[dms] Cfn lowercases the replicationSubnetGroupIdentifier on creation but CDK passes the same string without lowercasing.

While using Cfn level constructs to generate a DMS solution, we had an issue with the replicationSubnetGroupIdentifier being implicitly lower-cased by CloudFormation but then when we referenced it elsewhere, CDK did not also lower-case the value.

Reproduction Steps

Set your stackName to something with mixed case.

    this.subnetGroup = new dms.CfnReplicationSubnetGroup(this, 'DmsSubnetGroup', {
      // If you might be mixing cases, you're going to want to force this to lowercase...
      replicationSubnetGroupIdentifier: `${scope.stackName}-subnet-group`,
      replicationSubnetGroupDescription: `${scope.stackName} Subnet Group`,
      subnetIds: scope.vpc.privateSubnets.map((s): string => {
        return s.subnetId;
      }),
    });
    new cdk.CfnOutput(this, 'TheDmsSubnetGroupArn', { value: this.subnetGroup.ref });

    this.instance = new dms.CfnReplicationInstance(this, 'Instance', {
      replicationInstanceIdentifier: `${scope.stackName}-lims-instance`,
      replicationInstanceClass: 'dms.t2.micro', // WRITEME: do we need this configurable?
      kmsKeyId: this.kmsKey.keyId,
      // Cfn will .toLowerCase this, and CDK won't do that for you.
      replicationSubnetGroupIdentifier: this.subnetGroup.replicationSubnetGroupIdentifier,
      vpcSecurityGroupIds: [sg.securityGroupId],
      publiclyAccessible: false,
    });
    this.instance.addDependsOn(this.subnetGroup);

What did you expect to happen?

I expected replicationSubnetGroupIdentifier: this.subnetGroup.replicationSubnetGroupIdentifier to provide a working reference.

What actually happened?

The SubnetGroup resource is created with all lower-case names. The Instance resource is passed a replicationSubnetGroupIdentifier with mixed case. The Instance resource then fails to create.

Environment

• CDK CLI Version : 1.66.0
• Framework Version: 1.66.0
• Node.js Version: v14.3.0
• OS: Darwin andrew-hammond-C02XKGFMJG5H-SM 19.6.0 Darwin Kernel Version 19.6.0: Mon Aug 31 22:12:52 PDT 2020; root:xnu-6153.141.2~1/RELEASE_X86_64 x86_64
• Language (Version): TypeScript (4.0.3)

---

This is 🐛 Bug Report

[NetaNir]

What you could do is use the subnetGroup Ref:

    this.instance = new dms.CfnReplicationInstance(this, 'Instance', {
      replicationInstanceIdentifier: `${scope.stackName}-lims-instance`,
      replicationInstanceClass: 'dms.t2.micro',
      // Cfn will .toLowerCase this, and CDK won't do that for you.
      replicationSubnetGroupIdentifier: this.subnetGroup.ref,
      vpcSecurityGroupIds: [sg.securityGroupId],
      publiclyAccessible: false,
    });

The Ref is a value resolved by CloudFormation at deploy time, from DMS docs:

When you pass the logical ID of this resource to the intrinsic Ref function, Ref returns the name of the replication subnet group, such as mystack-myrepsubnetgroup-0a12bc456789de0fg.

Using Ref will also implicitly create the dependency between the subnetGroup and the instance, so you remove the line that adds it explicitly:

this.instance.addDependsOn(this.subnetGroup);

[github-actions[bot]]

⚠️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.