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website: Break up Resources page into smaller chunks 

- Resource behavior gets its own page.
- Meta-arguments all get their own pages.
- Stuff about resource syntax itself gets a page.

In the process of breaking the meta-arguments out into their own pages, I
revised them (with the exception of `provider`) so that they apply to both
resources and modules.

Like with Expressions, this commit repurposes the old resources.html URL as a
landing page for old links.
This commit is contained in:
Nick Fagerlund 2020-11-12 18:06:32 -08:00
parent a446ecb7b7
commit 6e2f5eb0be
10 changed files with 943 additions and 736 deletions
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---
layout: "language"
page_title: "Resource Behavior - Configuration Language"
---
# Resource Behavior
A `resource` block declares that you want a particular infrastructure object
to exist with the given settings. If you are writing a new configuration for
the first time, the resources it defines will exist _only_ in the configuration,
and will not yet represent real infrastructure objects in the target platform.
_Applying_ a Terraform configuration is the process of creating, updating,
and destroying real infrastructure objects in order to make their settings
match the configuration.
## How Terraform Applies a Configuration
When Terraform creates a new infrastructure object represented by a `resource`
block, the identifier for that real object is saved in Terraform's
[state](/docs/state/index.html), allowing it to be updated and destroyed
in response to future changes. For resource blocks that already have an
associated infrastructure object in the state, Terraform compares the
actual configuration of the object with the arguments given in the
configuration and, if necessary, updates the object to match the configuration.
In summary, applying a Terraform configuration will:
- _Create_ resources that exist in the configuration but are not associated with a real infrastructure object in the state.
- _Destroy_ resources that exist in the state but no longer exist in the configuration.
- _Update in-place_ resources whose arguments have changed.
- _Destroy and re-create_ resources whose arguments have changed but which cannot be updated in-place due to remote API limitations.
This general behavior applies for all resources, regardless of type. The
details of what it means to create, update, or destroy a resource are different
for each resource type, but this standard set of verbs is common across them
all.
The meta-arguments within `resource` blocks, documented in the
sections below, allow some details of this standard resource behavior to be
customized on a per-resource basis.
## Accessing Resource Attributes
[Expressions](/docs/configuration/expressions/index.html) within a Terraform module can access
information about resources in the same module, and you can use that information
to help configure other resources. Use the `<RESOURCE TYPE>.<NAME>.<ATTRIBUTE>`
syntax to reference a resource attribute in an expression.
In addition to arguments specified in the configuration, resources often provide
read-only attributes with information obtained from the remote API; this often
includes things that can't be known until the resource is created, like the
resource's unique random ID.
Many providers also include [data sources](./data-sources.html), which are a
special type of resource used only for looking up information.
For a list of the attributes a resource or data source type provides, consult
its documentation; these are generally included in a second list below its list
of configurable arguments.
For more information about referencing resource attributes in expressions, see
[Expressions: References to Resource Attributes](/docs/configuration/expressions/references.html#references-to-resource-attributes).
## Resource Dependencies
Most resources in a configuration don't have any particular relationship, and
Terraform can make changes to several unrelated resources in parallel.
However, some resources must be processed after other specific resources;
sometimes this is because of how the resource works, and sometimes the
resource's configuration just requires information generated by another
resource.
Most resource dependencies are handled automatically. Terraform analyses any
[expressions](/docs/configuration/expressions/index.html) within a `resource` block to find references
to other objects, and treats those references as implicit ordering requirements
when creating, updating, or destroying resources. Since most resources with
behavioral dependencies on other resources also refer to those resources' data,
it's usually not necessary to manually specify dependencies between resources.
However, some dependencies cannot be recognized implicitly in configuration. For
example, if Terraform must manage access control policies _and_ take actions
that require those policies to be present, there is a hidden dependency between
the access policy and a resource whose creation depends on it. In these rare
cases, [the `depends_on` meta-argument](./depends_on.html) can explicitly specify a
dependency.
## Local-only Resources
While most resource types correspond to an infrastructure object type that
is managed via a remote network API, there are certain specialized resource
types that operate only within Terraform itself, calculating some results and
saving those results in the state for future use.
For example, local-only resource types exist for
[generating private keys](/docs/providers/tls/r/private_key.html),
[issuing self-signed TLS certificates](/docs/providers/tls/r/self_signed_cert.html),
and even [generating random ids](/docs/providers/random/r/id.html).
While these resource types often have a more marginal purpose than those
managing "real" infrastructure objects, they can be useful as glue to help
connect together other resources.
The behavior of local-only resources is the same as all other resources, but
their result data exists only within the Terraform state. "Destroying" such
a resource means only to remove it from the state, discarding its data.

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@ -12,8 +12,21 @@ Each resource block describes one or more infrastructure objects, such
as virtual networks, compute instances, or higher-level components such as virtual networks, compute instances, or higher-level components such
as DNS records. as DNS records.
- [Resource Blocks](/docs/configuration/resources.html) documents how to declare - [Resource Blocks](/docs/configuration/blocks/resources/syntax.html) documents
resources, including information about meta-arguments like `for_each`. the syntax for declaring resources.
- [Resource Behavior](/docs/configuration/resources/behavior.html) explains in
more detail how Terraform handles resource declarations when applying a
configuration.
- The Meta-Arguments section documents special arguments that can be used with
every resource type, including
[`depends_on`](/docs/configuration/meta-arguments/depends_on.html),
[`count`](/docs/configuration/meta-arguments/count.html),
[`for_each`](/docs/configuration/meta-arguments/for_each.html),
[`provider`](/docs/configuration/meta-arguments/resource-provider.html),
and [`lifecycle`](/docs/configuration/meta-arguments/lifecycle.html).
- [Provisioners](/docs/configuration/blocks/resources/provisioners/index.html) - [Provisioners](/docs/configuration/blocks/resources/provisioners/index.html)
documents configuring post-creation actions for a resource using the documents configuring post-creation actions for a resource using the
`provisioner` and `connection` blocks. Since provisioners are non-declarative `provisioner` and `connection` blocks. Since provisioners are non-declarative

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---
layout: "language"
page_title: "Resources - Configuration Language"
sidebar_current: "docs-config-resources"
description: |-
Resources are the most important element in a Terraform configuration.
