terraform/website/docs/language/modules/develop/refactoring.html.md

---
layout: "language"
page_title: "Refactoring"
sidebar_current: "docs-modules-recactoring"
description: |-
  How to make backward-compatible changes to modules already in use.
---

# Refactoring

~> **Note:** Explicit refactoring declarations with `moved` blocks will work
only in Terraform v1.1 and later. When working with earlier Terraform versions,
or for refactoring actions too complex to express as `moved` blocks, you could
use
[the `terraform state mv` CLI command](/docs/cli/commands/state/mv.html)
as a separate step instead.

In shared modules and long-lived configurations, you may eventually outgrow
your initial module structure and resource names. For example, you might decide
that what was previously one child module makes more sense as two separate
modules and move a subset of the existing resources to the new one.

Terraform compares previous state with new configuration, correlating by
each module or resource's unique address. Therefore _by default_ Terraform
understands moving or renaming an object as an intent to destroy the object
at the old address and to create a new object at the new address.

When you add `moved` blocks in your configuration to record where you've
historically moved or renamed an object, Terraform treats an existing object at
the old address as if it now belongs to the new address.

## `moved` Block Syntax

A `moved` block expects no labels and contains only `from` and `to` arguments:

```hcl
moved {
  from = aws_instance.a
  to   = aws_instance.b
}
```

The example above records that the resource currently known as `aws_instance.b`
was known as `aws_instance.a` in a previous version of this module.

Before creating a new plan for `aws_instance.b`, Terraform first checks
whether there is an existing object for `aws_instance.a` recorded in the state.
If there is an existing object, Terraform renames that object to
`aws_instance.b` and then proceeds with creating a plan. The resulting plan is
as if the object had originally been created at `aws_instance.b`, avoiding any
need to destroy it during apply.

The `from` and `to` addresses both use a special addressing syntax that allows
selecting modules, resources, and resources inside child modules. Below, we
describe several refactoring use-cases and the appropriate addressing syntax
for each situation.

* [Renaming a Resource](#renaming-a-resource)
* [Enabling `count` or `for_each` For a Resource](#enabling-count-or-for_each-for-a-resource)
* [Renaming a Module Call](#renaming-a-module-call)
* [Enabling `count` or `for_each` For a Module Call](#enabling-count-or-for_each-for-a-module-call)
* [Splitting One Module into Multiple](#splitting-one-module-into-multiple)
* [Removing `moved` blocks](#removing-moved-blocks)

## Renaming a Resource

Consider this example module with a resource configuration:

```hcl
resource "aws_instance" "a" {
  count = 2

  # (resource-type-specific configuration)
}
```

Applying this configuration for the first time would cause Terraform to
create `aws_instance.a[0]` and `aws_instance.a[1]`.

If you later choose a different name for this resource, then you can change the
name label in the `resource` block and record the old name inside a `moved` block:

```hcl
resource "aws_instance" "b" {
  count = 2

  # (resource-type-specific configuration)
}

moved {
  from = aws_instance.a
  to   = aws_instance.b
}
```

When creating the next plan for each configuration using this module, Terraform
treats any existing objects belonging to `aws_instance.a` as if they had
been created for `aws_instance.b`: `aws_instance.a[0]` will be treated as
`aws_instance.b[0]`, and `aws_instance.a[1]` as `aws_instance.b[1]`.

New instances of the module, which _never_ had an
`aws_instance.a`, will ignore the `moved` block and propose to create
`aws_instance.b[0]` and `aws_instance.b[1]` as normal.

Both of the addresses in this example referred to a resource as a whole, and
so Terraform recognizes the move for all instances of the resource. That is,
it covers both `aws_instance.a[0]` and `aws_instance.a[1]` without the need
to identify each one separately.

Each resource type has a separate schema and so objects of different types
are not compatible. Therefore, although you can use `moved` to change the name
of a resource, you _cannot_ use `moved` to change to a different resource type
or to change a managed resource (a `resource` block) into a data resource
(a `data` block).

## Enabling `count` or `for_each` For a Resource

Consider this example module containing a single-instance resource:

```hcl
resource "aws_instance" "a" {
  # (resource-type-specific configuration)
}
```

Applying this configuration would lead to Terraform creating an object
bound to the address `aws_instance.a`.

