The for_each Meta-Argument

By default, a resource block configures one real infrastructure object (and similarly, a module block includes a child module's contents into the configuration one time). However, sometimes you want to manage several similar objects (like a fixed pool of compute instances) without writing a separate block for each one. OpenTF has two ways to do this: count and for_each.

If a resource or module block includes a for_each argument whose value is a map or a set of strings, OpenTF creates one instance for each member of that map or set.

Basic Syntax​

for_each is a meta-argument defined by the OpenTF language. It can be used with modules and with every resource type.

The for_each meta-argument accepts a map or a set of strings, and creates an instance for each item in that map or set. Each instance has a distinct infrastructure object associated with it, and each is separately created, updated, or destroyed when the configuration is applied.

Map:

resource "azurerm_resource_group" "rg" {
  for_each = {
    a_group = "eastus"
    another_group = "westus2"
  }
  name     = each.key
  location = each.value
}

Set of strings:

resource "aws_iam_user" "the-accounts" {
  for_each = toset( ["Todd", "James", "Alice", "Dottie"] )
  name     = each.key
}

Child module:

# my_buckets.tf
module "bucket" {
  for_each = toset(["assets", "media"])
  source   = "./publish_bucket"
  name     = "${each.key}_bucket"
}

# publish_bucket/bucket-and-cloudfront.tf
variable "name" {} # this is the input parameter of the module

resource "aws_s3_bucket" "example" {
  # Because var.name includes each.key in the calling
  # module block, its value will be different for
  # each instance of this module.
  bucket = var.name

  # ...
}

resource "aws_iam_user" "deploy_user" {
  # ...
}

The each Object​

In blocks where for_each is set, an additional each object is available in expressions, so you can modify the configuration of each instance. This object has two attributes:

• each.key — The map key (or set member) corresponding to this instance.
• each.value — The map value corresponding to this instance. (If a set was provided, this is the same as each.key.)

Limitations on values used in for_each​

The keys of the map (or all the values in the case of a set of strings) must be known values, or you will get an error message that for_each has dependencies that cannot be determined before apply, and a -target may be needed.

for_each keys cannot be the result (or rely on the result of) of impure functions, including uuid, bcrypt, or timestamp, as their evaluation is deferred during the main evaluation step.

Sensitive values, such as sensitive input variables, sensitive outputs, or sensitive resource attributes, cannot be used as arguments to for_each. The value used in for_each is used to identify the resource instance and will always be disclosed in UI output, which is why sensitive values are not allowed. Attempts to use sensitive values as for_each arguments will result in an error.

If you transform a value containing sensitive data into an argument to be used in for_each, be aware that most functions in OpenTF will return a sensitive result if given an argument with any sensitive content. In many cases, you can achieve similar results to a function used for this purpose by using a for expression. For example, if you would like to call keys(local.map), where local.map is an object with sensitive values (but non-sensitive keys), you can create a value to pass to for_each with toset([for k,v in local.map : k]).

Using Expressions in for_each​

The for_each meta-argument accepts map or set expressions. However, unlike most arguments, the for_each value must be known before OpenTF performs any remote resource actions. This means for_each can't refer to any resource attributes that aren't known until after a configuration is applied (such as a unique ID generated by the remote API when an object is created).

The for_each value must be a map or set with one element per desired resource instance. To use a sequence as the for_each value, you must use an expression that explicitly returns a set value, like the toset function. To prevent unwanted surprises during conversion, the for_each argument does not implicitly convert lists or tuples to sets. If you need to declare resource instances based on a nested data structure or combinations of elements from multiple data structures you can use OpenTF expressions and functions to derive a suitable value. For example:

• Transform a multi-level nested structure into a flat list by using nested for expressions with the flatten function.
• Produce an exhaustive list of combinations of elements from two or more collections by using the setproduct function inside a for expression.

Chaining for_each Between Resources​

Because a resource using for_each appears as a map of objects when used in expressions elsewhere, you can directly use one resource as the for_each of another in situations where there is a one-to-one relationship between two sets of objects.

For example, in AWS an aws_vpc object is commonly associated with a number of other objects that provide additional services to that VPC, such as an "internet gateway". If you are declaring multiple VPC instances using for_each then you can chain that for_each into another resource to declare an internet gateway for each VPC:

variable "vpcs" {
  type = map(object({
    cidr_block = string
  }))
}

resource "aws_vpc" "example" {
  # One VPC for each element of var.vpcs
  for_each = var.vpcs

  # each.value here is a value from var.vpcs
  cidr_block = each.value.cidr_block
}

resource "aws_internet_gateway" "example" {
  # One Internet Gateway per VPC
  for_each = aws_vpc.example

  # each.value here is a full aws_vpc object
  vpc_id = each.value.id
}

output "vpc_ids" {
  value = {
    for k, v in aws_vpc.example : k => v.id
  }

  # The VPCs aren't fully functional until their
  # internet gateways are running.
  depends_on = [aws_internet_gateway.example]
}

This chaining pattern explicitly and concisely declares the relationship between the internet gateway instances and the VPC instances, which tells OpenTF to expect the instance keys for both to always change together, and typically also makes the configuration easier to understand for human maintainers.

Referring to Instances​

When for_each is set, OpenTF distinguishes between the block itself and the multiple resource or module instances associated with it. Instances are identified by a map key (or set member) from the value provided to for_each.

• <TYPE>.<NAME> or module.<NAME> (for example, azurerm_resource_group.rg) refers to the block.
• <TYPE>.<NAME>[<KEY>] or module.<NAME>[<KEY>] (for example, azurerm_resource_group.rg["a_group"], azurerm_resource_group.rg["another_group"], etc.) refers to individual instances.

This is different from resources and modules without count or for_each, which can be referenced without an index or key.

Similarly, resources from child modules with multiple instances are prefixed with module.<NAME>[<KEY>] when displayed in plan output and elsewhere in the UI. For a module without count or for_each, the address will not contain the module index as the module's name suffices to reference the module.

Using Sets​

The OpenTF language doesn't have a literal syntax for set values, but you can use the toset function to explicitly convert a list of strings to a set:

locals {
  subnet_ids = toset([
    "subnet-abcdef",
    "subnet-012345",
  ])
}

resource "aws_instance" "server" {
  for_each = local.subnet_ids

  ami           = "ami-a1b2c3d4"
  instance_type = "t2.micro"
  subnet_id     = each.key # note: each.key and each.value are the same for a set

  tags = {
    Name = "Server ${each.key}"
  }
}

Conversion from list to set discards the ordering of the items in the list and removes any duplicate elements. toset(["b", "a", "b"]) will produce a set containing only "a" and "b" in no particular order; the second "b" is discarded.

If you are writing a module with an input variable that will be used as a set of strings for for_each, you can set its type to set(string) to avoid the need for an explicit type conversion:

variable "subnet_ids" {
  type = set(string)
}

resource "aws_instance" "server" {
  for_each = var.subnet_ids

  # (and the other arguments as above)
}