MLS-C01 Data Engineering Practice Question
Exhibit
Refer to the exhibit.
```
{
"Records": [
{
"eventVersion": "2.0",
"eventSource": "aws:s3",
"awsRegion": "us-east-1",
"eventName": "ObjectCreated:Put",
"s3": {
"s3SchemaVersion": "1.0",
"bucket": {
"name": "my-input-bucket",
"arn": "arn:aws:s3:::my-input-bucket"
},
"object": {
"key": "data/file.csv",
"size": 1024,
"eTag": "abc123"
}
}
}
]
}A data engineer configures an S3 event notification to trigger an AWS Lambda function when a new object is created in 'my-input-bucket'. The Lambda function processes the CSV file and writes results to 'my-output-bucket'. The engineer notices that the Lambda function is not triggered for some objects. Which step should the engineer take to diagnose the issue?
⚠ Common exam trap
The trap here is that candidates often focus on the Lambda function's execution role (Option A) or the event notification filters (Option D), overlooking the critical resource-based policy that grants S3 permission to invoke the function.
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
✓
Check the Lambda function's resource-based policy to ensure S3 has permission to invoke the function.
The most likely cause of the Lambda function not being triggered for some objects is that S3 lacks the necessary permission to invoke the function. S3 event notifications require a resource-based policy (also known as a Lambda function policy) that explicitly grants the S3 service principal permission to invoke the function. Without this policy, S3 will not be able to trigger the Lambda function, even if the event notification configuration is correct.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Check the Lambda function's execution role for permissions to write to the output bucket.
Why it's wrong here
This would cause processing errors, not trigger failures.
- ✗
Review the CloudWatch Logs for the Lambda function to see if there are errors.
Why it's wrong here
If not triggered, there would be no logs.
- ✓
Check the Lambda function's resource-based policy to ensure S3 has permission to invoke the function.
Why this is correct
Missing invoke permission is a common cause of trigger failure.
- ✗
Verify that the S3 event notification is configured with the correct prefix and suffix filters.
Why it's wrong here
Filters could cause missing objects, but the engineer should check if notification is set up.
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
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