How to exclude AWS subnet ids based on name in Terraform

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I retrieve the subnet ids from the dev account to attach NLB to them afterwards. There are two subnets per each AZ. Therefore, I get the "Error creating network Load Balancer: InvalidConfigurationRequest: A load balancer cannot be attached to multiple subnets in the same Availability Zone". I need to filter on the subnet names to exclude certain characters ("-lb-") in the name and only use the ones without the "-lb-". I know Terraform can use for_each argument and toset() function but does it have something like contains or where clauses similar to LINQ?

data "aws_subnet_ids" "dev_subnets" {
  vpc_id = data.aws_vpc.dev_account_vpc.id
}

and

resource "aws_lb" "dev_network_load_balancer" {
  name               = "my-nlb"
  internal           = true
  load_balancer_type = "network"

  subnets            = data.aws_subnet_ids.dev_subnets.ids
  ...
}
terraform amazon-web-services
1 Answers

Assuming that dev_subnets provides a list of subnet ids from which some may contain -lb- substring, others may not. For example:

# Returns ["subnet-1", "subnet-lb-1", "subnet-2", "subnet-lb-2"]
data "aws_subnet_ids" "dev_subnets" {
  vpc_id = data.aws_vpc.dev_account_vpc.id
}

In order to filter this list we can do the following:

resource "aws_lb" "dev_network_load_balancer" {
  name               = "my-nlb"
  internal           = true
  load_balancer_type = "network"

  # ["subnet-1", "subnet-2"]
  subnets            = [for s in data.aws_subnet_ids.dev_subnets.ids : s if replace(s, "-lb-", "") == s]
  ...
}

There are no advanced filter functions in Terraform as of today. replace(s, "-lb-", "") will attempt to replace the -lb- with an empty string which is compared to the original string. If they are equal, it means the original string did not contain -lb- substring.

Update:

If you want to filter strings which do contain -lb- substring:

[for s in data.aws_subnet_ids.dev_subnets.ids : s if replace(s, "-lb-", "") != s]
user_76611190
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