(RDS): deploy-time value can't be used for nr of instances in Cluster

[TLadd]

I'm attempting to store some configuration variables in parameter store for my RDS cluster but am running into this validation error about instance size needing to be at least 1. It's comparing the token's value, which is a big negative number.

Reproduction Steps

import * as rds from "@aws-cdk/aws-rds";
import * as ec2 from "@aws-cdk/aws-ec2";
import * as ssm from "@aws-cdk/aws-ssm";


class DbStack extends Stack {
    vpc: ec2.Vpc;

    constructor(scope: Construct, id: string, props?: StackProps) {
      this.vpc = new ec2.Vpc(this, "GrowMeVpc", {
        maxAzs: 3,
        enableDnsSupport: true,
        enableDnsHostnames: true,
      });

      const RDS_INSTANCES = Token.asNumber(getSsmStringParam("RDS_INSTANCES"));

        new rds.DatabaseCluster(this, "DbCluster", {
          engine: rds.DatabaseClusterEngine.auroraPostgres({
            version: rds.AuroraPostgresEngineVersion.of("12.4", "12", {
              s3Import: true,
              s3Export: true,
            }),
          }),
          defaultDatabaseName: "db",
          instances: RDS_INSTANCES,
          instanceProps: {
            instanceType: ec2.InstanceType.of(
             "t3",
             "medium"
            ),
            vpcSubnets: {
              subnetType: ec2.SubnetType.PRIVATE,
            },
            vpc: this.vpc,
          },
      });
  }
    

What did you expect to happen?

I would expect the validation to run after the token has resolved only

What actually happened?

Validation ran on the token's value itself

Environment

• CDK CLI Version : 1.92.0
• Framework Version: 1.92.0
• Node.js Version: 14.16.0
• OS : Max OS
• Language (Version): TypeScript 4.2.2

---

This is 🐛 Bug Report

[skinny85]

Hey @TLadd ,

thanks for opening the issue. Unfortunately, what you're trying to do is not possible. Since every instance is a separate resource that the DatabaseCluster Construct has to create, it's not possible to do it with a value only available at deploy time, like with the getSsmStringParam("RDS_INSTANCES") value.

I agree that the error message could be better here though! Keeping this open to fix that (PR are welcome, as always!).

BTW. We have a constant for the 12.4 version of the Aurora PostgreSQL engine defined here, so no need to use the of method in your code.

Thanks,
Adam

[BLasan]

@skinny85 Will work on this

[github-actions]

⚠️COMMENT VISIBILITY WARNING⚠️

Comments on closed issues are hard for our team to see.
If you need more assistance, please either tag a team member or open a new issue that references this one.
If you wish to keep having a conversation with other community members under this issue feel free to do so.