MLS-C01 Data Engineering Practice Question
A company uses Amazon EMR with Spark to process data daily. The job reads from S3 and writes to S3. Recently, the job started failing with 'S3AccessDenied' errors. The IAM role used by EMR has not changed. What is the MOST likely cause?
⚠ Common exam trap
A common mix-up: candidates assume the IAM role is the only factor in access control, overlooking that S3 bucket policies can be modified independently and can explicitly deny access to a specific role, causing 'AccessDenied' errors even when the role itself is unchanged.
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
✓
The S3 bucket policy was updated to deny access to the EMR role
The IAM role used by EMR has not changed, but the S3 bucket policy can be updated independently to deny access to that specific role. An explicit deny in a bucket policy overrides any allow in the IAM policy, causing the 'S3AccessDenied' error even though the role itself remains unchanged. This is the most likely cause because the job was previously working and only the bucket policy could have been modified without touching the EMR configuration.
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 EMR cluster's security group blocks outbound traffic
Why it's wrong here
Security groups affect network, not S3 access denied.
- ✓
The S3 bucket policy was updated to deny access to the EMR role
Why this is correct
Bucket policies can deny access even if IAM allows.
- ✗
The S3 bucket was deleted and recreated
Why it's wrong here
Would cause 'NoSuchBucket' error, not access denied.
- ✗
The EMR service role was rotated
Why it's wrong here
Role rotation would change permissions, but the question says the role hasn't changed.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.