"subnet_mapping": only one of subnet_mapping,subnets can be specified, but subnet_mapping,subnets were specified.

[andyli]

Description

AWS provider since v5.31.0, validates that exactly one of either subnets or subnet_mapping is configured.

Currently, the terraform-aws-alb module uses [] as the default of subnets. It means module users who want to use subnet_mapping have to set subnets to null in order to avoid triggering validation error:

"subnet_mapping": only one of `subnet_mapping,subnets` can be specified, but `subnet_mapping,subnets` were specified.

• ✋ I have searched the open/closed issues and my issue is not listed.

Versions

• Module version [Required]:
  9.4.0
• Terraform version:
  v1.5.7
• Provider version(s):
  provider registry.terraform.io/hashicorp/aws v5.31.0

Reproduction Code [Required]

terraform {
  required_version = "~> 1.0"
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "= 5.31.0"
    }
  }
}

data "aws_availability_zones" "zones" {}

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "5.4.0"

  name = "alb-test"
  cidr = "10.0.0.0/16"

  azs             = data.aws_availability_zones.zones.names
  public_subnets  = ["10.0.101.0/24", "10.0.102.0/24", "10.0.103.0/24"]
  private_subnets = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]

  enable_nat_gateway = true
  single_nat_gateway = true
}

resource "aws_eip" "lb" {
  for_each = toset(data.aws_availability_zones.zones.names)

  domain = "vpc"
  tags = {
    Name = "alb-test-${each.key}"
  }
}

module "aws-load-balancer" {
  source  = "terraform-aws-modules/alb/aws"
  version = "9.4.0"

  name = "alb-test"

  load_balancer_type               = "network"
  enable_cross_zone_load_balancing = true
  vpc_id                           = module.vpc.vpc_id

  subnet_mapping = [
    for i, az in data.aws_availability_zones.zones.names:
    {
      subnet_id     = module.vpc.public_subnets[i]
      allocation_id = aws_eip.lb[az].id
    }
  ]

  enable_deletion_protection = false
}

Expected behavior

terraform apply successfully creates the load balancer.

Actual behavior

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: Invalid combination of arguments
│ 
│   with module.aws-load-balancer.aws_lb.this[0],
│   on .terraform/modules/aws-load-balancer/main.tf line 12, in resource "aws_lb" "this":
│   12: resource "aws_lb" "this" {
│ 
│ "subnet_mapping": only one of `subnet_mapping,subnets` can be specified, but `subnet_mapping,subnets` were specified.
╵
╷
│ Error: Invalid combination of arguments
│ 
│   with module.aws-load-balancer.aws_lb.this[0],
│   on .terraform/modules/aws-load-balancer/main.tf line 65, in resource "aws_lb" "this":
│   65:   subnets                    = var.subnets
│ 
│ "subnets": only one of `subnet_mapping,subnets` can be specified, but `subnet_mapping,subnets` were specified.

Additional context

A workaround is to add subnets = null when using the module. Alternatively, use AWS provider < 5.31 (with terraform-aws-alb <= 9.2.0, because the later versions depends on AWS provider >= 5.31).

[antonbabenko]

This issue has been resolved in version 9.4.1 🎉

[github-actions]

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