Is it necessary for a Lambda to delete messages from an SQS queue after processing?

2
votes

I'm looking at the the AWS SQS documentation here: https://docs.aws.amazon.com/sdk-for-net/v3/developer-guide/ReceiveMessage.html#receive-sqs-message

My understanding is that we need to delete the message using AmazonSQSClient.DeleteMessage() once we're done processing it, but is this necessary when we're working with an SQS triggered Lambda?

I'm testing with a Lambda function that's triggered by an SQSEvent, and unless I'm mistaken, it appears that if the Lambda function runs to completion without throwing any errors, the message does NOT return to the SQS queue. If this is true, the I would rather avoid making that unnecessary call to AmazonSQSClient.DeleteMessage().

Here is a similar question from 2019 with the top answer saying that the SDK does not delete messages automatically and that they need to be explicitly deleted within the code. I'm wondering if anything has changed since then.

Thoughts?

3
amazon-web-servicesaws-lambdaaws-sdkamazon-sqs
Minor note on terminology. In the case of the Lambda function failing to process the SQS messages, those messages aren't returned to the SQS queue. Instead, they become visible again (once their visibility timeout occurs). Those messages never left the SQS queue. - jarmod

3 Answers

7
votes

The key here is that you are using the AWS Lambda integration with SQS. In that instance AWS Lambda handles retrieving the messages from the queue (making them available via the event object), and automatically deletes the message from the queue for you if the Lambda function returns a success status. It will not delete the message from the queue if the Lambda function throws an error.

When using AWS Lambda integration with SQS you should not be using the AWS SDK to interact with the SQS queue at all.

1
votes

Amazon SQS doesn't automatically delete a message after retrieving it for you, in case you don't successfully receive the message (for example, if the consumers fail or you lose connectivity). To delete a message, you must send a separate request which acknowledges that you've successfully received and processed the message.

This has not changed and likely won’t change in the future as there us no way for SQS to definitively know in all cases if messages have successfully been processed. If SQS started to “assume” what happens downstream it risk becoming unreliable in many scenarios.

1
votes

Yes, otherwise the next time you ask for a set of messages, you will get the same messages back - maybe not on the next call, but eventually you will. You likely don't want to keep processing the same set of messages over and over.