Later, you use [`for_each`](../../meta-arguments/for_each.html) with this
resource to systematically declare multiple instances. To preserve an object
that was previously associated with `aws_instance.a` alone, you must add a
`moved` block to specify which instance key the object will take in the new
configuration:

```hcl
locals {
  instances = tomap({
    big = {
      instance_type = "m3.large"
    }
    small = {
      instance_type = "t2.medium"
    }
  })
}

resource "aws_instance" "a" {
  for_each = local.instances

  instance_type = each.value.instance_type
  # (other resource-type-specific configuration)
}

moved {
  from = aws_instance.a
  to   = aws_instance.a["small"]
}
```

The above will keep Terraform from planning to destroy any existing object at
`aws_instance.a`, treating that object instead as if it were originally
created as `aws_instance.a["small"]`.

When at least one of the two addresses includes an instance key, like
`["small"]` in the above example, Terraform understands both addresses as
referring to specific _instances_ of a resource rather than the resource as a
whole. That means you can use `moved` to switch between keys and to add and
remove keys as you switch between `count`, `for_each`, or neither.

The following are some other examples of valid `moved` blocks that record
changes to resource instance keys in a similar way:

```hcl
# Both old and new configuration used "for_each", but the
# "small" element was renamed to "tiny".
moved {
  from = aws_instance.b["small"]
  to   = aws_instance.b["tiny"]
}

# The old configuration used "count" and the new configuration
# uses "for_each", with the following mappings from
# index to key:
moved {
  from = aws_instance.c[0]
  to   = aws_instance.c["small"]
}
moved {
  from = aws_instance.c[1]
  to   = aws_instance.c["tiny"]
}

# The old configuration used "count", and the new configuration
# uses neither "count" nor "for_each", and you want to keep
# only the object at index 2.
moved {
  from = aws_instance.d[2]
  to   = aws_instance.d
}
```

-> **Note:** When you add `count` to an existing resource that didn't use it,
Terraform automatically proposes to move the original object to instance zero,
unless you write an `moved` block explicitly mentioning that resource.
However, we recommend still writing out the corresponding `moved` block
explicitly, to make the change clearer to future readers of the module.

## Renaming a Module Call

You can rename a call to a module in a similar way as renaming a resource.
Consider the following original module version:

```hcl
module "a" {
  source = "../modules/example"

  # (module arguments)
}
```

When applying this configuration, Terraform would prefix the addresses for
any resources declared in this module with the module path `module.a`.
For example, a resource `aws_instance.example` would have the full address
`module.a.aws_instance.example`.

If you later choose a better name for this module call, then you can change the
name label in the `module` block and record the old name inside a `moved` block:

```hcl
module "b" {
  source = "../modules/example"

  # (module arguments)
}
```

When creating the next plan for each configuration using this module, Terraform
will treat any existing object addresses beginning with `module.a` as if
they had instead been created in `module.b`. `module.a.aws_instance.example`
would be treated as `module.b.aws_instance.example`.

Both of the addresses in this example referred to a module call as a whole, and
so Terraform recognizes the move for all instances of the call. If this
module call used `count` or `for_each` then it would apply to all of the
instances, without the need to specify each one separately.

## Enabling `count` or `for_each` For a Module Call

Consider this example of a single-instance module:

```hcl
module "a" {
  source = "../modules/example"

  # (module arguments)
}
```

Applying this configuration would cause Terraform to create objects whose
addresses begin with `module.a`.

In later module versions, you may need to use
[`count`](../../meta-arguments/count.html) with this resource to systematically
declare multiple instances. To preserve an object that was previously associated
with `aws_instance.a` alone, you can add a `moved` block to specify which
instance key that object will take in the new configuration:

```hcl
module "a" {
  source = "../modules/example"
  count  = 3

  # (module arguments)
}

moved {
  from = module.a
  to   = module.a[2]
}
```

The configuration above directs Terraform to treat all objects in `module.a` as
if they were originally created in `module.a[2]`. As a result, Terraform plans
to create new objects only for `module.a[0]` and `module.a[1]`.

When at least one of the two addresses includes an instance key, like
`[2]` in the above example, Terraform will understand both addresses as
referring to specific _instances_ of a module call rather than the module
call as a whole. That means you can use `moved` to switch between keys and to
add and remove keys as you switch between `count`, `for_each`, or neither.

For more examples of recording moves associated with instances, refer to
the similar section
[Enabling `count` and `for_each` For a Resource](#enabling-count-or-for_each-for-a-resource).

