AWS Lambda > what happens when reach concurrent limit

Question

Lambda has a 100 function limit.

What happens when you submit a 101 function when 100 are already running?

Will it:

• fail with an error
• queue up

Answer 1

If you are talking about Concurrent executions there isn't a limit of 100. The limit depends on the region but by default it's 1000 Concurrent executions .

To answer your question: As soon as the Concurrent executions limit is reached the next execution gets throttled. Each throttled invocation increases the Amazon CloudWatch Throttles metric for the function.

If your AWS Lambda is invoked asynchronous AWS Lambda automatically retries the throttled event for up to six hours, with delays between retries. If you didn't setup a Dead Letter Queue (DLQ) for your AWS Lambda your event is lost as soon as all retries fails.

For more information please check the AWS Lambda - Throttling Behavior

Answer 2

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. However, it might be longer if the queue is backed up. Lambda also reduces the rate at which it reads events from the queue.

As mentioned here .