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

Question

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?

Answer 1

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.

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Update: Lambda now supports partial batch failure for SQS whereby the Lambda function can return a list of failed messages and only those will become visible again.

Answer 2

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.