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April 4, 2022 03:47 pm GMT

Terraform - Filter results using 'for' loops

Overview

In todays tutorial we will take a look at a fairly common question I often get from the community and it is around how to filter results in Terraform or even if it is possible. We will also look at a real world usage example so that we can see how and when we would use filters in Terraform.

Filtering in Terraform can be achieved using for loop expressions. Though for loop constructs in terraform performs looping, it can also be used for manipulating data structures such as the following to name a few:

Transform: Changing the data structure.
Filter: Filter only on desired items in combination with an if expression.
Group: Group elements together in a new list by key.

Filtering results

Let's take a look at the following example variable where we have a list of applications:

variable "apps" {  type = list(object({    app_name            = string    app_kind            = string    app_require_feature = bool  }))  default = [    {      app_name            = "App1"      app_kind            = "Linux"      app_require_feature = false    },    {      app_name            = "App2"      app_kind            = "Linux"      app_require_feature = false    },    {      app_name            = "App3"      app_kind            = "Windows"      app_require_feature = true    },    {      app_name            = "App4"      app_kind            = "Windows"      app_require_feature = false    }  ]}

Say you want to filter only on app_require_feature = true you could write a for loop with an if expression like in the following local variable:

locals {  apps_that_require_feature = toset([for each in var.apps : each.app_name if each.app_require_feature == true])}output "result" {  value = local.apps_that_require_feature}

This will return a set of app_names that have the objects key "app_require_feature" set to true

$ terraform applyOutputs:result = ["App3"]

So let's say you want to filter the same variable but this time you want to only see the apps that are windows, you could write a for loop with an if expression like in the following local variable:

locals {  windows_apps = toset([for each in var.apps : each.app_name if each.app_kind == "windows"])}output "result2" {  value = local.windows_apps}

This will return a set of app_names that have the objects key "app_kind" set to "windows"

$ terraform applyOutputs:result2 = ["App3", "App4"]

Real world example

Let's take a real world usage case where we would need such a for construct to filter and only configure something based on certain criteria.

Say we have a variable with three storage accounts we want to create, but we only want to configure private endpoints on certain storage accounts. We could create an extra object key item called requires_private_endpoint like in the following example:

## variables ##variable "storage_config" {  type = list(object({    name                      = string    account_kind              = string    account_tier              = string    account_replication_type  = string    access_tier               = string    enable_https_traffic_only = bool    is_hns_enabled            = bool    requires_private_endpoint = bool  }))  default = [    #V2 Storage without private endpoint    {      name                      = "pwd9000v2sa001"      account_kind              = "StorageV2"      account_tier              = "Standard"      account_replication_type  = "LRS"      enable_https_traffic_only = true      access_tier               = "Hot"      is_hns_enabled            = false      requires_private_endpoint = false    },    #V2 Storage with private endpoint    {      name                      = "pwd9000v2sa002"      account_kind              = "StorageV2"      account_tier              = "Standard"      account_replication_type  = "LRS"      enable_https_traffic_only = true      access_tier               = "Cool"      is_hns_enabled            = false      requires_private_endpoint = true    },    #ADLS2 Storage with private endpoint    {      name                      = "pwd9000adls2sa001"      account_kind              = "BlockBlobStorage"      account_tier              = "Premium"      account_replication_type  = "ZRS"      enable_https_traffic_only = false      access_tier               = "Hot"      is_hns_enabled            = true      requires_private_endpoint = true    }  ]}

We can then create all three storage accounts with the following resource config:

## storage resources ##resource "azurerm_resource_group" "RG" {  name     = "example-resources"  location = "uksouth"}resource "azurerm_storage_account" "SAS" {  for_each = { for n in var.storage_config : n.name => n }  #Implicit dependency from previous resource  resource_group_name = azurerm_resource_group.RG.name  location            = azurerm_resource_group.RG.location  #values from variable storage_config objects  name                      = each.value.name  account_kind              = each.value.account_kind  account_tier              = each.value.account_tier  account_replication_type  = each.value.account_replication_type  access_tier               = each.value.access_tier  enable_https_traffic_only = each.value.enable_https_traffic_only  is_hns_enabled            = each.value.is_hns_enabled}

In the following resource block we can now configure private endpoints, but we will only do so for storage accounts that have an object key of requires_private_endpoint set to true like in the following resource config:

## private endpoint resources ##resource "azurerm_private_endpoint" "SASPE" {  for_each            = toset([for pe in var.storage_config : pe.name if pe.requires_private_endpoint == true])  name                = "${each.value.name}-pe"  location            = azurerm_resource_group.RG.location  resource_group_name = azurerm_resource_group.RG.name  subnet_id           = data.azurerm_subnet.data_subnet.id  private_service_connection {    name                           = "${each.value.name}-pe-sc"    private_connection_resource_id = azurerm_storage_account.SAS[each.value.name].id    is_manual_connection           = false  }

If you take a closer look at the for_each in the azurerm_private_endpoint resource we are using the filter there as follow:

for_each = toset([for pe in var.storage_config : pe.name if pe.requires_private_endpoint == true])

This for loop will filter and return a set of storage account names we can use to loop the resource creation of the private endpoints on the selected storage accounts.

Thus we can then use selected list(object)/storage config key that represents the storage account name as shown by the each.value.name to specify the values used for each storage account to create private endpoints for the selected storage accounts only that matches the filter: requires_private_endpoint == true.

I hope you have enjoyed this post and have learned something new.

Author

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Marcel.LFollow

Microsoft DevOps MVP | Cloud Solutions & DevOps Architect | Technical speaker focussed on Microsoft technologies, IaC and automation in Azure. Find me on GitHub: https://github.com/Pwd9000-ML

Original Link: https://dev.to/pwd9000/terraform-filter-results-using-for-loops-4n75

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