[Enhancement] Add ability to provide a list of Availability Zones for subnet placement

data.tf

@@ -1,5 +1,6 @@
 locals {
-  azs = length(var.azs) == 0 ? slice(data.aws_availability_zones.current.names, 0, var.az_count) : var.azs
+  # az_count has been provided slice based on az_count otherwise return those provided
+  azs = var.az_count != null ? slice(data.aws_availability_zones.current.names, 0, var.az_count) : data.aws_availability_zones.current.names
 
   # references to module.calculate_subnets output
   calculated_subnets       = module.calculate_subnets.subnets_by_type
@@ -123,6 +124,34 @@ data "aws_availability_zones" "current" {
     name   = "opt-in-status"
     values = ["opt-in-not-required"]
   }
+
+  dynamic "filter" {
+    for_each = var.azs != null ? [1] : []
+    content {
+      name   = "zone-name"
+      values = var.azs
+    }
+  }
+
+  lifecycle {
+    # Check at least one variable `var.azs` or `var.az_count` has been provided
+    precondition {
+      condition     = (var.azs != null && var.az_count != null) ? false : true
+      error_message = "Both `var.azs` and `var.az_count` provided. You must provide a value for `var.azs` or `var.az_count` but not both."
+    }
+
+    # Check that only `var.azs` or `var.az_count` was provided (and not both)
+    precondition {
+      condition     = (var.azs == null && var.az_count == null) ? false : true
+      error_message = "You must provide a value for the variable `var.az_count` or `var.azs`."
+    }
+
+    # Check if `var.azs` was provided that the filter returns the correct number of AZs (i.e. all exist in the current partition/region)
+    postcondition {
+      condition     = (var.azs == null) ? true : (length(self.names) == length(var.azs) ? true : false)
+      error_message = "One or more of the Availability Zones provided in `var.azs` does not exist. Please check the availability zones are available in the current region."
+    }
+  }
 }
 
 # search for existing vpc with var.vpc_id if not creating

examples/provide_specific_azs/.header.md

@@ -0,0 +1,6 @@
+# Deploy VPC subnets to a specific set of Availability Zones
+
+This example shows how you can use this module to deploy a VPC defining a list of specific Availability Zones where subnets should be deployed. This example creates the following:
+
+* VPC with IPv4 CIDR block named "specific-azs"
+* Private subnets deployed in each of the Availability Zones defined in the input variable `azs` with the name prefix "vpc-specific-azs-private" and a /18 netmask

examples/provide_specific_azs/README.md

@@ -0,0 +1,37 @@
+<!-- BEGIN_TF_DOCS -->
+# Deploy VPC subnets to a specific set of Availability Zones
+
+This example shows how you can use this module to deploy a VPC defining a list of specific Availability Zones where subnets should be deployed. This example creates the following:
+
+* VPC with IPv4 CIDR block named "specific-azs"
+* Private subnets deployed in each of the Availability Zones defined in the input variable `azs` with the name prefix "vpc-specific-azs-private" and a /18 netmask
+
+## Requirements
+
+No requirements.
+
+## Providers
+
+No providers.
+
+## Modules
+
+| Name | Source | Version |
+|------|--------|---------|
+| <a name="module_vpc_specific_azs"></a> [vpc\_specific\_azs](#module\_vpc\_specific\_azs) | ../.. | n/a |
+
+## Resources
+
+No resources.
+
+## Inputs
+
+| Name | Description | Type | Default | Required |
+|------|-------------|------|---------|:--------:|
+| <a name="input_azs"></a> [azs](#input\_azs) | A list of AZs to use. e.g. `azs = ["us-east-1a","us-east-1c"]` Incompatible with `az_count` | `list(string)` | <pre>[<br>  "us-east-1a",<br>  "us-east-1c"<br>]</pre> | no |
+| <a name="input_cidr_block"></a> [cidr\_block](#input\_cidr\_block) | IPv4 CIDR range to assign to VPC if creating VPC or to associate as a secondary IPv6 CIDR. Overridden by var.vpc\_id output from data.aws\_vpc. | `string` | `"10.0.0.0/16"` | no |
+
+## Outputs
+
+No outputs.
+<!-- END_TF_DOCS -->

examples/provide_specific_azs/main.tf

@@ -0,0 +1,17 @@
+module "vpc_specific_azs" {
+  # source  = "aws-ia/vpc/aws"
+  # version = ">= 4.3.1"
+  source = "../.."
+
+  name       = "specific-azs"
+  cidr_block = var.cidr_block
+  azs        = var.azs
+
+
+  subnets = {
+    private = {
+      name_prefix = "vpc-specific-azs-private"
+      netmask     = 18
+    }
+  }
+}

examples/provide_specific_azs/variables.tf

@@ -0,0 +1,15 @@
+variable "azs" {
+  type        = list(string)
+  description = "A list of AZs to use. e.g. `azs = [\"us-east-1a\",\"us-east-1c\"]` Incompatible with `az_count`"
+  default = [
+    "us-east-1a",
+    "us-east-1c",
+  ]
+}
+
+variable "cidr_block" {
+  type        = string
+  description = "IPv4 CIDR range to assign to VPC if creating VPC or to associate as a secondary IPv6 CIDR. Overridden by var.vpc_id output from data.aws_vpc."
+  default     = "10.0.0.0/16"
+}
+

test/examples_provide_specific_azs_test.go

@@ -0,0 +1,17 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/gruntwork-io/terratest/modules/terraform"
+)
+
+func TestExamplesSpecificAzs(t *testing.T) {
+
+	terraformOptions := &terraform.Options{
+		TerraformDir: "../examples/provide_specific_azs",
+	}
+
+	defer terraform.Destroy(t, terraformOptions)
+	terraform.InitAndApply(t, terraformOptions)
+}

variables.tf

@@ -23,8 +23,14 @@ variable "create_vpc" {
 
 variable "az_count" {
   type        = number
-  default     = 0
-  description = "Searches region for # of AZs to use and takes a slice based on count. Assume slice is sorted a-z."
+  description = "Searches region for # of AZs to use and takes a slice based on count. Assume slice is sorted a-z. Required if `azs` is not provided."
+  default     = null
+}
+
+variable "azs" {
+  type        = list(string)
+  description = "(Optional) A list of AZs to use. e.g. `azs = [\"us-east-1a\",\"us-east-1c\"]` Incompatible with `az_count`"
+  default     = null
 }
 
 variable "azs" {