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Question 640 of 1,616

Troubleshooting and Optimization →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that S3 event notifications are delivered on an at least once basis, and the Lambda function lacks idempotency. This is the most likely cause because Amazon S3 can resend the same event notification multiple times, and without idempotent handling, each invocation of the Lambda function creates a new DynamoDB record. The function’s use of the uuid library to generate a unique ID on every invocation actually worsens the problem—it ensures each duplicate event produces a different ID, so the same image is stored as a separate item each time, rather than deduplicating against a known key. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of event-driven architectures and the critical need for idempotency when dealing with at least once delivery guarantees. A common trap is assuming that generating a random UUID inside the function solves duplicates; in reality, idempotency requires using a deterministic identifier from the event payload itself, such as the S3 object key or ETag. Memory tip: think “S3 sends twice, Lambda must think twice”—always use the event’s own ID to make your function idempotent.

DVA-C02 Troubleshooting and Optimization Practice Question

This DVA-C02 practice question tests your understanding of troubleshooting and optimization. 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.

A developer is troubleshooting an AWS Lambda function that is invoked from an Amazon S3 bucket via event notifications. The function processes images and stores metadata in Amazon DynamoDB. The developer notices that some images are being processed multiple times, resulting in duplicate entries in DynamoDB. The S3 event notification is configured to send events to the Lambda function with the 's3:ObjectCreated:*' event type. The function uses the 'uuid' library to generate a unique ID for each image upon processing. What is the most likely cause of the duplicate processing?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1hardmultiple choice

Full question →

A
S3 event notifications are delivered at least once, and the Lambda function is not idempotent.
S3 can send the same event multiple times. Without idempotency checks (e.g., using the S3 object key as the DynamoDB primary key), each event creates a new item, causing duplicates.
B
The Lambda function's concurrency is set too high, causing race conditions.
Why wrong: High concurrency can cause race conditions but not duplicate event delivery. Duplicate events originate from S3, not from concurrency.
C
The DynamoDB table does not have a primary key that prevents duplicates.
Why wrong: Even with a primary key, the UUID generated per invocation differs, so each duplicate event creates a new item. The real issue is lack of idempotency, not table schema.
D
The S3 bucket is configured with versioning, causing multiple object creation events.
Why wrong: Versioning can generate multiple events (e.g., PutObject and CopyObject) for the same object, but the most common cause of duplicate processing is the 'at least once' delivery of notifications.

Full breakdown with real-world context →

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, and the Lambda function is not idempotent.

Amazon S3 event notifications are delivered on an 'at least once' basis, meaning the same event can be sent to Lambda multiple times. If the Lambda function is not idempotent—i.e., processing the same event multiple times produces duplicate side effects—then duplicate DynamoDB entries will occur. The use of a 'uuid' library inside the function does not help because a new UUID is generated on each invocation, so the same image gets different IDs and is stored as a separate item each time.

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.

✓
S3 event notifications are delivered at least once, and the Lambda function is not idempotent.
Why this is correct
S3 can send the same event multiple times. Without idempotency checks (e.g., using the S3 object key as the DynamoDB primary key), each event creates a new item, causing duplicates.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The Lambda function's concurrency is set too high, causing race conditions.
Why it's wrong here
High concurrency can cause race conditions but not duplicate event delivery. Duplicate events originate from S3, not from concurrency.
✗
The DynamoDB table does not have a primary key that prevents duplicates.
Why it's wrong here
Even with a primary key, the UUID generated per invocation differs, so each duplicate event creates a new item. The real issue is lack of idempotency, not table schema.
✗
The S3 bucket is configured with versioning, causing multiple object creation events.
Why it's wrong here
Versioning can generate multiple events (e.g., PutObject and CopyObject) for the same object, but the most common cause of duplicate processing is the 'at least once' delivery of notifications.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates assume generating a unique ID inside the function solves duplication, but they miss that idempotency requires using a stable, external identifier (like the S3 object key) to detect and skip already-processed events.

Detailed technical explanation

How to think about this question

S3 event notifications use an internal queue that guarantees at-least-once delivery, meaning the same event can be delivered multiple times if the first attempt fails or if there is a transient error. Lambda functions should be designed to be idempotent by using a deterministic identifier (e.g., the S3 object key and ETag) to check for existing records in DynamoDB before inserting, or by using conditional writes with a unique constraint. In practice, this is a common pitfall when migrating from synchronous to event-driven architectures.

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.

TExam Day Tips

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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this DVA-C02 question test?

Troubleshooting and Optimization — This question tests Troubleshooting and Optimization — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: S3 event notifications are delivered at least once, and the Lambda function is not idempotent. — Amazon S3 event notifications are delivered on an 'at least once' basis, meaning the same event can be sent to Lambda multiple times. If the Lambda function is not idempotent—i.e., processing the same event multiple times produces duplicate side effects—then duplicate DynamoDB entries will occur. The use of a 'uuid' library inside the function does not help because a new UUID is generated on each invocation, so the same image gets different IDs and is stored as a separate item each time.

What should I do if I get this DVA-C02 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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