# Splitting One Module into Multiple

As a module grows to support new requirements, it might eventually grow big
enough to warrant splitting into two separate modules.

Consider this example module:

```hcl
resource "aws_instance" "a" {
  # (other resource-type-specific configuration)
}

resource "aws_instance" "b" {
  # (other resource-type-specific configuration)
}

resource "aws_instance" "c" {
  # (other resource-type-specific configuration)
}
```

You can split this into two modules as follows:

* `aws_instance.a` now belongs to module "x".
* `aws_instance.b` also belongs to module "x".
* `aws_instance.c` belongs module "y".

To achieve this refactoring without replacing existing objects bound to the
old resource addresses, you must:

1. Write module "x", copying over the two resources it should contain.
2. Write module "y", copying over the one resource it should contain.
3. Edit the original module to no longer include any of these resources, and
   instead to contain only shim configuration to migrate existing users.

The new modules "x" and "y" should contain only `resource` blocks:

```hcl
# module "x"

resource "aws_instance" "a" {
  # (other resource-type-specific configuration)
}

resource "aws_instance" "b" {
  # (other resource-type-specific configuration)
}
```

```hcl
# module "y"

resource "aws_instance" "c" {
  # (other resource-type-specific configuration)
}
```

The original module, now only a shim for backward-compatibility, calls the
two new modules and indicates that the resources moved into them:

```hcl
module "x" {
  source = "../modules/x"

  # ...
}

module "y" {
  source = "../modules/y"

  # ...
}

moved {
  from = aws_instance.a
  to   = module.x.aws_instance.a
}

moved {
  from = aws_instance.b
  to   = module.x.aws_instance.b
}

moved {
  from = aws_instance.c
  to   = module.y.aws_instance.c
}
```

When an existing user of the original module upgrades to the new "shim"
version, Terraform notices these three `moved` blocks and behaves
as if the objects associated with the three old resource addresses were
originally created inside the two new modules.

New users of this family of modules may use either the combined shim module
_or_ the two new modules separately. You may wish to communicate to your
existing users that the old module is now deprecated and so they should use
the two separate modules for any new needs.

The multi-module refactoring situation is unusual in that it violates the
typical rule that a parent module sees its child module as a "closed box",
unaware of exactly which resources are declared inside it. This compromise
assumes that all three of these modules are maintained by the same people
and distributed together in a single
[module package](../sources.html#modules-in-package-sub-directories).

To reduce [coupling](https://en.wikipedia.org/wiki/Coupling_(computer_programming))
between separately-packaged modules, Terraform only allows declarations of
moves between modules in the same package. In other words, Terraform would
not have allowed moving into `module.x` above if the `source` address of
that call had not been a [local path](../sources.html#local-paths).

Terraform resolves module references in `moved` blocks relative to the module
instance they are defined in. For example, if the original module above were
already a child module named `module.original`, the reference to
`module.x.aws_instance.a` would resolve as
`module.original.module.x.aws_instance.a`. A module may only make `moved`
statements about its own objects and objects of its child modules.

If you need to refer to resources within a module that was called using
`count` or `for_each` meta-arguments, you must specify a specific instance
key to use in order to match with the new location of the resource
configuration:

```hcl
moved {
  from = aws_instance.example
  to   = module.new[2].aws_instance.example
}
```

## Removing `moved` Blocks

Over time, a long-lasting module may accumulate many `moved` blocks. 

Removing a `moved` block is a generally breaking change because any configurations that refer to the old address will plan to delete that existing object instead of move it. We strongly recommend that you retain all historical `moved` blocks from earlier versions of your modules to preserve the upgrade path for users of any previous version. 

If you do decide to remove `moved` blocks, proceed with caution. It can be safe to remove `moved` blocks when you are maintaining private modules within an organization and you are certain that all users have successfully run `terraform apply` with your new module version.

If you need to rename or move the same object twice, we recommend documenting the full history
using _chained_ `moved` blocks, where the new block refers to the existing block:


```hcl
moved {
  from = aws_instance.a
  to   = aws_instance.b
}

moved {
  from = aws_instance.b
  to   = aws_instance.c
}
```

Recording a sequence of moves in this way allows for successful upgrades for
both configurations with objects at `aws_instance.a` _and_ configurations with
objects at `aws_instance.b`. In both cases, Terraform treats the existing
object as if it had been originally created as `aws_instance.c`.