Serverless RDS MySQL will not scale to 0

Question

I am experimenting with a serverless RDS instance. It has the following configuration:

* a minimum Aurora capacity unit (ACU) of 1 (there is no option for 0)
* autoscaling timeout of 2 minutes
* "scale to 0 ACUs when cluster is idle" is checked
* amount of time that the cluster can be idle before scaling is 15 minutes

However, over the past 3 hours, with zero connections to the database, the "serverless database capacity (count)" graph remains at 1. It has brief moments when it dips to 0, but it does not stay at 0, despite no active connections for hours. How do I ensure that the database can scale to 0?

Accepted Answer

Hi there.

From [Serverless documentation](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless.how-it-works.html#aurora-serverless.how-it-works.pause-resume), if any "*database connections are* ***requested*** *when an Aurora Serverless DB cluster is paused, the DB cluster automatically resumes and services the connection requests*", so I'd recommend checking not only that the **DatabaseConnections** CW metric is zero (which indicates no persistent connections), but also that the **ConnectionAttempts** CW metric is zero for longer than the SecondsUntilAutoPause value (15 minutes in your setup). Any connection request/attempt within the SecondsUntilAutoPause time frame resets the AutoPause counter.

If ConnectionAttempts is greater than zero at any point in time and you're not sure where it comes from, I'd then recommend [enabling audit for the CONNECT event on Aurora Advanced Audit settings for your cluster](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless.how-it-works.html#aurora-serverless.logging), and looking into [the logged events in the CW Logs log group](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraMySQL.Integrating.CloudWatch.html#AuroraMySQL.Integrating.CloudWatch.Monitor) when it happens again after enabling audit.

I hope this helps. If you'd like or need a more detailed investigation into your cluster, I'd recommend [opening a technical case with AWS Support](https://console.aws.amazon.com/support/home), as they'll be able to work with you through it, step by step.

Answer

My recent billing statement for December 2021 shows that RDS is not scaling down at all over the course of the month:

```
Amazon Relational Database Service for Aurora MySQL $44.42
$0.06 per Aurora Capacity Unit hour running Amazon Aurora MySQL Serverless740.341 ACU-Hr $44.42
USD 0.021 per GB-month of backup storage exceeding free allocation for Aurora MySQL0.169 GB-Mo $0.00
```

Answer

Hello!

Good question.  You're right on the configuration.  You cannot have the minimum capacity set to less than 1.

Check out the option to "Pause compute capacity after consecutive minutes of inactivity".  Default is 5 minutes, but you can configure this to your needs: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless.how-it-works.html#aurora-serverless.how-it-works.pause-resume.

For example, if you using CloudFormation - make sure the AutoPause is set to **true**.

```

"ScalingConfiguration" : {
                  "AutoPause" : true,
                  "MinCapacity" : 4,
                  "MaxCapacity" : 32,
                  "SecondsUntilAutoPause" : 1000
}
```

Another thing to pay attention to: if autoscaling doesn't find a scaling point before timing out, Aurora will keep the cluster at the current capacity.  To change this behavior, you can select the **Force the capacity change** option as well.  Check that at the ForceApplyCapacityChange property.  More information here: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless.how-it-works.html#aurora-serverless.how-it-works.timeout-action

Serverless RDS MySQL will not scale to 0

Relevant content