(vpc.selectSubnets): Cannot find a subnet by subnetGroupName in a lookup VPC

Unable to get subnets by their group name in a lookup VPC

Reproduction Steps

I have created a vpc in my infrastructure repo and is referenced in another repo. When I try to get subnets with group name rds-private-subnet, I get following error.

Error: There are no subnet groups with name 'rds-private-subnet' in this VPC. Available names: public-subnet,private-subnet
    at LookedUpVpc.selectSubnetObjectsByName (/Users/user/projects/auth/node_modules/@aws-cdk/aws-ec2/lib/vpc.ts:343:13)

Below is the code snippet that caused it.

// @ infrastructure/stack.ts
   const privateSubnectConfiguration = {
      cidrMask: 24,
      name: "private-subnet",
      subnetType: ec2.SubnetType.PRIVATE,
    };

    const publicSubnectConfiguration = {
      cidrMask: 24,
      name: "public-subnet",
      subnetType: ec2.SubnetType.PUBLIC,
    };

    const rdsPrivateSubnetConfiguration = {
      cidrMask: 24,
      name: "rds-private-subnet",
      subnetType: ec2.SubnetType.PRIVATE,
    };

const vpc = new ec2.Vpc(this, "test-vpc", {
      cidr: "10.0.0.0/16",
      subnetConfiguration: [
        publicSubnectConfiguration,
        privateSubnectConfiguration,
        rdsPrivateSubnetConfiguration,
      ],
      maxAzs: 2,
      natGateways: 1,
      natGatewayProvider,
    });

// @ auth/stack.ts
const vpc = ec2.Vpc.fromLookup(this, "vpc", {
      vpcId: <vpc-id>
 });

const rdsSubnetGroup = new rds.SubnetGroup(this, "RDS subnet group", {
      description: "RDS subnet group",
      vpc: vpc,
      vpcSubnets: vpc.selectSubnets({
        subnetGroupName: "rds-private-subnet",
      }),
    });

What did you expect to happen?

I expected to get the subnets with group name rds-private-subnet.

What actually happened?

cdk threw an error saying the subnets with group name rds-private-subnet does not exist.

Error: There are no subnet groups with name 'rds-private-subnet' in this VPC. Available names: public-subnet,private-subnet
    at LookedUpVpc.selectSubnetObjectsByName (/Users/user/projects/auth/node_modules/@aws-cdk/aws-ec2/lib/vpc.ts:343:13)

Environment

• **CDK CLI Version :1.112.0
• Framework Version:
• **Node.js Version:15.3.0
• **OS :macos
• **Language (Version):~3.9.7

Other

---

This is 🐛 Bug Report

[peterwoodworth]

Hey @ludbek,

I was able to reproduce this error if I:

• First lookup the VPC before configuring the RDS private subnet
  • this will cache public-subnet and private-subnet in your cdk.context.json
• Then add the RDS subnet to the VPC and deploy
• Add the RDS subnet group to the stack with the looked up VPC. Your cdk.context.json already has the data cached for your VPC from when we first looked up the VPC, so it will use this cached data instead of actually getting data.

To fix this, you can simply delete your cdk.context.json (this will update it the next time you run the stack) or you can manage your cached data

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