Each resource corresponds to an infrastructure object, such as a virtual
network or compute instance.
---
# Resource Blocks
-> **Note:** This page is about Terraform 0.12 and later. For Terraform 0.11 and
earlier, see
[0.11 Configuration Language: Resources](../configuration-0-11/resources.html).
> **Hands-on:** Try the [Terraform: Get Started](https://learn.hashicorp.com/collections/terraform/aws-get-started?utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) collection on HashiCorp Learn.
_Resources_ are the most important element in the Terraform language.
Each resource block describes one or more infrastructure objects, such
as virtual networks, compute instances, or higher-level components such
as DNS records.
## Resource Syntax
Resource declarations can include a number of advanced features, but only
a small subset are required for initial use. More advanced syntax features,
such as single resource declarations that produce multiple similar remote
objects, are described later in this page.
```hcl
resource "aws_instance" "web" {
ami = "ami-a1b2c3d4"
instance_type = "t2.micro"
}
```
A `resource` block declares a resource of a given type ("aws_instance")
with a given local name ("web"). The name is used to refer to this resource
from elsewhere in the same Terraform module, but has no significance outside
that module's scope.
The resource type and name together serve as an identifier for a given
resource and so must be unique within a module.
Within the block body (between `{` and `}`) are the configuration arguments
for the resource itself. Most arguments in this section depend on the
resource type, and indeed in this example both `ami` and `instance_type` are
arguments defined specifically for [the `aws_instance` resource type](/docs/providers/aws/r/instance.html).
-> **Note:** Resource names must start with a letter or underscore, and may
contain only letters, digits, underscores, and dashes.
## Resource Types
Each resource is associated with a single _resource type_, which determines
the kind of infrastructure object it manages and what arguments and other
attributes the resource supports.
### Providers
Each resource type is implemented by a [provider](/docs/configuration/provider-requirements.html),
which is a plugin for Terraform that offers a collection of resource types. A
provider usually provides resources to manage a single cloud or on-premises
infrastructure platform. Providers are distributed separately from Terraform
itself, but Terraform can automatically install most providers when initializing
a working directory.
In order to manage resources, a Terraform module must specify which providers it
requires. Additionally, most providers need some configuration in order to
access their remote APIs, and the root module must provide that configuration.
For more information, see:
- [Provider Requirements](/docs/configuration/provider-requirements.html), for declaring which
providers a module uses.
- [Provider Configuration](/docs/configuration/providers.html), for configuring provider settings.
Terraform usually automatically determines which provider to use based on a
resource type's name. (By convention, resource type names start with their
provider's preferred local name.) When using multiple configurations of a
provider (or non-preferred local provider names), you must use the `provider`
meta-argument to manually choose an alternate provider configuration. See
[the `provider` meta-argument](/docs/configuration/meta-arguments/resource-provider.html) for more details.
### Resource Arguments
Most of the arguments within the body of a `resource` block are specific to the
selected resource type. The resource type's documentation lists which arguments
are available and how their values should be formatted.
The values for resource arguments can make full use of
[expressions](/docs/configuration/expressions/index.html) and other dynamic Terraform
language features.
There are also some _meta-arguments_ that are defined by Terraform itself
and apply across all resource types. (See [Meta-Arguments](#meta-arguments) below.)
### Documentation for Resource Types
Every Terraform provider has its own documentation, describing its resource
types and their arguments.
Most publicly available providers are distributed on the
[Terraform Registry](https://registry.terraform.io/browse/providers), which also
hosts their documentation. When viewing a provider's page on the Terraform
Registry, you can click the "Documentation" link in the header to browse its
documentation. Provider documentation on the registry is versioned, and you can
use the dropdown version menu in the header to switch which version's
documentation you are viewing.
To browse the publicly available providers and their documentation, see
[the providers section of the Terraform Registry](https://registry.terraform.io/browse/providers).
-> **Note:** Provider documentation used to be hosted directly on terraform.io,
as part of Terraform's core documentation. Although some provider documentation
might still be hosted here, the Terraform Registry is now the main home for all
public provider docs. (The exception is the built-in
[`terraform` provider](/docs/providers/terraform/index.html) for reading state
data, since it is not available on the Terraform Registry.)
## Resource Behavior
For more information about how Terraform manages resources when applying a
configuration, see
[Resource Behavior](/docs/configuration/blocks/resources/behavior.html).
## Meta-Arguments
The Terraform language defines several meta-arguments, which can be used with
any resource type to change the behavior of resources.
The following meta-arguments are documented on separate pages:
- [`depends_on`, for specifying hidden dependencies](/docs/configuration/meta-arguments/depends_on.html)
- [`count`, for creating multiple resource instances according to a count](/docs/configuration/meta-arguments/count.html)
- [`for_each`, to create multiple instances according to a map, or set of strings](/docs/configuration/meta-arguments/for_each.html)
- [`provider`, for selecting a non-default provider configuration](/docs/configuration/meta-arguments/resource-provider.html)
- [`lifecycle`, for lifecycle customizations](/docs/configuration/meta-arguments/lifecycle.html)
- [`provisioner` and `connection`, for taking extra actions after resource creation](/docs/configuration/blocks/resources/provisioners/index.html)
## Operation Timeouts
Some resource types provide a special `timeouts` nested block argument that
allows you to customize how long certain operations are allowed to take
before being considered to have failed.
For example, [`aws_db_instance`](/docs/providers/aws/r/db_instance.html)
allows configurable timeouts for `create`, `update` and `delete` operations.
Timeouts are handled entirely by the resource type implementation in the
provider, but resource types offering these features follow the convention
of defining a child block called `timeouts` that has a nested argument
named after each operation that has a configurable timeout value.
