- A
Enable DynamoDB auto scaling to increase write capacity and reduce throttling, which causes retries and duplicates.
Why wrong: Option A is wrong because idempotent writes would require a unique identifier; using PutItem with a condition expression on a unique attribute (like the sequence number) is effectively the same as option B but transactions provide atomicity.
- B
Use DynamoDB TransactWriteItems with a condition check that the record's Kinesis sequence number does not already exist in the table.
Option B is correct because using a DynamoDB transaction with a condition check on the Kinesis sequence number ensures that each record is written only once.
- C
Place an Amazon SQS FIFO queue between the KCL application and DynamoDB to deduplicate messages.
Why wrong: Option C is wrong because placing an SQS FIFO queue adds latency and complexity without solving the duplicate write issue. The KCL already provides at-least-once delivery, so the consumer must handle deduplication using conditional writes based on the Kinesis sequence number.
- D
Modify the application to use DynamoDB BatchWriteItem instead of PutItem to reduce the number of write requests.
Why wrong: Option D is wrong because adding a FIFO queue adds latency and complexity without guaranteeing exactly-once processing in the consumer.
Prevent Duplicate Writes from Kinesis to DynamoDB Using Transactions
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 a real-time analytics platform that ingests IoT sensor data from millions of devices. The data is sent to Amazon Kinesis Data Streams with 16 shards. A custom Java application using the Kinesis Client Library (KCL) processes the data and writes aggregated results to Amazon DynamoDB. The application runs on a fleet of EC2 instances in an Auto Scaling group. Recently, the team noticed that some records are being processed multiple times, resulting in duplicate entries in DynamoDB. The application uses the DynamoDB PutItem API to write records. The team needs to eliminate duplicates without significantly increasing latency. Which solution should the team implement?
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
Use DynamoDB TransactWriteItems with a condition check that the record's Kinesis sequence number does not already exist in the table.
Option B is correct because using a DynamoDB transaction with a condition check on the Kinesis sequence number ensures that each record is written only once. Option A is wrong because increasing write capacity does not address duplicate processing; duplicates arise from the KCL consumer processing records multiple times, not from throttling. Option C is wrong because while SQS FIFO provides deduplication at the queue level, it does not guarantee exactly-once processing downstream in DynamoDB; the consumer could still write duplicates if it fails after writing but before deleting the message. Additionally, adding an extra queue increases latency and complexity. Option D is wrong because BatchWriteItem does not decrease duplicates; it only batches multiple put requests into one API call and still requires idempotency measures.
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.
- ✗
Enable DynamoDB auto scaling to increase write capacity and reduce throttling, which causes retries and duplicates.
Why it's wrong here
Option A is wrong because idempotent writes would require a unique identifier; using PutItem with a condition expression on a unique attribute (like the sequence number) is effectively the same as option B but transactions provide atomicity.
- ✓
Use DynamoDB TransactWriteItems with a condition check that the record's Kinesis sequence number does not already exist in the table.
Why this is correct
Option B is correct because using a DynamoDB transaction with a condition check on the Kinesis sequence number ensures that each record is written only once.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Place an Amazon SQS FIFO queue between the KCL application and DynamoDB to deduplicate messages.
Why it's wrong here
Option C is wrong because placing an SQS FIFO queue adds latency and complexity without solving the duplicate write issue. The KCL already provides at-least-once delivery, so the consumer must handle deduplication using conditional writes based on the Kinesis sequence number.
- ✗
Modify the application to use DynamoDB BatchWriteItem instead of PutItem to reduce the number of write requests.
Why it's wrong here
Option D is wrong because adding a FIFO queue adds latency and complexity without guaranteeing exactly-once processing in the consumer.
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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
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: Use DynamoDB TransactWriteItems with a condition check that the record's Kinesis sequence number does not already exist in the table. — Option B is correct because using a DynamoDB transaction with a condition check on the Kinesis sequence number ensures that each record is written only once. Option A is wrong because increasing write capacity does not address duplicate processing; duplicates arise from the KCL consumer processing records multiple times, not from throttling. Option C is wrong because while SQS FIFO provides deduplication at the queue level, it does not guarantee exactly-once processing downstream in DynamoDB; the consumer could still write duplicates if it fails after writing but before deleting the message. Additionally, adding an extra queue increases latency and complexity. Option D is wrong because BatchWriteItem does not decrease duplicates; it only batches multiple put requests into one API call and still requires idempotency measures.
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.
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
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