Terraform引用带count的子网模块创建AKS集群时类型报错求助

问题场景 #

通过Terraform模块创建3个AKS子网（测试/预发布/生产），模块调用代码：

# AKS Subnet - Test / Staging / Production
module "aks_vpn_subnets" {
  source                  = "./modules/subnets"
  count                   = 3
  azure_subnet_name       = var.aks_azure_subnet_names[count.index]
  rg_name                 = module.rg.rg_name
  network_name            = module.network.name
  subnet_address_prefixes = [var.aks_subnet_address_prefixes[count.index]]
}

子网模块的main.tf和outputs.tf：

# ./modules/subnets/main.tf
resource "azurerm_subnet" "subnet" {
  name                 = var.azure_subnet_name
  resource_group_name  = var.rg_name
  virtual_network_name = var.network_name
  address_prefixes     = var.subnet_address_prefixes
}

# ./modules/subnets/outputs.tf
output "subnet_id" {
  description = "ID of the subnet"
  value = azurerm_subnet.subnet.*.id
}

尝试创建1个AKS集群并引用第一个子网，代码：

# AKS cluster only create in the first subnet created?
module "aks_cluster" {
  source = "./modules/aks"

  count = 1

  . . . 

  vnet_subnet_id    = module.aks_vpn_subnets[count.index].subnet_id
  
  . . .

  depends_on = [
    module.aks_vpn_subnets
  ]
}

出现错误：

The given value is not suitable for module.aks_cluster[0].var.vnet_subnet_id declared at
│ modules/aks/variables.tf:47,1-26: string required.

AKS模块的变量定义：

variable "vnet_subnet_id" {
  type        = string
  description = "The ID of a Subnet where the Kubernetes Node Pool should exist"
}

错误原因 #

• 子网模块的subnet_id输出使用了azurerm_subnet.subnet.*.id语法，这会返回列表类型（即使每个模块实例只创建1个子网，*会把单个资源的属性包装成列表）
• AKS模块的vnet_subnet_id变量要求的是字符串类型，类型不匹配导致报错

解决方案 #

1. 修正子网模块的输出 #

修改./modules/subnets/outputs.tf，去掉*语法，直接返回单个子网ID的字符串：

output "subnet_id" {
  description = "ID of the subnet"
  value = azurerm_subnet.subnet.id
}

2. 修正AKS模块的子网引用 #

因为module.aks_vpn_subnets是包含3个实例的列表，直接通过索引[0]引用第一个子网的ID即可。如果只创建1个AKS集群，也可以去掉count = 1让代码更简洁：

保留count的写法

module "aks_cluster" {
  source = "./modules/aks"

  count = 1

  . . . 

  vnet_subnet_id    = module.aks_vpn_subnets[0].subnet_id
  
  . . .

  depends_on = [
    module.aks_vpn_subnets
  ]
}

去掉count的简洁写法

module "aks_cluster" {
  source = "./modules/aks"

  . . . 

  vnet_subnet_id    = module.aks_vpn_subnets[0].subnet_id
  
  . . .

  depends_on = [
    module.aks_vpn_subnets
  ]
}

内容的提问来源于stack exchange，提问作者Jananath Banuka