Each of these arguments takes a string representation of a duration, such
as `"60m"` for 60 minutes, `"10s"` for ten seconds, or `"2h"` for two hours.
```hcl
resource "aws_db_instance" "example" {
# ...
timeouts {
create = "60m"
delete = "2h"
}
}
```
The set of configurable operations is chosen by each resource type. Most
resource types do not support the `timeouts` block at all. Consult the
documentation for each resource type to see which operations it offers
for configuration, if any.

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---
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/configuration/blocks/resources/syntax.html) configures one real
infrastructure object. (Similarly, a
[module block](/docs/configuration/blocks/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/configuration/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/configuration/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.

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---
layout: "language"
page_title: "The depends_on Meta-Argument - Configuration Language"
---
# The `depends_on` Meta-Argument
-> **Version note:** Module support for `depends_on` was added in Terraform 0.13, and
previous versions can only use it with resources.
Use the `depends_on` meta-argument to handle hidden resource or module dependencies that
Terraform can't automatically infer.
Explicitly specifying a dependency is only necessary when a resource or module relies on
some other resource's behavior but _doesn't_ access any of that resource's data
in its arguments.
This argument is available in `module` blocks and in all `resource` blocks,
regardless of resource type. For example:
```hcl
resource "aws_iam_role" "example" {
name = "example"
# assume_role_policy is omitted for brevity in this example. See the
# documentation for aws_iam_role for a complete example.
assume_role_policy = "..."
}
resource "aws_iam_instance_profile" "example" {
# Because this expression refers to the role, Terraform can infer
# automatically that the role must be created first.
role = aws_iam_role.example.name
}
resource "aws_iam_role_policy" "example" {
name = "example"
role = aws_iam_role.example.name
policy = jsonencode({
"Statement" = [{
# This policy allows software running on the EC2 instance to
# access the S3 API.
"Action" = "s3:*",
"Effect" = "Allow",
}],
})
}
resource "aws_instance" "example" {
ami = "ami-a1b2c3d4"
instance_type = "t2.micro"
# Terraform can infer from this that the instance profile must
# be created before the EC2 instance.
iam_instance_profile = aws_iam_instance_profile.example
# However, if software running in this EC2 instance needs access
# to the S3 API in order to boot properly, there is also a "hidden"
# dependency on the aws_iam_role_policy that Terraform cannot
# automatically infer, so it must be declared explicitly:
depends_on = [
aws_iam_role_policy.example,
]
}
```
The `depends_on` meta-argument, if present, must be a list of references
to other resources or child modules in the same calling module.
Arbitrary expressions are not allowed in the `depends_on` argument value,
because its value must be known before Terraform knows resource relationships
and thus before it can safely evaluate expressions.
The `depends_on` argument should be used only as a last resort. When using it,
always include a comment explaining why it is being used, to help future
maintainers understand the purpose of the additional dependency.

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---
layout: "language"
page_title: "The for_each Meta-Argument - Configuration Language"
---
# The `for_each` Meta-Argument
-> **Version note:** `for_each` was added in Terraform 0.12.6. Module support
for `for_each` 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 For Each](https://learn.hashicorp.com/tutorials/terraform/for-each?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/configuration/blocks/resources/syntax.html) configures one real
infrastructure object. (Similarly, a
[module block](/docs/configuration/blocks/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`](/docs/configuration/meta-arguments/count.html) and `for_each`.
If a resource or module block includes a `for_each` argument whose value is a map or
a set of strings, Terraform will create one instance for each member of
that map or set.
## Basic Syntax
`for_each` is a meta-argument defined by the Terraform 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.
-> **Note:** 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.
Map:
```hcl
resource "azurerm_resource_group" "rg" {
for_each = {
a_group = "eastus"
another_group = "westus2"
}
name = each.key
location = each.value
}
```
Set of strings:
```hcl
resource "aws_iam_user" "the-accounts" {
for_each = toset( ["Todd", "James", "Alice", "Dottie"] )
name = each.key
}
```
Child module:
```hcl
# my_buckets.tf
module "bucket" {
for_each = toset(["assets", "media"])
source = "./publish_bucket"
name = "${each.key}_bucket"
}
```
```hcl
# 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`.)
## Using Expressions in `for_each`
The `for_each` meta-argument accepts map or set [expressions](/docs/configuration/expressions/index.html).
However, unlike most arguments, the `for_each` value must be known
_before_ Terraform 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. When providing a set, you must use an expression that
explicitly returns a set value, like the [`toset`](/docs/configuration/functions/toset.html)
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 Terraform 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](/docs/configuration/functions/flatten.html#flattening-nested-structures-for-for_each).
* Produce an exhaustive list of combinations of elements from two or more
collections by
[using the `setproduct` function inside a `for` expression](/docs/configuration/functions/setproduct.html#finding-combinations-for-for_each).
## Referring to Instances
When `for_each` is set, Terraform 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.
-> **Note:** Within nested `provisioner` or `connection` blocks, the special
`self` object refers to the current _resource instance,_ not the resource block
as a whole.
## Using Sets
The Terraform language doesn't have a literal syntax for
[set values](/docs/configuration/types.html#collection-types), but you can use the `toset`
function to explicitly convert a list of strings to a set:
```hcl
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](/docs/configuration/variables.html) 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:
```hcl
variable "subnet_ids" {
type = set(string)
}
resource "aws_instance" "server" {
for_each = var.subnet_ids
# (and the other arguments as above)
}
```

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---
layout: "language"
page_title: "The lifecycle Meta-Argument - Configuration Language"
---
# The `lifecycle` Meta-Argument
The general lifecycle for resources is described in the
[Resource Behavior](/docs/configuration/blocks/resources/behavior.html) page. Some details of
that behavior can be customized using the special nested `lifecycle` block
within a resource block body:
```hcl
resource "azurerm_resource_group" "example" {
# ...
lifecycle {
create_before_destroy = true
}
}
```
## Syntax and Arguments
`lifecycle` is a nested block that can appear within a resource block.
The `lifecycle` block and its contents are meta-arguments, available
for all `resource` blocks regardless of type.
