The answer is that the Lambda function is not idempotent and is being retried on failures. This is the most likely cause of duplicate processing from a DynamoDB stream because DynamoDB Streams guarantees at-least-once delivery, meaning the same record can be delivered multiple times if the Lambda invocation fails, times out, or the function crashes before completing. When the function lacks idempotency—meaning it cannot safely process the same event multiple times without side effects—each retry increments the counter again, leading to inaccuracies. On the AWS Certified Data Engineer Associate DEA-C01 exam, this concept tests your understanding of stream processing guarantees versus application-level design; a common trap is confusing at-least-once delivery with exactly-once processing, or assuming concurrency limits cause duplicates when they actually cause throttling. Remember: streams deliver at least once, so your Lambda must be idempotent to avoid double-counting—think “retry equals repeat, not reset.”
DEA-C01 Data Ingestion and Transformation Practice Question
This DEA-C01 practice question tests your understanding of data ingestion and transformation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 Amazon Kinesis Data Streams with a Lambda consumer to process real-time events. The Lambda function is triggered by a DynamoDB stream to update a counter. Recently, the counter has been inaccurate due to duplicate processing. What is the most likely cause?
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 Lambda function is not idempotent and is being retried on failures
Option D is correct because Lambda functions may be invoked multiple times for the same record if they fail or time out, leading to duplicates if the operation is not idempotent. Option A is wrong because DynamoDB streams guarantee at-least-once delivery, not exactly-once. Option B is wrong because Kinesis shard rebalancing does not cause duplicates. Option C is wrong because Lambda concurrency limits would cause throttling, not 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.
✗
The DynamoDB stream is configured with 'TRIM_HORIZON' iterator type
Why it's wrong here
Iterator type affects where to start reading, not duplication.
✗
The Lambda function's reserved concurrency is too low
Why it's wrong here
Low concurrency causes throttling, not duplicate processing.
✓
The Lambda function is not idempotent and is being retried on failures
Why this is correct
Retries cause duplicate updates if the function is not idempotent.
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 Kinesis stream has undergone a shard rebalance
Why it's wrong here
Rebalancing may cause reprocessing but not systematic duplicates.
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 DEA-C01 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.
Data Ingestion and Transformation — This question tests Data Ingestion and Transformation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Lambda function is not idempotent and is being retried on failures — Option D is correct because Lambda functions may be invoked multiple times for the same record if they fail or time out, leading to duplicates if the operation is not idempotent. Option A is wrong because DynamoDB streams guarantee at-least-once delivery, not exactly-once. Option B is wrong because Kinesis shard rebalancing does not cause duplicates. Option C is wrong because Lambda concurrency limits would cause throttling, not duplicates.
What should I do if I get this DEA-C01 question wrong?
Identify which DEA-C01 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.
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Question Discussion
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