Quick Answer
The answer is that the IAM role for Firehose lacks the s3:PutObject permission. This is correct because the CloudWatch metric 'S3.BucketExists' equals 1, confirming the bucket exists and is accessible, while 'DeliveryToS3.Success' is 0, indicating the write operation itself is failing. For Kinesis Firehose to deliver data to S3, its assumed IAM role must include the s3:PutObject action; without it, the stream cannot write objects, causing all delivery attempts to fail silently. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of how IAM permissions map to specific service operations—a common trap is assuming bucket existence or network issues are the root cause when the real problem is a missing write permission. Remember the memory tip: "BucketExists is 1, but Success is 0? Check PutObject, not the bucket."
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 engineer notices that an Amazon Kinesis Data Firehose delivery stream is failing to deliver data to an Amazon S3 bucket. The CloudWatch metrics show 'DeliveryToS3.Success' is 0 and 'S3.BucketExists' is 1. 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.
The S3 bucket has an ACL that denies access to Firehose.
Why wrong: Firehose uses bucket policies, not ACLs.
The Firehose delivery stream Lambda transformation function is failing.
Why wrong: Lambda transformation is optional; the error is in delivery to S3.
The IAM role for Firehose lacks s3:PutObject permission.
Write permission is required for delivery.
The S3 bucket does not exist.
Why wrong: S3.BucketExists is 1, so the bucket exists.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The IAM role for Firehose lacks s3:PutObject permission.
The metric 'S3.BucketExists' is 1, confirming the S3 bucket exists, so the issue is not bucket existence. With 'DeliveryToS3.Success' at 0, the failure is in the write operation. The IAM role assumed by Firehose must have the s3:PutObject permission to deliver data; lacking it would cause all delivery attempts to fail silently, matching the observed metrics.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The S3 bucket has an ACL that denies access to Firehose.
Why it's wrong here
Firehose uses bucket policies, not ACLs.
The Firehose delivery stream Lambda transformation function is failing.
Why it's wrong here
Lambda transformation is optional; the error is in delivery to S3.
The IAM role for Firehose lacks s3:PutObject permission.
Why this is correct
Write permission is required for delivery.
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 S3 bucket does not exist.
Why it's wrong here
S3.BucketExists is 1, so the bucket exists.
Common exam traps
The trap here is that candidates may confuse 'S3.BucketExists' with successful delivery, or assume a missing bucket is the issue when the metric clearly shows the bucket exists, leading them to overlook the IAM permission gap.
Detailed technical explanation
Under the hood, Kinesis Data Firehose uses a service-linked IAM role to assume permissions for S3 operations. The s3:PutObject permission is required for each individual object upload; if missing, Firehose retries up to the configured retry duration but never succeeds, leading to a persistent 0 in 'DeliveryToS3.Success'. In real-world scenarios, this often occurs after a role rotation or when a bucket policy explicitly denies the Firehose principal, but the metric 'S3.BucketExists' remains 1 because the bucket's existence is checked via a separate API call (HeadBucket) that does not require s3:PutObject.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Data Operations and Support — This question tests Data Operations and Support — Read the scenario before looking for a memorised answer..
The correct answer is: The IAM role for Firehose lacks s3:PutObject permission. — The metric 'S3.BucketExists' is 1, confirming the S3 bucket exists, so the issue is not bucket existence. With 'DeliveryToS3.Success' at 0, the failure is in the write operation. The IAM role assumed by Firehose must have the s3:PutObject permission to deliver data; lacking it would cause all delivery attempts to fail silently, matching the observed metrics.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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