The following arguments can be used within a `lifecycle` block:
* `create_before_destroy` (bool) - By default, when Terraform must change
a resource argument that cannot be updated in-place due to
remote API limitations, Terraform will instead destroy the existing object
and then create a new replacement object with the new configured arguments.
The `create_before_destroy` meta-argument changes this behavior so that
the new replacement object is created _first,_ and the prior object
is destroyed after the replacement is created.
This is an opt-in behavior because many remote object types have unique
name requirements or other constraints that must be accommodated for
both a new and an old object to exist concurrently. Some resource types
offer special options to append a random suffix onto each object name to
avoid collisions, for example. Terraform CLI cannot automatically activate
such features, so you must understand the constraints for each resource
type before using `create_before_destroy` with it.
* `prevent_destroy` (bool) - This meta-argument, when set to `true`, will
cause Terraform to reject with an error any plan that would destroy the
infrastructure object associated with the resource, as long as the argument
remains present in the configuration.
This can be used as a measure of safety against the accidental replacement
of objects that may be costly to reproduce, such as database instances.
However, it will make certain configuration changes impossible to apply,
and will prevent the use of the `terraform destroy` command once such
objects are created, and so this option should be used sparingly.
Since this argument must be present in configuration for the protection to
apply, note that this setting does not prevent the remote object from
being destroyed if the `resource` block were removed from configuration
entirely: in that case, the `prevent_destroy` setting is removed along
with it, and so Terraform will allow the destroy operation to succeed.
* `ignore_changes` (list of attribute names) - By default, Terraform detects
any difference in the current settings of a real infrastructure object
and plans to update the remote object to match configuration.
The `ignore_changes` feature is intended to be used when a resource is
created with references to data that may change in the future, but should
not affect said resource after its creation. In some rare cases, settings
of a remote object are modified by processes outside of Terraform, which
Terraform would then attempt to "fix" on the next run. In order to make
Terraform share management responsibilities of a single object with a
separate process, the `ignore_changes` meta-argument specifies resource
attributes that Terraform should ignore when planning updates to the
associated remote object.
The arguments corresponding to the given attribute names are considered
when planning a _create_ operation, but are ignored when planning an
_update_. The arguments are the relative address of the attributes in the
resource. Map and list elements can be referenced using index notation,
like `tags["Name"]` and `list[0]` respectively.
```hcl
resource "aws_instance" "example" {
# ...
lifecycle {
ignore_changes = [
# Ignore changes to tags, e.g. because a management agent
# updates these based on some ruleset managed elsewhere.
tags,
]
}
}
```
Instead of a list, the special keyword `all` may be used to instruct
Terraform to ignore _all_ attributes, which means that Terraform can
create and destroy the remote object but will never propose updates to it.
Only attributes defined by the resource type can be ignored.
`ignore_changes` cannot be applied to itself or to any other meta-arguments.
## Literal Values Only
The `lifecycle` settings all affect how Terraform constructs and traverses
the dependency graph. As a result, only literal values can be used because
the processing happens too early for arbitrary expression evaluation.

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---
layout: "language"
page_title: "The Resource provider Meta-Argument - Configuration Language"
---
# The Resource `provider` Meta-Argument
The `provider` meta-argument specifies which provider configuration to use for a resource,
overriding Terraform's default behavior of selecting one based on the resource
type name. Its value should be an unquoted `<PROVIDER>.<ALIAS>` reference.
As described in [Provider Configuration](/docs/configuration/providers.html), you can optionally
create multiple configurations for a single provider (usually to manage
resources in different regions of multi-region services). Each provider can have
one default configuration, and any number of alternate configurations that
include an extra name segment (or "alias").
By default, Terraform interprets the initial word in the resource type name
(separated by underscores) as the local name of a provider, and uses that
provider's default configuration. For example, the resource type
`google_compute_instance` is associated automatically with the default
configuration for the provider named `google`.
By using the `provider` meta-argument, you can select an alternate provider
configuration for a resource:
```hcl
# default configuration
provider "google" {
region = "us-central1"
}
# alternate configuration, whose alias is "europe"
provider "google" {
alias = "europe"
region = "europe-west1"
}
resource "google_compute_instance" "example" {
# This "provider" meta-argument selects the google provider
# configuration whose alias is "europe", rather than the
# default configuration.
provider = google.europe
# ...
}
```
A resource always has an implicit dependency on its associated provider, to
ensure that the provider is fully configured before any resource actions
are taken.
The `provider` meta-argument expects
[a `<PROVIDER>.<ALIAS>` reference](/docs/configuration/providers.html#referring-to-alternate-providers),
which does not need to be quoted. Arbitrary expressions are not permitted for
`provider` because it must be resolved while Terraform is constructing the
dependency graph, before it is safe to evaluate expressions.

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--- ---
layout: "language" layout: "language"
page_title: "Resources - Configuration Language" page_title: "Resources Landing Page - Configuration Language"
sidebar_current: "docs-config-resources"
description: |-
Resources are the most important element in a Terraform configuration.
Each resource corresponds to an infrastructure object, such as a virtual
network or compute instance.
--- ---
# Resources # Resources Landing Page
-> **Note:** This page is about Terraform 0.12 and later. For Terraform 0.11 and To improve navigation, we've split the old Resources page into several smaller
earlier, see pages.
[0.11 Configuration Language: Resources](../configuration-0-11/resources.html).
> **Hands-on:** Try the [Terraform: Get Started](https://learn.hashicorp.com/collections/terraform/aws-get-started?utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) collection on HashiCorp Learn. <a id="resource-syntax"></a>
<a id="resource-types"></a>
<a id="providers"></a>
<a id="resource-arguments"></a>
<a id="documentation-for-resource-types"></a>
<a id="meta-arguments"></a>
<a id="operation-timeouts"></a>
_Resources_ are the most important element in the Terraform language. ## Syntax and Elements of Resource Blocks
Each resource block describes one or more infrastructure objects, such
as virtual networks, compute instances, or higher-level components such
as DNS records.
