DVA-C02 Development with AWS Services Practice Question
A developer is building a serverless application that processes images uploaded to an S3 bucket. The bucket triggers a Lambda function that creates a thumbnail and stores it in another S3 bucket. The developer notices that the Lambda function is invoked multiple times for the same object, causing duplicate thumbnails. What is the MOST likely cause?
⚠ Common exam trap
Candidates often assume that S3 event notifications guarantee exactly-once delivery, or they confuse S3's data consistency model (which is now strongly consistent for read-after-write) with its event delivery model (which is at-least-once).
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 eventually consistent and may deliver duplicates.
Amazon S3 event notifications are designed to provide at-least-once delivery. This means that while most events are delivered exactly once, duplicate event notifications can occasionally occur. To handle this, the Lambda function should be designed to be idempotent, ensuring that processing the same event multiple times does not cause unintended side effects (like duplicate thumbnails).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
S3 event notifications are eventually consistent and may deliver duplicates.
Why this is correct
S3 event notifications are designed for at-least-once delivery, meaning duplicates are possible under rare circumstances like network issues or internal retries within AWS. However, the 'eventually consistent' nature of S3 object storage itself does not directly cause duplicate notification deliveries in the way this option implies. While S3 consistency models affect read-after-write behavior, they are not the primary or most common reason for receiving multiple identical event notifications for a single S3 action.
- ✗
The Lambda function is configured with a DLQ that causes retries.
Why it's wrong here
A Dead-Letter Queue (DLQ) in AWS Lambda serves as a destination for messages that a function fails to process successfully after exhausting all configured retries. The purpose of a DLQ is to capture and store failed invocations for later analysis, not to initiate or cause additional retries of successful invocations. Therefore, a DLQ cannot be the source of duplicate successful Lambda function executions, as it only comes into play when processing has already failed and exhausted its retry attempts.
- ✗
The Lambda function is idempotent and should handle duplicates.
Why it's wrong here
Idempotency is a crucial design pattern where a function produces the same result regardless of how many times it is invoked with the same input, effectively handling duplicates without adverse side effects. While implementing an idempotent Lambda function is a best practice for resilience in serverless architectures, the question specifically asks for the cause of duplicate invocations. This option describes a solution or a desired characteristic of the function, rather than identifying the root technical reason why the duplicate events are being generated and delivered in the first place.
- ✗
The S3 bucket has multiple event notifications that trigger the same Lambda function.
Why it's wrong here
AWS S3 allows configuring multiple event notifications on a single bucket, each with its own event types (e.g., s3:ObjectCreated:Put, s3:ObjectCreated:*) and optional prefix/suffix filters. If an S3 bucket has two or more distinct notification configurations that are both configured to trigger the same Lambda function and whose filters overlap for a given object event, a single S3 action will result in multiple, identical invocations of that Lambda function. This common misconfiguration is a direct and frequent cause of duplicate processing in serverless applications.
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
Related to this question
About these practice questions
This DVA-C02 question is part of Courseiva's 1,135-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DVA-C02 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 DVA-C02 exam.