Cannot set reserve concurrency for a lambda function

Question

The `Unreserved account concurrency` of my account is  `100`

I have four lambda function in `eu-central-1` and I want to limit the concurrency of one of them to a very small value. Specifically, I do not want more than 3 instances of this function running concurrently. This function is not expected to receive a lot of traffic so setting its `Reserve concurrency` to a low value makes a lot of sense.

However, when I try to do I get a rejection message as follows: `The unreserved account concurrency can't go below 100.` (see attached screenshot).

What should I do to be able to restrict the concurrency of this function?

![Enter image description here](/media/postImages/original/IMSGVDNcZ5SKe6tU7TcbqxCw)

Answer

You have to leave a minimum of 100 for lambda functions in an account that don't have reserved concurrency - https://docs.aws.amazon.com/lambda/latest/dg/configuration-concurrency.html that mentions "You can reserve up to the Unreserved account concurrency value that is shown, minus 100 for functions that don't have reserved concurrency."

You have to request an increase in the account concurrency to a number higher than 100, so you can set reserved concurrency for certain lambda functions in your account.

Answer

Lambda provisioned concurrency can be reserved up to 100 less than the number of unreserved account concurrencies. [1]
 
[1] Managing Lambda reserved concurrency - AWS Lambda  
https://docs.aws.amazon.com/lambda/latest/dg/configuration-concurrency.html

Therefore, to set the provisioned concurrency to 3, the account concurrency must be greater than 103.

The concurrency limit can be requested via service quotas. [2]

[2] AWS service quotas - AWS General Reference  
https://docs.aws.amazon.com/general/latest/gr/aws_service_limits.html

Cannot set reserve concurrency for a lambda function

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