## Resource Syntax This information has moved to
[Resource Blocks](/docs/configuration/blocks/resources/syntax.html).
Resource declarations can include a number of advanced features, but only <a id="resource-behavior"></a>
a small subset are required for initial use. More advanced syntax features, <a id="accessing-resource-attributes"></a>
such as single resource declarations that produce multiple similar remote <a id="resource-dependencies"></a>
objects, are described later in this page. <a id="local-only-resources"></a>
```hcl ## Details of Resource Behavior
resource "aws_instance" "web" {
ami = "ami-a1b2c3d4"
instance_type = "t2.micro"
}
```
A `resource` block declares a resource of a given type ("aws_instance") This information has moved to
with a given local name ("web"). The name is used to refer to this resource [Resource Behavior](/docs/configuration/blocks/resources/behavior.html).
from elsewhere in the same Terraform module, but has no significance outside
that module's scope.
The resource type and name together serve as an identifier for a given ## Resource Meta-Arguments
resource and so must be unique within a module.
Within the block body (between `{` and `}`) are the configuration arguments Each resource meta-argument has moved to its own page.
for the resource itself. Most arguments in this section depend on the
resource type, and indeed in this example both `ami` and `instance_type` are
arguments defined specifically for [the `aws_instance` resource type](/docs/providers/aws/r/instance.html).
-> **Note:** Resource names must start with a letter or underscore, and may <a id="depends_on-explicit-resource-dependencies"></a>
contain only letters, digits, underscores, and dashes.
## Resource Types ### `depends_on`
Each resource is associated with a single _resource type_, which determines This information has moved to
the kind of infrastructure object it manages and what arguments and other [`depends_on`](/docs/configuration/meta-arguments/depends_on.html).
attributes the resource supports.
### Providers <a id="count-multiple-resource-instances-by-count"></a>
<a id="the-count-object"></a>
<a id="referring-to-instances"></a>
<a id="using-expressions-in-count"></a>
<a id="when-to-use-for_each-instead-of-count"></a>
Each resource type is implemented by a [provider](./provider-requirements.html), ### `count`
which is a plugin for Terraform that offers a collection of resource types. A
provider usually provides resources to manage a single cloud or on-premises
infrastructure platform. Providers are distributed separately from Terraform
itself, but Terraform can automatically install most providers when initializing
a working directory.
In order to manage resources, a Terraform module must specify which providers it This information has moved to
requires. Additionally, most providers need some configuration in order to [`count`](/docs/configuration/meta-arguments/count.html).
access their remote APIs, and the root module must provide that configuration.
For more information, see: <a id="for_each-multiple-resource-instances-defined-by-a-map-or-set-of-strings"></a>
<a id="the-each-object"></a>
<a id="using-expressions-in-for_each"></a>
<a id="referring-to-instances-1"></a>
<a id="using-sets"></a>
- [Provider Requirements](./provider-requirements.html), for declaring which ### `for_each`
providers a module uses.
- [Provider Configuration](./providers.html), for configuring provider settings.
Terraform usually automatically determines which provider to use based on a This information has moved to
resource type's name. (By convention, resource type names start with their [`for_each`](/docs/configuration/meta-arguments/for_each.html).
provider's preferred local name.) When using multiple configurations of a
provider (or non-preferred local provider names), you must use the `provider`
meta-argument to manually choose an alternate provider configuration. See
[the section on `provider` below][inpage-provider] for more details.
### Resource Arguments <a id="provider-selecting-a-non-default-provider-configuration"></a>
Most of the arguments within the body of a `resource` block are specific to the ### `provider`
selected resource type. The resource type's documentation lists which arguments
are available and how their values should be formatted.
The values for resource arguments can make full use of This information has moved to
[expressions](./expressions.html) and other dynamic Terraform [`provider`](/docs/configuration/meta-arguments/resource-provider.html).
language features.
There are also some _meta-arguments_ that are defined by Terraform itself <a id="lifecycle-lifecycle-customizations"></a>
and apply across all resource types. (See [Meta-Arguments](#meta-arguments) below.)
### Documentation for Resource Types ### `lifecycle`
Every Terraform provider has its own documentation, describing its resource This information has moved to
types and their arguments. [`lifecycle`](/docs/configuration/meta-arguments/lifecycle.html).
Most publicly available providers are distributed on the <a id="provisioner-and-connection-resource-provisioners"></a>
[Terraform Registry](https://registry.terraform.io/browse/providers), which also
hosts their documentation. When viewing a provider's page on the Terraform
Registry, you can click the "Documentation" link in the header to browse its
documentation. Provider documentation on the registry is versioned, and you can
use the dropdown version menu in the header to switch which version's
documentation you are viewing.
To browse the publicly available providers and their documentation, see ### Provisioners
[the providers section of the Terraform Registry](https://registry.terraform.io/browse/providers).
-> **Note:** Provider documentation used to be hosted directly on terraform.io, This information has moved to
as part of Terraform's core documentation. Although some provider documentation [Provisioners](/docs/configuration/blocks/resources/provisioners/index.html).
might still be hosted here, the Terraform Registry is now the main home for all
public provider docs. (The exception is the built-in
[`terraform` provider](/docs/providers/terraform/index.html) for reading state
data, since it is not available on the Terraform Registry.)
## Resource Behavior
A `resource` block declares that you want a particular infrastructure object
to exist with the given settings. If you are writing a new configuration for
the first time, the resources it defines will exist _only_ in the configuration,
and will not yet represent real infrastructure objects in the target platform.
_Applying_ a Terraform configuration is the process of creating, updating,
and destroying real infrastructure objects in order to make their settings
match the configuration.
When Terraform creates a new infrastructure object represented by a `resource`
block, the identifier for that real object is saved in Terraform's
[state](/docs/state/index.html), allowing it to be updated and destroyed
in response to future changes. For resource blocks that already have an
associated infrastructure object in the state, Terraform compares the
actual configuration of the object with the arguments given in the
configuration and, if necessary, updates the object to match the configuration.
