Terraform Iterators

Comparing Itertors: for, for_each and count

Feature	for	for_each	count
Purpose	Used to transform and filter collections (lists, sets, maps).	Iterates over each element in a map or set to create multiple resource instances.	Creates a specified number of instances of a resource or module.
Use Case	Modifying collections, such as filtering a list of strings in a collection.	Creating resources with unique configurations based on the default value for each item.	Creating a fixed number of similar resources without a unique resource configuration.
Supported Collection Types	Lists, Sets, Maps	Sets and Maps	Numeric expressions
Accessing Elements	Access to both element and their index (if needed).	Access to key-value pairs of resources each.key and each.value.	Access to index of the current resource using count.index.
Resource Creation	Used for data manipulation, not direct resource creation.	Enables dynamic resource creation with unique configurations derived from each item.	Allows for simple replication with slight variations (e.g., naming) based on an index.

1. Using for_each on a list of strings:

    locals {
      ip_addresses = ["10.0.0.1", "10.0.0.2"]
    }
   
    resource "example" "example" {
      for_each   = toset(local.ip_addresses)
      ip_address = each.key
    }
   

2. Using for_each on a list of objects

    locals {
      virtual_machines = [
        {
          ip_address = "10.0.0.1"
          name       = "vm-1"
        },
        {
          ip_address = "10.0.0.1"
          name       = "vm-2"
        }
      ]
    }    
   
    resource "example" "example" {
      for_each   = {
        for index, vm in local.virtual_machines:
        vm.name => vm
      }
      name       = each.value.name
      ip_address = each.value.ip_address
    }
   

3. Using for_each to combine two lists

    locals {
      domains = [
        "https://abc.com",
        "https://def.com"
      ]
      paths = [
        "/one",
        "/two",
        "/three"
      ]
    }
           
    resource "example" "example" {
      # Loop over both lists and flatten the result
      urls = flatten([
        for domain in local.domains : [
          for path in local.paths : {
            domain = domain
            path   = path
          }
        ]
      ])
    }
   

4. Using for_each on a nested block

    # Using the optional() keyword makes fields null if not present
    variable "routes" {
      type = list(
        name   = string
        path   = string
        config = optional(object({
          cache_enabled = bool
          https_only    = bool
        }))
      default = []
    }
   
    resource "example" "example" {
      name = ...
         
      dynamic "route" {
        for_each = {
          for route in var.routes :
          route.name => route
        }
        content {
          # Note: <top_level_block>.value.<object_key>
          name = route.value.name
        }
           
        dynamic "configuration" {
          # Note: <top_level_block>.value.<optional_object_key>
          for_each = route.value.config != null ? [1] : []
          content {
            cache_enabled = route.value.config.cache_enabled
            https_only    = route.value.config.https_only
          }
        }
      }
   

5. Using for_each as a conditional

``` variable “deploy_example” { type = bool description = “Indicates whether to deploy something.” default = true }

# Using count and a conditional, for_each is also possible here.
# See the next solution using a for_each with a conditional.
resource "example" "example" {
  count      = var.deploy_example ? 0 : 1
  name       = ...
  ip_address = ...
}

variable "enable_logs" {
  type        = bool
  description = "Indicates whether to enable something."
  default     = false
}

resource "example" "example" {
  name       = ...
  ip_address = ...

  # Note: dynamic blocks cannot use count!
  # Using for_each with an empty list and list(1) as a readable alternative. 
  dynamic "logs" {
    for_each = var.enable_logs ? [] : [1]
    content {
      name     = "logging"
    }
  }
}
```