RDS: Cannot set ServerlessCluster minCapacity to 0.5 ACU

[zmillman]

Describe the bug

The Aurora Serverless v2 docs say that you can set capacity to 0.5 ACUs:

The smallest Aurora Serverless v2 capacity that you can define is 0.5 ACUs. You can specify a higher number if it's less than or equal to the maximum capacity value. Setting the minimum capacity to a small number lets lightly loaded DB clusters consume minimal compute resources. At the same time, they stay ready to accept connections immediately and scale up when they become busy.

However, this can't be configured using the CDK node library (I'm using 2.140.0) because AuroraCapacityUnit is hardcoded to a fixed set of powers of 2 in aws-rds/lib/serverless-cluster.ts

Expected Behavior

I expected to be able to set minCapacity to 0.5

Current Behavior

Reproduction Steps

With aws-cdk-lib@2.140.0:

import {
  AuroraCapacityUnit,
  AuroraPostgresEngineVersion,
  DatabaseClusterEngine,
  ServerlessCluster,
} from "aws-cdk-lib/aws-rds";
import { Construct } from "constructs";

export class DB extends Construct {
  constructor(scope: Construct, id: string) {
    super(scope, id);

    new ServerlessCluster(this, "Database", {
      engine: DatabaseClusterEngine.auroraPostgres({
        version: AuroraPostgresEngineVersion.VER_13_12,
      }),
      scaling: {
        minCapacity: 0.5,
        maxCapacity: AuroraCapacityUnit.ACU_4,
      },
    });
  }
}

Possible Solution

Provide ACU_0_5 = 0.5 as an option for AuroraCapacityUnit

Additional Information/Context

No response

CDK CLI Version

2.140.0

Framework Version

No response

Node.js Version

v20.10.0

OS

MacOs

Language

TypeScript

Language Version

5.4.5

Other information

No response

[khushail]

Hi @zmillman , thanks for reporting this. I see that in the docs its mentioned one can give minCapacity as 0.5

but in the actual code, these are the authorized values -

aws-cdk/packages/aws-cdk-lib/aws-rds/lib/serverless-cluster.ts

Line 217 in dfea721

export enum AuroraCapacityUnit {

I reproduced the error with given code and it shows error while mentioning the value as 0.5.

[mazyu36]

Hi.

Using the ServerlessCluster class indicates that you are trying to use Serveless v1, is that correct?

If so, I think it is expected behavior that you cannot set the ACU to 0.5, as this is not supported in CloudFormation either.

If you want to use ServerlessV2, you should use the DatabaseCluster class instead. For this class, the ACU option is a number, so setting it to 0.5 would be possible.

[zmillman]

Oh! Switching to DatabaseCluster works for me.

I assumed that ServerlessCluster would start up a v2 cluster instead of a v1 cluster (there's nothing on that page to indicate which version it creates)

Perhaps someone could update the library docs so it's clearer that ServerlessCluster configures a v1 and DatabaseCluster handles v2?

[mazyu36]

I also had the same misunderstanding before. 😅
I agree it is unclear, so I submitted a PR to improve the documentation.

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