123 lines
4.7 KiB
Markdown
123 lines
4.7 KiB
Markdown
---
|
||
layout: "language"
|
||
page_title: "The count Meta-Argument - Configuration Language"
|
||
---
|
||
|
||
# The `count` Meta-Argument
|
||
|
||
-> **Version note:** Module support for `count` was added in Terraform 0.13, and
|
||
previous versions can only use it with resources.
|
||
|
||
-> **Note:** A given resource or module block cannot use both `count` and `for_each`.
|
||
|
||
> **Hands-on:** Try the [Manage Similar Resources With Count](https://learn.hashicorp.com/tutorials/terraform/count?in=terraform/0-13&utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) tutorial on HashiCorp Learn.
|
||
|
||
By default, a [resource block](/docs/language/resources/syntax.html) configures one real
|
||
infrastructure object. (Similarly, a
|
||
[module block](/docs/language/modules/syntax.html) 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.
|
||
Terraform has two ways to do this:
|
||
`count` and [`for_each`](/docs/language/meta-arguments/for_each.html).
|
||
|
||
If a resource or module block includes a `count` argument whose value is a whole number,
|
||
Terraform will create that many instances.
|
||
|
||
## Basic Syntax
|
||
|
||
`count` is a meta-argument defined by the Terraform language. It can be used
|
||
with modules and with every resource type.
|
||
|
||
The `count` meta-argument accepts a whole number, and creates that many
|
||
instances of the resource or module. Each instance has a distinct infrastructure object
|
||
associated with it, and each is separately created,
|
||
updated, or destroyed when the configuration is applied.
|
||
|
||
```hcl
|
||
resource "aws_instance" "server" {
|
||
count = 4 # create four similar EC2 instances
|
||
|
||
ami = "ami-a1b2c3d4"
|
||
instance_type = "t2.micro"
|
||
|
||
tags = {
|
||
Name = "Server ${count.index}"
|
||
}
|
||
}
|
||
```
|
||
|
||
## The `count` Object
|
||
|
||
In blocks where `count` is set, an additional `count` object is
|
||
available in expressions, so you can modify the configuration of each instance.
|
||
This object has one attribute:
|
||
|
||
- `count.index` — The distinct index number (starting with `0`) corresponding
|
||
to this instance.
|
||
|
||
## Using Expressions in `count`
|
||
|
||
The `count` meta-argument accepts numeric [expressions](/docs/language/expressions/index.html).
|
||
However, unlike most arguments, the `count` value must be known
|
||
_before_ Terraform performs any remote resource actions. This means `count`
|
||
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).
|
||
|
||
## Referring to Instances
|
||
|
||
When `count` is set, Terraform distinguishes between the block itself
|
||
and the multiple _resource or module instances_ associated with it. Instances are
|
||
identified by an index number, starting with `0`.
|
||
|
||
- `<TYPE>.<NAME>` or `module.<NAME>` (for example, `aws_instance.server`) refers to the resource block.
|
||
- `<TYPE>.<NAME>[<INDEX>]` or `module.<NAME>[<INDEX>]` (for example, `aws_instance.server[0]`,
|
||
`aws_instance.server[1]`, 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.
|
||
|
||
-> **Note:** Within nested `provisioner` or `connection` blocks, the special
|
||
`self` object refers to the current _resource instance,_ not the resource block
|
||
as a whole.
|
||
|
||
## When to Use `for_each` Instead of `count`
|
||
|
||
If your instances are almost identical, `count` is appropriate. If some
|
||
of their arguments need distinct values that can't be directly derived from an
|
||
integer, it's safer to use `for_each`.
|
||
|
||
Before `for_each` was available, it was common to derive `count` from the
|
||
length of a list and use `count.index` to look up the original list value:
|
||
|
||
```hcl
|
||
variable "subnet_ids" {
|
||
type = list(string)
|
||
}
|
||
|
||
resource "aws_instance" "server" {
|
||
# Create one instance for each subnet
|
||
count = length(var.subnet_ids)
|
||
|
||
ami = "ami-a1b2c3d4"
|
||
instance_type = "t2.micro"
|
||
subnet_id = var.subnet_ids[count.index]
|
||
|
||
tags = {
|
||
Name = "Server ${count.index}"
|
||
}
|
||
}
|
||
```
|
||
|
||
This was fragile, because the resource instances were still identified by their
|
||
_index_ instead of the string values in the list. If an element was removed from
|
||
the middle of the list, every instance _after_ that element would see its
|
||
`subnet_id` value change, resulting in more remote object changes than intended.
|
||
Using `for_each` gives the same flexibility without the extra churn.
|