opentf/website/docs/language/resources/behavior.mdx

---
description: >-
  Learn how OpenTofu uses resource blocks to create infrastructure objects.
  Also learn about resource dependencies and how to access resource attributes.
---

# 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_ an OpenTofu configuration is the process of creating, updating,
and destroying real infrastructure objects in order to make their settings
match the configuration.

## How OpenTofu Applies a Configuration

When OpenTofu creates a new infrastructure object represented by a `resource`
block, the identifier for that real object is saved in OpenTofu's
[state](../../language/state/index.mdx), 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, OpenTofu 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 an OpenTofu 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.
- _Forget_ resources that exist in the state but no longer in the configuration and are referenced in a `removed` block within 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](../../language/expressions/index.mdx) within an OpenTofu 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](../../language/data-sources/index.mdx),
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](../../language/expressions/references.mdx#references-to-resource-attributes).

## Resource Dependencies

Most resources in a configuration don't have any particular relationship, and
OpenTofu 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. OpenTofu analyses any
[expressions](../../language/expressions/index.mdx) 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 OpenTofu 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](../../language/meta-arguments/depends_on.mdx)
can explicitly specify a dependency.

You can also use the [`replace_triggered_by` lifecycle argument](#replace_triggered_by) to add dependencies between otherwise independent resources. It forces OpenTofu to replace the parent resource when there is a change to a referenced resource or resource attribute.

## Lifecycle Customizations

A `lifecycle` block inside a `resource` block allows some customization of
OpenTofu's behavior relating to instances of a resource at different phases
of its lifecycle.

```hcl
resource "example" "example" {
  # ...normal resource arguments...

  lifecycle {
    # ...lifecycle arguments...
  }
}
```

The following arguments and nested block types are supported in the `lifecycle`
block for a managed resource:

* `create_before_destroy` (bool) - By default, when OpenTofu must change
  a resource argument that cannot be updated in-place due to
  remote API limitations, OpenTofu 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. OpenTofu CLI cannot automatically activate
  such features, so you must understand the constraints for each resource
  type before using `create_before_destroy` with it.

  Note that OpenTofu propagates and applies the `create_before_destroy` meta-attribute
  behaviour to all resource dependencies. For example, if `create_before_destroy`
  is enabled on resource A but not on resource B, but resource A is dependent on
  resource B, then OpenTofu enables `create_before_destroy` for resource B
  implicitly by default and stores it in state snapshots. You cannot override
  `create_before_destroy` to `false` on resource B because that would cause
  dependency cycles in the graph.

  Destroy provisioners of this resource do not run during replacement if
  `create_before_destroy` is set to `true`. [GitHub issue #13549](https://github.com/hashicorp/terraform/issues/13549) contains more details.

* `enabled` (bool) - Controls whether a resource is created and managed by OpenTofu.
  When set to `false`, the resource is excluded from the configuration as if it didn't exist,
  and any existing infrastructure object will be destroyed. When set to `true` (the default),
  the resource operates normally.

  For more information, refer to [the `enabled` meta-argument](../../language/meta-arguments/enabled.mdx).

* `prevent_destroy` (bool) - This meta-argument, when set to `true`, will
  cause OpenTofu 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 `tofu 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
  be active, this setting cannot prevent the remote object from being destroyed
  when the `resource` block is removed from configuration entirely: in that
  case, the `prevent_destroy` setting is removed along with it, and so OpenTofu
  would allow the destroy operation to succeed.

* <span id="ignore_changes">`ignore_changes`</span> (list of attribute names) - By default, OpenTofu 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 OpenTofu, which
  OpenTofu would then attempt to "fix" on the next run. In order to make
  OpenTofu share management responsibilities of a single object with a
  separate process, the `ignore_changes` meta-argument specifies resource
  attributes that OpenTofu 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,
      ]
    }
  }
  ```

  Use the special keyword `all` instead of an attribute list to instruct
  OpenTofu to ignore changes to _all_ attributes, which means that OpenTofu 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.

* `precondition` and `postcondition` blocks, as described in
  [Custom Conditions](../../language/expressions/custom-conditions.mdx#preconditions-and-postconditions).

* <span id="replace_triggered_by">`replace_triggered_by`</span> (list of resource or attribute references) -
  Replaces the resource when any of the referenced
  items change. Write a list of expressions referencing managed resources,
  resource instances, or instance attributes. When used in a resource that
  uses `count` or `for_each`, you can use `count.index` or `each.key` in the
  expression to refer to specific instances of other resources that are
  configured with the same count or collection.

  References trigger replacement in the following conditions:

  - If the reference is to a resource with multiple instances, a plan to
    update or replace any instance will trigger replacement.
  - If the reference is to a single resource instance, a plan to update or
    replace that instance will trigger replacement.
  - If the reference is to a single attribute of a resource instance, any
    change to the attribute value will trigger replacement.

  You can only refer to managed resources in `replace_triggered_by`
  expressions. This lets you modify these expressions without forcing
  replacement.

  ```hcl
  resource "aws_appautoscaling_target" "ecs_target" {
    # ...
    lifecycle {
      replace_triggered_by = [
        # Replace `aws_appautoscaling_target` each time this instance of
        # the `aws_ecs_service` is replaced.
        aws_ecs_service.svc.id
      ]
    }
  }
  ```

  `replace_triggered_by` allows only resource addresses because the decision is
  based on the planned actions for all of the given resources. Plain values such
  as local values or input variables do not have planned actions of their own,
  but you can treat them with a resource-like lifecycle by using them with
  [the `terraform_data` resource type](tf-data.mdx).

## 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 OpenTofu 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](https://registry.terraform.io/providers/hashicorp/tls/latest/docs/resources/private_key),
[issuing self-signed TLS certificates](https://registry.terraform.io/providers/hashicorp/tls/latest/docs/resources/self_signed_cert),
and even [generating random ids](https://registry.terraform.io/providers/hashicorp/random/latest/docs/resources/id).
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 OpenTofu state. "Destroying" such
a resource means only to remove it from the state, discarding its data.