The answer is that the IAM policy is missing kms:Encrypt permission for the output bucket’s KMS key. This is because when a Glue ETL job writes to an SSE-KMS encrypted S3 bucket, it must have explicit kms:Encrypt (or kms:GenerateDataKey) permission on the key used to encrypt that output bucket. The provided policy only grants kms:Decrypt and kms:GenerateDataKey for the input bucket’s key (abc123), which allows reading the source data, but it lacks any KMS actions for the output key (xyz789), causing the access denied error during the write phase. On the AWS Certified Machine Learning Specialty MLS-C01 exam, this scenario tests your understanding of how KMS permissions must be scoped separately for read and write operations across different encrypted S3 buckets—a common trap is assuming s3:PutObject alone is sufficient when the bucket uses SSE-KMS. Remember the memory tip: “Read needs Decrypt, Write needs Encrypt—never mix the keys.”
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.
Refer to the exhibit. A data engineer has attached this IAM policy to an IAM role used by an AWS Glue ETL job. The job reads from an S3 bucket (data-bucket) that is encrypted with SSE-KMS using the key arn:aws:kms:us-east-1:123456789012:key/abc123, transforms the data, and writes the result to a different S3 bucket (output-bucket) encrypted with a different KMS key (arn:aws:kms:us-east-1:123456789012:key/xyz789). When the job runs, it fails with an access denied error. What is the cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The policy does not grant kms:Encrypt permission for the output bucket's KMS key.
Option C is correct because the policy grants kms:Decrypt and kms:GenerateDataKey for the input bucket's key, but does not grant the equivalent permissions for the output bucket's key (xyz789). The job needs to encrypt the output, so it needs kms:Encrypt (or GenerateDataKey) for the output key. Option A is wrong because the policy includes s3:PutObject. Option B is wrong because the Glue catalog permissions are sufficient. Option D is wrong because the policy includes s3:GetObject.
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 policy does not include s3:GetObject permission for the output bucket.
Why it's wrong here
The job reads from input bucket, not output; output requires PutObject.
✗
The policy does not include glue:CreateTable permission.
Why it's wrong here
The policy includes glue:UpdateTable, but the job may need create; however, the error is likely KMS.
✗
The policy does not include s3:PutObject permission for the output bucket.
Why it's wrong here
The policy grants s3:PutObject on data-bucket, but the output bucket is different.
✓
The policy does not grant kms:Encrypt permission for the output bucket's KMS key.
Why this is correct
To write to an SSE-KMS encrypted bucket, the role needs kms:Encrypt or kms:GenerateDataKey for that key.
Related concept
Read the scenario before looking for a memorised answer.
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.
Trap categories for this question
Command / output trap
The job reads from input bucket, not output; output requires PutObject.
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.
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: The policy does not grant kms:Encrypt permission for the output bucket's KMS key. — Option C is correct because the policy grants kms:Decrypt and kms:GenerateDataKey for the input bucket's key, but does not grant the equivalent permissions for the output bucket's key (xyz789). The job needs to encrypt the output, so it needs kms:Encrypt (or GenerateDataKey) for the output key. Option A is wrong because the policy includes s3:PutObject. Option B is wrong because the Glue catalog permissions are sufficient. Option D is wrong because the policy includes s3:GetObject.
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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Question Discussion
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