A developer is building a serverless application that uses an Amazon SQS FIFO queue to process orders. The orders must be processed exactly once and in the order they are received. The developer uses an AWS Lambda function as the consumer. Despite using a FIFO queue, the developer notices that some orders are being processed more than once. What is the most likely cause of this behavior?

The Lambda function is configured with a batch size greater than 1, causing multiple messages to be processed at once and some to be duplicated.

Batch processing is supported but does not cause duplicates. All messages in a batch must be deleted individually. If one message fails, the entire batch is not deleted, but duplicates are not inherent.

The SQS queue has a message retention period set too low, causing messages to be deleted automatically and re-queued.

Message retention period only controls how long a message stays in the queue before being deleted permanently. It does not cause duplicates.

The Lambda function's execution role does not have permission to delete messages from the queue.

If the role lacks delete permission, the function will fail but will not cause duplicates; the message would become visible again after visibility timeout. However, this is less likely than the function simply not calling DeleteMessage for other reasons.

What does this DVA-C02 question test?

Read the scenario before looking for a memorised answer.

What is the correct answer to this question?

The correct answer is: The Lambda function is not deleting messages from the queue after successful processing, so they become visible again after the visibility timeout. — FIFO queues guarantee exactly-once processing only when the consumer uses the correct message deduplication ID and visibility timeout settings. If the Lambda function fails to delete the message after processing (e.g., due to a timeout or error), the message becomes visible again after the visibility timeout expires, leading to duplicate processing. Additionally, if the Lambda function's retention period or DLQ is misconfigured, it may cause re-processing. The most common cause is that the visibility timeout is set too short relative to the function's execution time, or the function is not deleting the message successfully. However, the question asks about the most likely cause given that it's a FIFO queue. FIFO queues do not guarantee exactly-once delivery; they ensure that a message is delivered at least once, but the consumer must handle idempotency. The developer should implement idempotent processing using, for example, a DynamoDB table to track processed message IDs.

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

Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.