The answer is that the Lambda function's reserved concurrency is set too low, as this directly limits how many shards the consumer can process in parallel. When a Kinesis data stream has 10 shards, each shard can be processed by one Lambda invocation at a time, but if reserved concurrency is capped below 10, some shards will queue up waiting for an available execution environment, causing high consumer lag. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of how Lambda’s concurrency model interacts with Kinesis stream parallelism—a common trap is to blame DynamoDB write capacity or suggest adding more shards, but the root cause is almost always an artificial concurrency ceiling. Remember the 1:1 rule: one shard needs one concurrent Lambda invocation to keep lag at zero, so if your reserved concurrency is less than your shard count, you will see lag.
DEA-C01 Data Operations and Support Practice Question
This DEA-C01 practice question tests your understanding of data operations and support. 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 data pipeline using Amazon Kinesis Data Streams is experiencing high consumer lag. The stream has 10 shards. The consumer is an AWS Lambda function that processes each record and writes to Amazon DynamoDB. What is the MOST likely cause of the lag?
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's reserved concurrency is set too low
Option C is correct because if the Lambda function's concurrency limit is reached, it cannot process all shards simultaneously, causing lag. Option A is wrong because increasing shards would increase parallelism but is not the root cause. Option B is wrong because Lambda can process from multiple shards. Option D is wrong because DynamoDB write capacity could be a bottleneck but is less likely than Lambda concurrency.
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 Lambda function's reserved concurrency is set too low
Why this is correct
Low concurrency limits parallel processing of shards.
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 DynamoDB table's write capacity is throttling writes
Why it's wrong here
Could cause lag but Lambda would retry; concurrency is more direct.
✗
The number of shards is insufficient for the data volume
Why it's wrong here
Could be a cause, but not the most likely given existing setup.
✗
The Lambda function is not authorized to read from Kinesis
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 Operations and Support — This question tests Data Operations and Support — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Lambda function's reserved concurrency is set too low — Option C is correct because if the Lambda function's concurrency limit is reached, it cannot process all shards simultaneously, causing lag. Option A is wrong because increasing shards would increase parallelism but is not the root cause. Option B is wrong because Lambda can process from multiple shards. Option D is wrong because DynamoDB write capacity could be a bottleneck but is less likely than Lambda concurrency.
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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