Question

A developer builds a payment-processing Lambda function triggered by an SQS queue. During load testing, the team notices that occasionally the same payment message is processed twice, resulting in duplicate charges. This happens because SQS standard queues can deliver a message more than once. Which approach BEST prevents duplicate charges while keeping the function reliable?

Accepted Answer

Store a unique transaction ID from each message in a DynamoDB table with a conditional write, and skip processing if the ID already exists

Answer

Increase the SQS visibility timeout to a value larger than the Lambda timeout so messages are never redelivered

Answer

Switch the Lambda invocation type from event to request-response so each message is only processed once

Answer

Enable reserved concurrency of 1 on the Lambda function so only one message is processed at a time