This general behavior applies for all resources, regardless of type. The
details of what it means to create, update, or destroy a resource are different
for each resource type, but this standard set of verbs is common across them
all.
The meta-arguments within `resource` blocks, documented in the
sections below, allow some details of this standard resource behavior to be
customized on a per-resource basis.
### Accessing Resource Attributes
[Expressions](./expressions.html) within a Terraform module can access
information about resources in the same module, and you can use that information
to help configure other resources. Use the `<RESOURCE TYPE>.<NAME>.<ATTRIBUTE>`
syntax to reference a resource attribute in an expression.
In addition to arguments specified in the configuration, resources often provide
read-only attributes with information obtained from the remote API; this often
includes things that can't be known until the resource is created, like the
resource's unique random ID.
Many providers also include [data sources](./data-sources.html), which are a
special type of resource used only for looking up information.
For a list of the attributes a resource or data source type provides, consult
its documentation; these are generally included in a second list below its list
of configurable arguments.
For more information about referencing resource attributes in expressions, see
[Expressions: References to Resource Attributes](./expressions.html#references-to-resource-attributes).
### Resource Dependencies
Most resources in a configuration don't have any particular relationship, and
Terraform can make changes to several unrelated resources in parallel.
However, some resources must be processed after other specific resources;
sometimes this is because of how the resource works, and sometimes the
resource's configuration just requires information generated by another
resource.
Most resource dependencies are handled automatically. Terraform analyses any
[expressions](./expressions.html) within a `resource` block to find references
to other objects, and treats those references as implicit ordering requirements
when creating, updating, or destroying resources. Since most resources with
behavioral dependencies on other resources also refer to those resources' data,
it's usually not necessary to manually specify dependencies between resources.
However, some dependencies cannot be recognized implicitly in configuration. For
example, if Terraform must manage access control policies _and_ take actions
that require those policies to be present, there is a hidden dependency between
the access policy and a resource whose creation depends on it. In these rare
cases, [the `depends_on` meta-argument][inpage-depend] can explicitly specify a
dependency.
## Meta-Arguments
Terraform CLI defines the following meta-arguments, which can be used with
any resource type to change the behavior of resources:
- [`depends_on`, for specifying hidden dependencies][inpage-depend]
- [`count`, for creating multiple resource instances according to a count][inpage-count]
- [`for_each`, to create multiple instances according to a map, or set of strings][inpage-for_each]
- [`provider`, for selecting a non-default provider configuration][inpage-provider]
- [`lifecycle`, for lifecycle customizations][inpage-lifecycle]
- [`provisioner` and `connection`, for taking extra actions after resource creation][inpage-provisioner]
These arguments often have additional restrictions on what language features can
be used with them, which are described in each
### `depends_on`: Explicit Resource Dependencies
[inpage-depend]: #depends_on-explicit-resource-dependencies
Use the `depends_on` meta-argument to handle hidden resource dependencies that
Terraform can't automatically infer.
Explicitly specifying a dependency is only necessary when a resource relies on
some other resource's behavior but _doesn't_ access any of that resource's data
in its arguments.
This argument is available in all `resource` blocks, regardless of resource
type. For example:
```hcl
resource "aws_iam_role" "example" {
name = "example"
# assume_role_policy is omitted for brevity in this example. See the
# documentation for aws_iam_role for a complete example.
assume_role_policy = "..."
}
resource "aws_iam_instance_profile" "example" {
# Because this expression refers to the role, Terraform can infer
# automatically that the role must be created first.
role = aws_iam_role.example.name
}
resource "aws_iam_role_policy" "example" {
name = "example"
role = aws_iam_role.example.name
policy = jsonencode({
"Statement" = [{
# This policy allows software running on the EC2 instance to
# access the S3 API.
"Action" = "s3:*",
"Effect" = "Allow",
}],
})
}
resource "aws_instance" "example" {
ami = "ami-a1b2c3d4"
instance_type = "t2.micro"
# Terraform can infer from this that the instance profile must
# be created before the EC2 instance.
iam_instance_profile = aws_iam_instance_profile.example
# However, if software running in this EC2 instance needs access
# to the S3 API in order to boot properly, there is also a "hidden"
# dependency on the aws_iam_role_policy that Terraform cannot
# automatically infer, so it must be declared explicitly:
depends_on = [
aws_iam_role_policy.example,
]
}
```
The `depends_on` meta-argument, if present, must be a list of references
to other resources in the same module. Arbitrary expressions are not allowed
in the `depends_on` argument value, because its value must be known before
Terraform knows resource relationships and thus before it can safely
evaluate expressions.
The `depends_on` argument should be used only as a last resort. When using it,
always include a comment explaining why it is being used, to help future
maintainers understand the purpose of the additional dependency.
### `count`: Multiple Resource Instances By Count
[inpage-count]: #count-multiple-resource-instances-by-count
-> **Note:** A given resource block cannot use both `count` and `for_each`.
By default, a `resource` block configures one real infrastructure object.
However, sometimes you want to manage several similar objects, such as a fixed
pool of compute instances. Terraform has two ways to do this:
`count` and [`for_each`][inpage-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.
The `count` meta-argument accepts a whole number, and creates that many
instances of the resource. Each instance has a distinct infrastructure object
associated with it (as described above in
[Resource Behavior](#resource-behavior)), 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 resource 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.
#### Referring to Instances
When `count` is set, Terraform distinguishes between the resource block itself
and the multiple _resource instances_ associated with it. Instances are
identified by an index number, starting with `0`.
- `<TYPE>.<NAME>` (for example, `aws_instance.server`) refers to the resource block.
- `<TYPE>.<NAME>[<INDEX>]` (for example, `aws_instance.server[0]`,
`aws_instance.server[1]`, etc.) refers to individual instances.
This is different from resources without `count` or `for_each`, which can be
referenced without an index or key.
-> **Note:** Within nested `provisioner` or `connection` blocks, the special
`self` object refers to the current _resource instance,_ not the resource block
as a whole.
