Terraform loops

Terraform loops

There are a couple of ways to loop in Terraform. Let’s have a look.

Count

This is the “oldest” method. It’s quite powerful.

resource "some_resource" "some_name" {
  count = 3
  name  = "my_resource_${count.index}"
}

As you can see, the resource some_resource is being created 3 (count = 3) times. The name should be unique, so the count.index variable is used. This variable is available when using count.

The variable count.index has these values:

itteration	count.index value
first	0
second	1
third	2

And so on.

Counting and looking up values

The parameter count sounds simple, but is actually quite powerful. Lets have a look at this example.

Here is a sample .tfvars file:

virtual_machines = [
  {
    name = "first"
    size = 16
  },
  {
    name = "second"
    size = 32
  }
]

The above structure is a “list of maps”:

• List is indicated by the [ and ] character.
• Map is indicated by the { and } character.

Now lets loop over that list:

resource "fake_virtual_machine" "default" {
  count = length(var.virtual_machines)
  name  = var.virtual_machines[count.index].name
  size  = var.virtual_machines[count.index].size
}

A couple of tricks happen here:

1. count is calculated by the length function. It basically counts how many maps there are in the list virtual_machines.
2. name is looked-up in the variable var.virtual_machines. Pick the first (0) entry from var.virtual_machines in the first itteration.
3. Similarly size is looked up.

This results in two resources:

resource "fake_virtual_machine" "default" {
  name  = "first"
  size  = 16
}

# NOTE: This code does not work; `default` may not be repeated. It's just to explain what happens.
resource "fake_virtual_machine" "default" {
  name  = "second"
  size  = 32
}

For each

The looping mechanism for_each can also be used. Similar to the count example, let’s think of a data structure to make virtual machines:

virtual_machines = [
  {
    name = "first"
    size = 16
  },
  {
    name = "second"
    size = 32
  }
]

And let’s use for_each to loop over this structure.

resource "fake_virtual_machine" "default" {
  for_each = var.virtual_machines
  name = each.value.name
  size = each.value.size
}

This pattern creates exactly the same resources as the count example.

Dynamic block

Imagine there is some block in the fake_virtual_machine resource, like this:

resource "fake_virtual_machine" "default" {
  name = "example"
  disk {
    name = "os"
    size = 32
  }
  disk {
    name = "data"
    size = 128
  }
}

The variable structure we’ll use looks like this:

virtual_machines = [
  {
    name = "first"
    disks [
      {
        name = "os"
        size = 32
      },
      {
        name = "data"
        size = 128
      }
    ]
  },
  {
    name = "second"
    disks [
      {
        name = "os"
        size = 64
      },
      {
        name = "data"
        size = 256
      }
    ]
  }
]

As you can see:

• A list of virtual_machines.
• Each virtual_machine has a list of disks.

Now let’s introduce the dynamic block:

resource "fake_virtual_machine" "default" {
  for_each = var.virtual_machines
  name = each.value.name
  dynamic "disk" {
    for_each = each.value.disks
    content {
      name = disk.value.name
      size = disk.value.size
    }
  }
}

Wow, that’s a lot to explain:

1. The dynamic "disk" { starts a dynamic block. The name (“disk”) must reflect the parameter in the resource, not juts any name. Now a new object is available; disk.
2. The for_each = each.value.disks loops the dynamic block. The loop uses disks from an already looped value var.virtual_machines.
3. The content { block will be rendered by Terraform.
4. The name = disk.value.name uses the disk variable (created by the block iterator disk) to find the value from the disks map.

Hope that helps a bit when writing loops in Terraform!