How to reference the the AWSEBV2LoadBalancer Subnets property?

Question

I'm following the AWS guide for deploying an HA Wordpress site to Elastic Beanstalk which includes using the eb-php-wordpress extension. The process requires editing a couple of configuration files with known resource IDs prior to deploying the application.

In particular, the instructions say to edit the efs-create.config file with a VPC ID, and Subnet IDs. The file, among other things, helps set the OptionSettings property of the AWS::ElasticBeanstalk::Environment resource. Thanks to danimal, I now know how to pull the VpcId into the file using Fn::GetAtt:

option_settings:
  aws:elasticbeanstalk:customoption:
    EFSVolumeName: "EB-EFS-Volume"
    VPCId: { "Fn::GetAtt" : [ "AWSEBSecurityGroup", "VpcId" ] }
  ## Subnet Options
    SubnetA: "subnet-XXXXXXXX"
    SubnetB: "subnet-XXXXXXXX"
    SubnetC: "subnet-XXXXXXXX"
    SubnetD: "subnet-XXXXXXXX"

Now my obstacle is getting those subnet IDs added. I tried applying the same, but slightly expanded, concept to the AWSEBV2LoadBalancer resource to pull the Subnets array, but I got an error.

The code:

SubnetA: { "Fn::Select" : [ "0", { "Fn::GetAtt" : [ "AWSEBV2LoadBalancer", "Subnets" ] } ] }
SubnetB: { "Fn::Select" : [ "1", { "Fn::GetAtt" : [ "AWSEBV2LoadBalancer", "Subnets" ] } ] }
SubnetC: { "Fn::Select" : [ "2", { "Fn::GetAtt" : [ "AWSEBV2LoadBalancer", "Subnets" ] } ] }
SubnetD: { "Fn::Select" : [ "3", { "Fn::GetAtt" : [ "AWSEBV2LoadBalancer", "Subnets" ] } ] }

The error:

 Service:AmazonCloudFormation, Message:Template error: resource AWSEBV2LoadBalancer does not support attribute type Subnets in Fn::GetAtt

I found this odd since the generated template shows Subnets under that resouce :

"AWSEBV2LoadBalancer": {
    "Properties": {
        "SecurityGroups": [
            {
                "Ref": "AWSEBLoadBalancerSecurityGroup"
            }
        ],
        "Subnets": [
            "subnet-0ec0699f08ff45e6e",
            "subnet-0a4fec611d42b062f",
            "subnet-09a4c28b8f330c0c3",
            "subnet-0ea8d69e46ce87afc"
        ]
    },
    "Type": "AWS::ElasticLoadBalancingV2::LoadBalancer"
},

I also tried assigning the array using Subnets: { "Fn::GetAtt" : [ "AWSEBV2LoadBalancer", "Subnets" ] and then using Fn::Select and Ref on that: SubnetA: { "Fn::Select" : [ "0", [ "Ref" : "Subnets" ] ] } but that created its own error:

Service:AmazonCloudFormation, Message:Template format error: Unresolved resource dependencies [Subnets] in the Resources block of the template

How do I extract that array and assign each element to a different resource?

Answer 1

Per the docs (per danimal), this is not possible. Leaves me wondering how others have done it as I can't imagine I'm the first to try.