Lambda Throttling Behaviour with SNS

Question

While reading the SNS FAQ part concerning the retrying behaviour with Lambda functions, I've encountered the following statement:

Q: What happens to Amazon SNS messages if the subscribing endpoint is not available? Lambda: If Lambda is not available, SNS will retry 2 times at 1 seconds apart, then 10 times exponentially backing off from 1 seconds to 20 minutes and finally 38 times every 20 minutes for a total 50 attempts over more than 13 hours before the message is discarded from SNS.

As far as I know, Lambda function implements its own retry mechanism for throttling, as mentioned in Lambda Throttling Bevaviour Documentation:

Asynchronous invocation: If your Lambda function is invoked asynchronously and is throttled, AWS Lambda automatically retries the throttled event for up to six hours, with delays between retries.

So what exactly happens when the function becomes throttled and another SNS message appears? Does SNS treat the Lambda as "available" and aborts retry mechanism, allowing Lambda to automatically retry, or does it keep retrying delivering the message?

Answer 1

The word "available" refers to the ability of SNS to contact the Lambda service and submit a single request to invoke the function.

The key to understanding this requires that you first know that SNS invokes Lambda functions asynchnously, and then that you understand the implications of that.

An asynchronous invocation request does not provide any feedback to the caller (SNS, in this case) whether the function ran immediately or was throttled, or whether it succeeded or threw an exception.

SNS >> Lambda: "Hi, run this Lambda function asynchronously, with this payload."
Lambda >> SNS: "Okay, I received your request and will do that as soon as it is possible. Goodbye."

The caller (SNS) is unconcerned with the details of what follows. Having successfully made the request, SNS is finished processing that message, and it is now up to the Lambda service to invoke the function immediately and/or engage in the documented Lambda retry behavior.

SNS only actually contacts the Lambda service once per message. When it can't do that, Lambda is not "available." This should happen very, very rarely... but if SNS can't make contact, that is when SNS engages in the behavior described in the SNS FAQ -- trying to submit the request to invoke the function. Once that has been accomplished, SNS's role is complete, and the rest is handled by the Lambda service.

Each message is handled independently across the SNS/Lambda integration, with SNS handing each message off as soon as possible, with no awareness on the part of SNS of whether function invocations are subsequently being throttled.