Why Are Duplicate Email Messages Sent During High Traffic with SQS, Lambda, and SES?
A developer is investigating an issue in part of a company's application. In the application, messages are sent to an Amazon Simple Queue Service (Amazon SQS) queue. The AWS Lambda function polls messages from the SQS queue and sends email messages by using Amazon Simple Email Service (Amazon SES). Users have been receiving duplicate email messages during periods of high traffic. Which reasons could explain the duplicate email messages? (Choose two.)
Community Votes
100% of anonymous learners picked answer AD. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
The question tests understanding of SQS delivery semantics and how visibility timeout impacts message processing, with the trap being to confuse standard SQS with FIFO queues or to overlook Lambda's polling behavior.
Duplicate emails in an SQS-Lambda-SES pipeline are typically caused by standard SQS at-least-once delivery combined with a visibility timeout that is too short relative to the Lambda timeout. Community consensus confirms options A and D are correct.
Choosing only A (standard SQS at-least-once delivery) or incorrectly assuming SQS provides exactly-once processing, while missing that a low visibility timeout can cause the same message to be processed again by Lambda, leading to duplicates.
Community Discussion (4 comments)
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Expert Analysis
Why the Answer Is Correct
Standard SQS queues provide at-least-once delivery, meaning a message can be delivered more than once, especially during high traffic. Additionally, if the SQS visibility timeout is lower than or equal to the Lambda function's timeout, Lambda may not delete the message before the visibility timeout expires, causing another poller to receive and process the same message again. Together, these two factors explain duplicate email messages.Why the Other Options Are Wrong
Option B is wrong because Standard SQS queues do not support exactly-once processing; FIFO queues provide exactly-once but are not mentioned. Option C is wrong because DKIM misconfiguration affects email authentication, not duplication. Option E is wrong because a high bounce rate is a delivery outcome, not a cause of duplicate messages.Community Comment Notes
All commenters unanimously chose AD. One comment referenced the AWS Lambda documentation for SQS, which confirms that Lambda polls the queue and that a visibility timeout shorter than the function timeout can cause duplicates. Commenters did not debate other options, reinforcing that AD is the clear consensus.Official Reference
Exam Strategy
For SQS-related questions, always consider both delivery semantics (at-least-once vs exactly-once) and the relationship between visibility timeout and function timeout. If the scenario involves duplicates, check for FIFO queue usage and whether the visibility timeout might be too short, as these are the most common causes tested.
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