How do I troubleshoot Lambda reserved concurrency issues?

Last updated: 2022-10-26

I’ve configured reserved concurrency for my AWS Lambda function but the function isn't working. How do I fix this issue?

Resolution

The following are possible causes for the failure of your Lambda function after you set up reserved concurrency fails:

  • There isn’t sufficient concurrency left to reserve.
  • Reserved concurrency settings are leading to throttling behavior, or a 429 status code.

Follow these steps to troubleshoot issues with reserved concurrency:

1.    Confirm that you've configured reserved concurrency for your Lambda function. Check the setting using the Lambda console or by calling the GetFunction API.
Note: A Lambda function configured to have zero reserved concurrency is throttled because it can't process any events. Make sure that you increase the value to a number greater than zero.

2.    Check if the Lambda function is set up so that the function can’t scale, resulting in a 429 status code.
For more information, see How do I troubleshoot Lambda function throttling with "Rate exceeded" and 429 "TooManyRequestsException" errors?

3.    Review the Amazon CloudWatch metrics for ConcurrentExecutions with the Maximum statistic for your Lambda function. Check whether the ConcurrentExecutions metric is reaching the configured Reserved Concurrency value.
For more information about concurrency metrics, see Working with Lambda function metrics and review the Using concurrency metrics section.

4.    Increase the reserved concurrency for your Lambda function to a concurrency value that keeps the function from being throttled. Change the setting using the Lambda console, or by calling the PutFunctionConcurrency API.

5.    If you still can’t resolve the issue, open a case with AWS Support. Provide the following information in the case:

  • The Lambda function ARN.
  • The workflow on the Lambda function setup with all included services.
  • Details about whether the issue is intermittent or continuous.
  • The exact timestamp of the issue with the time zone or timestamp in UTC.

Note: AWS Support representatives don’t have access to customer Amazon CloudWatch logs due to security and privacy reasons.