#### Using Expressions in `count`
The `count` meta-argument accepts numeric [expressions](./expressions.html).
However, unlike most resource 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).
#### When to Use `for_each` Instead of `count`
If your resource 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.
### `for_each`: Multiple Resource Instances Defined By a Map, or Set of Strings
[inpage-for_each]: #for_each-multiple-resource-instances-defined-by-a-map-or-set-of-strings
-> **Version note:** `for_each` was added in Terraform 0.12.6.
-> **Note:** A given resource block cannot use both `count` and `for_each`.
By default, a `resource` block configures one real infrastructure object.
However, sometimes you want to manage several similar objects, such as a fixed
pool of compute instances. Terraform has two ways to do this:
[`count`][inpage-count] and `for_each`.
> **Hands-on:** Try the [Manage Similar Resources With For Each](https://learn.hashicorp.com/tutorials/terraform/for-each?in=terraform/0-13&utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) tutorial on HashiCorp Learn.
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 (as described above in
[Resource Behavior](#resource-behavior)), and each is separately created,
updated, or destroyed when the configuration is applied.
-> **Note:** 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 resource during evaluation.
Map:
```hcl
resource "azurerm_resource_group" "rg" {
for_each = {
a_group = "eastus"
another_group = "westus2"
}
name = each.key
location = each.value
}
```
Set of strings:
```hcl
resource "aws_iam_user" "the-accounts" {
for_each = toset( ["Todd", "James", "Alice", "Dottie"] )
name = each.key
}
```
#### The `each` Object
In resource 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`.)
#### Using Expressions in `for_each`
The `for_each` meta-argument accepts map or set [expressions](./expressions.html).
However, unlike most resource arguments, the `for_each` value must be known
_before_ Terraform 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. When providing a set, you must use an expression that
explicitly returns a set value, like the [`toset`](/docs/configuration/functions/toset.html)
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 Terraform 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](./functions/flatten.html#flattening-nested-structures-for-for_each).
* Produce an exhaustive list of combinations of elements from two or more
collections by
[using the `setproduct` function inside a `for` expression](./functions/setproduct.html#finding-combinations-for-for_each).
#### Referring to Instances
When `for_each` is set, Terraform distinguishes between the resource block itself
and the multiple _resource instances_ associated with it. Instances are
identified by a map key (or set member) from the value provided to `for_each`.
- `<TYPE>.<NAME>` (for example, `azurerm_resource_group.rg`) refers to the resource block.
- `<TYPE>.<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 without `count` or `for_each`, which can be
referenced without an index or key.
-> **Note:** Within nested `provisioner` or `connection` blocks, the special
`self` object refers to the current _resource instance,_ not the resource block
as a whole.
#### Using Sets
The Terraform language doesn't have a literal syntax for
[set values](./types.html#collection-types), but you can use the `toset`
function to explicitly convert a list of strings to a set:
```hcl
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](./variables.html) 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)
}
```
### `provider`: Selecting a Non-default Provider Configuration
[inpage-provider]: #provider-selecting-a-non-default-provider-configuration
The `provider` meta-argument specifies which provider configuration to use,
overriding Terraform's default behavior of selecting one based on the resource
type name. Its value should be an unquoted `<PROVIDER>.<ALIAS>` reference.
As described in [Provider Configuration](./providers.html), you can optionally
create multiple configurations for a single provider (usually to manage
resources in different regions of multi-region services). Each provider can have
one default configuration, and any number of alternate configurations that
include an extra name segment (or "alias").
By default, Terraform interprets the initial word in the resource type name
(separated by underscores) as the local name of a provider, and uses that
provider's default configuration. For example, the resource type
`google_compute_instance` is associated automatically with the default
configuration for the provider named `google`.
By using the `provider` meta-argument, you can select an alternate provider
configuration for a resource:
```hcl
# default configuration
provider "google" {
region = "us-central1"
}
# alternate configuration, whose alias is "europe"
provider "google" {
alias = "europe"
region = "europe-west1"
}
resource "google_compute_instance" "example" {
# This "provider" meta-argument selects the google provider
# configuration whose alias is "europe", rather than the
# default configuration.
provider = google.europe
# ...
}
```
A resource always has an implicit dependency on its associated provider, to
ensure that the provider is fully configured before any resource actions
are taken.
The `provider` meta-argument expects
[a `<PROVIDER>.<ALIAS>` reference](./providers.html#referring-to-alternate-providers),
which does not need to be quoted. Arbitrary expressions are not permitted for
`provider` because it must be resolved while Terraform is constructing the
dependency graph, before it is safe to evaluate expressions.
### `lifecycle`: Lifecycle Customizations
[inpage-lifecycle]: #lifecycle-lifecycle-customizations
The general lifecycle for resources is described above in the
[Resource Behavior](#resource-behavior) section. Some details of that behavior
can be customized using the special nested `lifecycle` block within a resource
block body:
```
resource "azurerm_resource_group" "example" {
# ...
lifecycle {
create_before_destroy = true
}
}
```
The `lifecycle` block and its contents are meta-arguments, available
for all `resource` blocks regardless of type. The following lifecycle
meta-arguments are supported:
* `create_before_destroy` (bool) - By default, when Terraform must make a
change to a resource argument that cannot be updated in-place due to
remote API limitations, Terraform will instead destroy the existing object
and then create a new replacement object with the new configured arguments.
The `create_before_destroy` meta-argument changes this behavior so that
the new replacement object is created _first,_ and then the prior object
is destroyed only once the replacement is created.
This is an opt-in behavior because many remote object types have unique
name requirements or other constraints that must be accommodated for
both a new and an old object to exist concurrently. Some resource types
offer special options to append a random suffix onto each object name to
avoid collisions, for example. Terraform CLI cannot automatically activate
such features, so you must understand the constraints for each resource
type before using `create_before_destroy` with it.
* `prevent_destroy` (bool) - This meta-argument, when set to `true`, will
cause Terraform to reject with an error any plan that would destroy the
infrastructure object associated with the resource, as long as the argument
remains present in the configuration.
