An ecommerce frontend calls a POST API via API Gateway that invokes a Lambda function asynchronously to process orders. Occasionally orders are not processed, and Lambda logs show no errors. What should the developer do to diagnose and resolve the missing order processing?

The correct answer is to inspect the Lambda dead-letter queue (DLQ) for failed events. When a Lambda function is invoked asynchronously, if it fails after all retries (or if the event is discarded due to misconfiguration), the event can be sent to a configured DLQ (an SQS queue or SNS topic). Since the Lambda logs show no errors, the function itself might not be failing, but the asynchronous invocation could be failing to deliver the event, or the function might be timing out or encountering an unhandled error that prevents it from logging. The DLQ captures these undelivered or unprocessable events, allowing for diagnosis and reprocessing. Checking frontend logs for API failures is less direct; the API Gateway might successfully receive the request even if the Lambda invocation fails downstream. Examining Lambda logs for errors is already stated as not showing errors, so this is unlikely to reveal the problem. Ensuring API Gateway caching is disabled is irrelevant for POST requests, which typically bypass caching, and caching issues wouldn't cause silent processing failures without Lambda errors.