DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. 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 company has an AWS Lambda function that processes S3 events. The function is invoked multiple times for the same S3 object, causing duplicate processing. The engineer suspects the issue is related to retries from the S3 event notification or Lambda's built-in retry behavior. What is the MOST effective way to ensure idempotent processing?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a DynamoDB table to store a record of processed S3 object keys and check for existence before processing.
Option B is correct because storing processed S3 object keys in a DynamoDB table and checking for existence before processing ensures idempotency at the application level. This approach directly handles duplicate invocations caused by S3 event retries or Lambda's built-in retry behavior, as the function can conditionally skip processing if the key already exists in DynamoDB. It provides a durable, consistent, and scalable mechanism to prevent duplicate processing regardless of how many times the function is invoked for the same object.
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.
✗
Modify the S3 bucket event notification configuration to use a prefix filter that excludes duplicate objects.
Why it's wrong here
Prefix filters do not prevent duplicate event delivery.
✓
Use a DynamoDB table to store a record of processed S3 object keys and check for existence before processing.
Why this is correct
This pattern ensures idempotency by tracking processed objects.
Related concept
Read the scenario before looking for a memorised answer.
✗
Set the Lambda function's ReservedConcurrency to 1 to prevent concurrent executions.
Why it's wrong here
This limits concurrency but does not prevent duplicate invocations for the same object.
✗
Use an Amazon SQS FIFO queue as the event source and enable content-based deduplication.
Why it's wrong here
SQS FIFO can deduplicate messages but S3 events are delivered directly to Lambda, not SQS.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse concurrency control (ReservedConcurrency) with idempotency, or assume SQS FIFO deduplication is a drop-in solution without realizing S3 cannot directly send events to FIFO queues.
Detailed technical explanation
How to think about this question
Lambda's built-in retry behavior for asynchronous invocations (which S3 events use) retries twice with a 1-minute delay between attempts, and S3 event notifications may also retry for up to 10 minutes. DynamoDB's conditional writes (e.g., using `ConditionExpression: attribute_not_exists(PK)`) provide atomic check-and-insert operations, ensuring exactly-once semantics even under concurrent invocations. This pattern is recommended by AWS for idempotent processing and is resilient to both Lambda retries and S3 event duplication.
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.
Incident and Event Response — This question tests Incident and Event Response — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a DynamoDB table to store a record of processed S3 object keys and check for existence before processing. — Option B is correct because storing processed S3 object keys in a DynamoDB table and checking for existence before processing ensures idempotency at the application level. This approach directly handles duplicate invocations caused by S3 event retries or Lambda's built-in retry behavior, as the function can conditionally skip processing if the key already exists in DynamoDB. It provides a durable, consistent, and scalable mechanism to prevent duplicate processing regardless of how many times the function is invoked for the same object.
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.
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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