AWS lambda throttling retries

Question

I have a question about Lambda's asynchronous invocation: https://docs.aws.amazon.com/lambda/latest/dg/invocation-async.html

If the function doesn't have enough concurrency available to process all events, additional requests are throttled. For throttling errors (429) and system errors (500-series), Lambda returns the event to the queue and attempts to run the function again for up to 6 hours. The retry interval increases exponentially from 1 second after the first attempt to a maximum of 5 minutes. If the queue contains many entries, Lambda increases the retry interval and reduces the rate at which it reads events from the queue.

From the doc, it seems like if I set a reserved concurrency for a lambda function and it couldn't process events due to throttling, the event could be retried for up to 6 hours. However, it doesn't say anything about the total number of retries. How will it be different from the scenario where the lambda function returns an error (it will get retried a maximum of 2 times)?

Answer 1

If the queue contains many entries, Lambda increases the retry interval and reduces the rate at which it reads events from the queue.

It seems that lambda retry only if there are enouge concurrency for it. If not, It will wait up to 6 hours.