This can be used as a measure of safety against the accidental replacement
of objects that may be costly to reproduce, such as database instances.
However, it will make certain configuration changes impossible to apply,
and will prevent the use of the `terraform destroy` command once such
objects are created, and so this option should be used sparingly.
Since this argument must be present in configuration for the protection to
apply, note that this setting does not prevent the remote object from
being destroyed if the `resource` block were removed from configuration
entirely: in that case, the `prevent_destroy` setting is removed along
with it, and so Terraform will allow the destroy operation to succeed.
* `ignore_changes` (list of attribute names) - By default, Terraform detects
any difference in the current settings of a real infrastructure object
and plans to update the remote object to match configuration.
The `ignore_changes` feature is intended to be used when a resource is
created with references to data that may change in the future, but should
not effect said resource after its creation. In some rare cases, settings
of a remote object are modified by processes outside of Terraform, which
Terraform would then attempt to "fix" on the next run. In order to make
Terraform share management responsibilities of a single object with a
separate process, the `ignore_changes` meta-argument specifies resource
attributes that Terraform should ignore when planning updates to the
associated remote object.
The arguments corresponding to the given attribute names are considered
when planning a _create_ operation, but are ignored when planning an
_update_. The arguments are the relative address of the attributes in the
resource. Map and list elements can be referenced using index notation,
like `tags["Name"]` and `list[0]` respectively.
```hcl
resource "aws_instance" "example" {
# ...
lifecycle {
ignore_changes = [
# Ignore changes to tags, e.g. because a management agent
# updates these based on some ruleset managed elsewhere.
tags,
]
}
}
```
Instead of a list, the special keyword `all` may be used to instruct
Terraform to ignore _all_ attributes, which means that Terraform can
create and destroy the remote object but will never propose updates to it.
Only attributes defined by the resource type can be ignored.
`ignore_changes` cannot be applied to itself or to any other meta-arguments.
The `lifecycle` settings all effect how Terraform constructs and traverses
the dependency graph. As a result, only literal values can be used because
the processing happens too early for arbitrary expression evaluation.
### `provisioner` and `connection`: Resource Provisioners
[inpage-provisioner]: #provisioner-and-connection-resource-provisioners
> **Hands-on:** To learn about more declarative ways to handle provisioning actions, try the [Provision Infrastructure Deployed with Terraform](https://learn.hashicorp.com/collections/terraform/provision?utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) collection on HashiCorp Learn.
Some infrastructure objects require some special actions to be taken after they
are created before they can become fully functional. For example, compute
instances may require configuration to be uploaded or a configuration management
program to be run before they can begin their intended operation.
Create-time actions like these can be described using _resource provisioners_.
A provisioner is another type of plugin supported by Terraform, and each
provisioner takes a different kind of action in the context of a resource
being created.
Provisioning steps should be used sparingly, since they represent
non-declarative actions taken during the creation of a resource and so
Terraform is not able to model changes to them as it can for the declarative
portions of the Terraform language.
Provisioners can also be defined to run when a resource is _destroyed_, with
certain limitations.
The `provisioner` and `connection` block types within `resource` blocks are
meta-arguments available across all resource types. Provisioners and their
usage are described in more detail in
[the Provisioners section](/docs/provisioners/index.html).
## Local-only Resources
While most resource types correspond to an infrastructure object type that
is managed via a remote network API, there are certain specialized resource
types that operate only within Terraform itself, calculating some results and
saving those results in the state for future use.
For example, local-only resource types exist for
[generating private keys](/docs/providers/tls/r/private_key.html),
[issuing self-signed TLS certificates](/docs/providers/tls/r/self_signed_cert.html),
and even [generating random ids](/docs/providers/random/r/id.html).
While these resource types often have a more marginal purpose than those
managing "real" infrastructure objects, they can be useful as glue to help
connect together other resources.
The behavior of local-only resources is the same as all other resources, but
their result data exists only within the Terraform state. "Destroying" such
a resource means only to remove it from the state, discarding its data.
## Operation Timeouts
Some resource types provide a special `timeouts` nested block argument that
allows you to customize how long certain operations are allowed to take
before being considered to have failed.
For example, [`aws_db_instance`](/docs/providers/aws/r/db_instance.html)
allows configurable timeouts for `create`, `update` and `delete` operations.
Timeouts are handled entirely by the resource type implementation in the
provider, but resource types offering these features follow the convention
of defining a child block called `timeouts` that has a nested argument
named after each operation that has a configurable timeout value.
Each of these arguments takes a string representation of a duration, such
as `"60m"` for 60 minutes, `"10s"` for ten seconds, or `"2h"` for two hours.
```hcl
resource "aws_db_instance" "example" {
# ...
timeouts {
create = "60m"
delete = "2h"
}
}
```
The set of configurable operations is chosen by each resource type. Most
resource types do not support the `timeouts` block at all. Consult the
documentation for each resource type to see which operations it offers
for configuration, if any.

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</li> </li>
<li> <li>
<a href="/docs/configuration/resources.html">Resource Blocks</a> <a href="/docs/configuration/blocks/resources/syntax.html">Resource Blocks</a>
</li>
<li>
<a href="/docs/configuration/blocks/resources/behavior.html">Resource Behavior</a>
</li>
<li>
<a href="#">Meta-Arguments</a>
<ul class="nav nav-auto-expand">
<li>
<a href="/docs/configuration/meta-arguments/depends_on.html"><code>depends_on</code></a>
</li>
<li>
<a href="/docs/configuration/meta-arguments/count.html"><code>count</code></a>
</li>
<li>
<a href="/docs/configuration/meta-arguments/for_each.html"><code>for_each</code></a>
</li>
<li>
<a href="/docs/configuration/meta-arguments/resource-provider.html"><code>provider</code></a>
</li>
<li>
<a href="/docs/configuration/meta-arguments/lifecycle.html"><code>lifecycle</code></a>
</li>
</ul>
</li> </li>
<li> <li>