By increasing the reserved concurrency from 100 to 500, the function can handle more concurrent invocations, reducing the likelihood of throttling when traffic spikes. This directly addresses the scaling bottleneck without changing the event source or message processing pattern.
What should I do if I get this DOP-C02 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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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