How to get subnet IP for specific AZ with CDKTF

Terraform CDK for Terraform
1

I have a subnet declared as follows which creates 4 subnets, one in each of the 4 AZs in us-west-2 in AWS

    const largePublicSubnets = new aws.vpc.Subnet(this, "large_pub_subnet", {
      availabilityZone,
      cidrBlock:
        "${cidrsubnet(aws_vpc.vpc.cidr_block, 4, count.index + 2 * length(data.aws_availability_zones.all.names))}",
      mapPublicIpOnLaunch: true,
      tags: {
        name: environment + "-large-public-subnet-${(count.index + 1)}",
        ["kubernetes.io/cluster/" + eksClusterName]: "shared",
        "kubernetes.io/role/elb": "1",
        environment: environment,
        public: "true",
      },
      vpcId: "${aws_vpc.vpc.id}",
    });
    largePublicSubnets.addOverride("count", Fn.lengthOf(zoneNames));

In addition to this, I have a nodegroup declared as follows, which uses all 4 of the above subnets:

    const firehoseNodeGroup = new aws.eks.EksNodeGroup(
      this,
      "firehoseNodeGroup",
      {
        clusterName: eksCluster.name,
        nodeGroupName: `eksNodeGroup-${environment}-firehose`,
        nodeRoleArn: nodeGroupRole.arn,
        subnetIds: [],
        instanceTypes: ["m5.4xlarge"],
        labels: {
          app: "firehose",
        },
        scalingConfig: {
          minSize: 0,
          maxSize: 4,
          desiredSize: 1,
        },
        dependsOn: eksNodeGroupRoleAttachments,
        tags: {
          Name: "firehose_eks_node",
        },
        lifecycle: {
          ignoreChanges: ["scaling_config[0].desired_size"],
        },
        taint: [
          {
            key: "app",
            value: "firehose",
            effect: "NO_SCHEDULE",
          },
        ],
      }
    );
    firehoseNodeGroup.addOverride(
      "subnet_ids",
      "${ [ aws_subnet.large_pub_subnet[0].id, aws_subnet.large_pub_subnet[1].id, aws_subnet.large_pub_subnet[2].id, aws_subnet.large_pub_subnet[3].id ]}"
    );

I’d like to restrict my node group to only run in one specific AZ, us-west-2c, but I’m not sure how to correctly determine which subnet corresponds to that AZ. How can I do that? I’m open to redesigning my subnet declaration if needed since the current structure is some tech debt we’ve accumulated.

2

If you are open to restructuring, the simplest code would be to not use availability zones data source and instead just have them as a configuration. That way you would be able to simply create them in a way that you know which one is which.

You could probably chain a couple terraform functions together to filter the list down to just the one subnet you are looking for. I don’t have a working example of that though.

3

Got it. Yeah I figured manually making subnets for each AZ instead of using iterators and such with terraform would end up being easier. Thanks for the confirmation!