DEA-C01 Data Operations and Support Practice Question
A company runs a data pipeline that uses AWS Lambda to process files uploaded to an S3 bucket. Recently, some files have been processed multiple times. The Lambda function is triggered by S3 event notifications. What is the MOST likely cause of duplicate processing?
⚠ Common exam trap
The trap is choosing 'the function is not idempotent' as the cause — that is the reason duplicates cause damage, but the actual cause of duplicate invocations is S3's at-least-once event delivery.
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
✓
S3 event notifications are delivered at least once.
S3 event notifications are delivered using an at-least-once delivery model, meaning the same event can be delivered more than once, which causes Lambda to be invoked multiple times for the same object. This is the most likely cause of duplicate processing when the function itself is not idempotent. The root cause is the delivery guarantee, not the function's error handling.
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 has a high error rate and retries.
Why it's wrong here
A high error rate causes retries, but S3 event notifications are delivered at least once regardless of function errors, so duplicates occur even on success. Error-driven retries are the correct explanation when a synchronous caller or Lambda's own asynchronous retry policy redelivers after a failed invocation.
- ✗
The Lambda function is not idempotent.
Why it's wrong here
Non-idempotent code makes duplicate side effects visible, but it is not the cause of duplicate delivery; S3 event notifications are at-least-once by design. Idempotency is the right remedy when a pipeline must tolerate redelivery, such as writing to DynamoDB with conditional writes or tracking processed object keys.
- ✗
The Lambda function has a reserved concurrency setting.
Why it's wrong here
Reserved concurrency caps simultaneous invocations for a function; it neither creates nor duplicates S3 event deliveries. It is tempting because concurrency settings affect throttling and scaling, and reserved concurrency is the right control when guaranteeing capacity or limiting blast radius for a critical function.
- ✓
S3 event notifications are delivered at least once.
Why this is correct
Amazon S3 event notifications provide at-least-once delivery, so the same event can be published more than once, invoking Lambda repeatedly for one uploaded object. This inherent duplication, not Lambda retries or concurrency, explains the repeated file processing.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
This DEA-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 DEA-C01 exam.