- A
Increase the Lambda concurrency limit to 2000 to handle the load.
Why wrong: Concurrency is not the issue; duplicates are caused by retries on failure.
- B
Ensure the Lambda function is idempotent and uses the sequence number to deduplicate records.
If the function fails and retries, using sequence numbers allows it to skip already processed records, preventing duplicates.
- C
Decrease the batch size to 10 to reduce the impact of failures.
Why wrong: Smaller batch size reduces the number of records affected per failure but does not eliminate duplicates if the function is not idempotent.
- D
Use Amazon SQS FIFO queue as a buffer between Kinesis and Lambda to guarantee exactly-once processing.
Why wrong: Kinesis does not directly integrate with SQS; this adds complexity and latency.
Quick Answer
The answer is ensuring the Lambda function is idempotent and uses the Kinesis sequence number to deduplicate records. This is correct because when a Lambda function fails during processing—due to a timeout or error—the entire batch is retried, and any records that were already partially processed will be replayed, causing duplicates. The sequence number, unique per record in a shard, allows the function to track which records have been committed, preventing duplicate writes to S3. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your understanding of Kinesis Data Streams’ at-least-once delivery semantics and the need for idempotent consumers in streaming pipelines. A common trap is assuming that increasing concurrency or batch size solves duplicates, but those only affect throughput, not retry behavior. Remember: in Kinesis, retries are per batch, not per record—so always deduplicate by sequence number to avoid double counting.
MLS-C01 Data Engineering Practice Question
This MLS-C01 practice question tests your understanding of data engineering. 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 runs an e-commerce platform that generates clickstream data in real-time. The data is ingested into Amazon Kinesis Data Streams (100 shards) and processed by AWS Lambda functions, which aggregate data in 1-minute windows and write the results to Amazon S3. The Lambda functions are triggered by the Kinesis stream using the event source mapping. Recently, the company noticed that some records are being processed multiple times, leading to duplicate data in S3. The Lambda function is idempotent, but the duplicates are causing downstream issues. The Lambda function's concurrency limit is 1000, and the batch size is 100. The average processing time per record is 200 ms. What is the most likely cause of the duplicates, and how should it be fixed?
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Ensure the Lambda function is idempotent and uses the sequence number to deduplicate records.
Option B is correct. Lambda functions process records from Kinesis in batches. If the function fails (e.g., due to timeout or error), the entire batch is retried, causing duplicates if some records were already partially processed. To avoid duplicates, the function should be idempotent and should not commit partial results. Option A is wrong because the concurrency is sufficient. Option C is wrong because increasing batch size increases the risk of partial failure. Option D is wrong because a FIFO queue does not integrate with Kinesis.
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.
- ✗
Increase the Lambda concurrency limit to 2000 to handle the load.
Why it's wrong here
Concurrency is not the issue; duplicates are caused by retries on failure.
- ✓
Ensure the Lambda function is idempotent and uses the sequence number to deduplicate records.
Why this is correct
If the function fails and retries, using sequence numbers allows it to skip already processed records, preventing duplicates.
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.
- ✗
Decrease the batch size to 10 to reduce the impact of failures.
Why it's wrong here
Smaller batch size reduces the number of records affected per failure but does not eliminate duplicates if the function is not idempotent.
- ✗
Use Amazon SQS FIFO queue as a buffer between Kinesis and Lambda to guarantee exactly-once processing.
Why it's wrong here
Kinesis does not directly integrate with SQS; this adds complexity and latency.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Identify which MLS-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.
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FAQ
Questions learners often ask
What does this MLS-C01 question test?
Data Engineering — This question tests Data Engineering — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Ensure the Lambda function is idempotent and uses the sequence number to deduplicate records. — Option B is correct. Lambda functions process records from Kinesis in batches. If the function fails (e.g., due to timeout or error), the entire batch is retried, causing duplicates if some records were already partially processed. To avoid duplicates, the function should be idempotent and should not commit partial results. Option A is wrong because the concurrency is sufficient. Option C is wrong because increasing batch size increases the risk of partial failure. Option D is wrong because a FIFO queue does not integrate with Kinesis.
What should I do if I get this MLS-C01 question wrong?
Identify which MLS-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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Last reviewed: Jun 20, 2026
This MLS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the MLS-C01 exam.
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