How to import existing private subnets in a VPC in AWS CDK using TypeScript?

Question

I have few private subnets in my VPC and i'm trying to import them using typescript as shown below:

    vpc1 = ec2.Vpc.fromLookup(this, 'myVpc',{isDefault: false, vpcId:vpcId });
   
   // Iterate the private subnets
    const selection = vpc1.selectSubnets({
      subnetType: ec2.SubnetType.PRIVATE
    });
   
   for (const subnet of selection.subnets) {

   }

I managed to import the VPC. But, the private subnets are not listed.

How can this be done ?

try looking into `cdk.context.json` it should have pulled all the subnets of given vpc and the type to which they are categorized to! — Balu Vyamajala, Mar 02 '21 at 08:02
I added some details as an answer which might help debug the problem — Balu Vyamajala, Mar 02 '21 at 13:04

score 2 · Answer 1 · answered Mar 02 '21 at 13:00

When importing a vpc that is created outside CDK code to cdk, it will build a cdk.context.json file with vpc and subnet information and selectSubnets is merely extracting information from that vpc object that is built in that context file. Some documentation here.

vpc = ec2.Vpc.fromLookup(this, 'myVpc',{isDefault: false, vpcId:vpcId });

Subnet type is determined by CDK on multiple criteria

Presence of tag aws-cdk:subnet-type which will be there if vpc is created by CDK itself.
Based on presence of Internet Gateway (IGW), Nat Gateway or No Gateway. Ex: If there route to IGW, then it will be treated as PUBLIC.

We can observe what type cdk derived in cdk.context file.

If we don't like the subnet type by default or We need to specific subnets, cases where we have too many private subnets and we need import specefic one, we can always import them like this:

const subnet1 = ec2.Subnet.fromSubnetId(this, 'private-subnet-1', 'subnet-1234345');
const subnet2 = ec2.Subnet.fromSubnetId(this, 'private-subnet-2', 'subnet-456789');

Okay, thank you for the information. But, how do i import all the private subnets without specifying the subnet ID ? — Bharath Bharath, Mar 03 '21 at 06:30
Do you see your private subnets in `cdk.context.json` when you do cdk synth and if you see what is the type? — Balu Vyamajala, Mar 04 '21 at 03:50

score 0 · Answer 2 · answered Sep 21 '22 at 16:40

With cdk v2 you can get the private subnets this way :

vpc = ec2.Vpc.fromLookup(this, 'myVpc',{isDefault: false, vpcId:vpcId });
    const privateSubnets: ec2.ISubnet[] = [];
    for (const subnet of vpc.privateSubnets) {
      privateSubnets.push(ec2.Subnet.fromSubnetAttributes(scope, subnet.subnetId, { subnetId: subnet.subnetId }));
    }

How to import existing private subnets in a VPC in AWS CDK using TypeScript?

2 Answers2