The answer is to increase the Lambda function timeout to 60 seconds and the SQS visibility timeout to 90 seconds. This combination directly addresses the root cause: the Lambda function times out while waiting for the external API, and the SQS visibility timeout is too short, causing messages to reappear in the queue before processing completes, which creates a retry loop and inflates the ApproximateNumberOfMessagesVisible metric. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how Lambda and SQS timeouts must be coordinated to prevent message duplication and backlog growth. A common trap is to increase concurrency or reduce batch size, but those actions either amplify timeouts or reduce throughput without fixing the underlying latency issue. Remember the memory tip: “Lambda timeout + visibility timeout = processing window” — always ensure the visibility timeout exceeds the function timeout plus your function’s maximum execution time to avoid premature redrives.
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 is using AWS Lambda to process events from an Amazon SQS queue. The Lambda function is configured with a batch size of 10 and a maximum concurrency of 5. Recently, the function started experiencing high error rates and the SQS queue's ApproximateNumberOfMessagesVisible metric is increasing. The CloudWatch logs show that the function is timing out after 30 seconds. The function makes calls to an external API that sometimes takes more than 30 seconds to respond. The DevOps engineer needs to reduce the backlog and prevent message loss. The engineer is considering the following actions: A) Increase the Lambda function timeout to 60 seconds and increase the SQS visibility timeout to 90 seconds. B) Decrease the batch size to 1 to avoid processing multiple messages at once. C) Increase the Lambda function reserved concurrency to 100 to allow more concurrent executions. D) Use a dead-letter queue to capture messages that fail processing after all retries. Which combination of actions should the engineer take?
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 timeout to 60 seconds and increase the SQS visibility timeout to 90 seconds.
Option A is correct because increasing the timeout allows the function to wait longer for the external API, and increasing visibility timeout prevents messages from becoming visible again before the function completes. This reduces retries and backlog. Option B is wrong because decreasing batch size reduces throughput, worsening the backlog. Option C is wrong because increasing concurrency may cause more timeouts if the function still times out. Option D is good but alone does not reduce backlog; it only captures failed messages. The best approach is to fix the timeout first.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Use a dead-letter queue to capture messages that fail processing after all retries.
Why it's wrong here
Helps prevent loss but does not reduce backlog.
✗
Decrease the batch size to 1 to avoid processing multiple messages at once.
Why it's wrong here
Reduces throughput, making backlog worse.
✓
Increase the Lambda function timeout to 60 seconds and increase the SQS visibility timeout to 90 seconds.
Why this is correct
This allows the function to complete and prevents premature retries.
Related concept
Static NAT maps one inside address to one outside address.
✗
Increase the Lambda function reserved concurrency to 100 to allow more concurrent executions.
Why it's wrong here
More concurrent executions may still time out.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DOP-C02 NAT questions on configuration and troubleshooting.
Incident and Event Response — This question tests Incident and Event Response — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Increase the Lambda function timeout to 60 seconds and increase the SQS visibility timeout to 90 seconds. — Option A is correct because increasing the timeout allows the function to wait longer for the external API, and increasing visibility timeout prevents messages from becoming visible again before the function completes. This reduces retries and backlog. Option B is wrong because decreasing batch size reduces throughput, worsening the backlog. Option C is wrong because increasing concurrency may cause more timeouts if the function still times out. Option D is good but alone does not reduce backlog; it only captures failed messages. The best approach is to fix the timeout first.
What should I do if I get this DOP-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DOP-C02 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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