terraform/website/upgrade-guides/0-12.html.markdown

---
layout: "language"
page_title: "Upgrading to Terraform 0.12"
sidebar_current: "upgrade-guides-0-12"
description: |-
  Upgrading to Terraform v0.12
---

# Upgrading to Terraform v0.12

[Terraform v0.12 is a major release](https://hashicorp.com/blog/terraform-0-1-2-preview)
focused on configuration language improvements and thus includes some
changes that you'll need to consider when upgrading. The goal of this guide is
to cover the most common upgrade concerns and issues.

For most users, upgrading configuration should be completely automatic. Some
simple configurations will require no changes at all, and most other
configurations can be prepared by running
[the automatic upgrade tool](/docs/commands/0.12upgrade.html). Please read on
for more information and recommendations on the upgrade process.

-> If you are a developer maintaining a provider plugin, please see
[the documentation on 0.12 compatibility for providers](/docs/extend/terraform-0.12-compatibility.html)
to learn more about the changes that are required.


## Upgrade to Terraform 0.11 first

We strongly recommend completing an upgrade to the latest Terraform v0.11
release first. This will give you an opportunity to address any changes
required for the previous major version upgrades separately, rather than
making multiple changes at once.

In particular, if you are upgrading from a Terraform version prior to v0.9,
you _must_ first [upgrade to Terraform v0.9](/upgrade-guides/0-9.html) and
switch to initializing with `terraform init`, because v0.12 no longer includes
the functionality for automatically migrating from the legacy remote state
mechanism.

This guide focuses on changes from v0.11 to v0.12. Each previous major release
has its own upgrade guide, so please consult the other guides (available in the
navigation) to upgrade step-by-step to v0.11 first.

Terraform v0.11.14 (and any subsequent v0.11 releases) also include some
additional functionality to help smooth the upgrade, which we will use later
in this guide.

Prior versions of Terraform are available from
[the releases server](https://releases.hashicorp.com/terraform/).

## Pre-upgrade Checklist

Terraform v0.11.14 introduced a temporary helper command
`terraform 0.12checklist`, which analyzes your configuration to detect any
required steps that will be easier to perform before upgrading.

To use it, first upgrade to [Terraform v0.11.14](https://releases.hashicorp.com/terraform/0.11.14/).
Then, perform the following steps:

* `terraform init` to ensure your working directory is fully initialized and
  all required plugins are installed and selected.
* `terraform apply` to ensure that your real infrastructure and Terraform
  state are consistent with the current configuration. The instructions
  produced by the checklist command assume that configuration and state are
  synchronized.
* `terraform 0.12checklist` to see if there are any pre-upgrade steps in the
  checklist.

If all is well, the final command will produce a message like this:

```
Looks good! We did not detect any problems that ought to be
addressed before upgrading to Terraform v0.12

This tool is not perfect though, so please check the v0.12 upgrade
guide for additional guidance, and for next steps:
    https://www.terraform.io/upgrade-guides/0-12.html
```

As the message suggests, the next step in that case is to read the remainder
of this page to prepare for and carry out the upgrade.

However, the checklist command may instead produce a list of one or more tasks
that we recommend you perform before upgrading to Terraform 0.12, because they
are easier to perform with a fully-functional Terraform 0.11 than with a
Terraform 0.12 that has encountered compatibility problems.

The tasks it may suggest you perform could include:

* Upgrading any provider versions that are not compatible with Terraform v0.12.
  We recommend upgrading to the latest version of each provider before upgrading
  because that will avoid changing many things in one step.
* Renaming any resources or provider aliases that have names that start with
  digits, because that is no longer valid in Terraform 0.12.
* Upgrading any external modules the configuration uses which themselves have
  the above problems.

In each case, the tool will give some direction on how to perform the task it
is suggesting.

The output from `terraform 0.12checklist` is in Markdown format so that it can
easily be pasted into a Markdown-compatible issue tracker, should you want
to track the necessary tasks or share the work with other team members.

After all of the tasks are complete, run `terraform 0.12checklist` one more time
to verify that everything is complete. If so, continue reading the following
sections to complete the upgrade!

### Addendum: Invalid module names

There is one additional pre-upgrade checklist item that the Terraform team did
not become aware of until after the release of Terraform v0.11.14, and thus
cannot be detected automatically by the checklist tool: renaming modules which
have names that start with digits.

Terraform 0.11 inadvertently tolerated leading-digit names for modules as a
result of a validation bug, but Terraform 0.12 has corrected that bug and will
reject such module names. Unfortunately, module names are also recorded in
state snapshots and so a state snapshot created for a configuration with an
invalid module name will itself be invalid as far as Terraform 0.12 is
concerned.

You can address this in a similar way to what the checklist tool suggests for
invalid resource names and provider aliases:

* Rename the module in your configuration.
* Use `terraform state mv module.old module.new` _in Terraform 0.11.14_ to
  update the state to use the new name instead of the old name.

As with all of the pre-upgrade checklist items, be sure to run `terraform apply`
once more before upgrading in order to ensure that the latest state snapshot is
synchronized with the latest configuration.

## Upgrading to Terraform 0.12

Before switching to Terraform 0.12, we recommend using Terraform v0.11.14 (or
any later v0.11 release) to perform one last `terraform init` and
`terraform apply` to ensure that everything is initialized and synchronized.

Once `terraform apply` shows no changes pending, switch over to a Terraform
v0.12 release and run `terraform init` again to upgrade the working directory
metadata to v0.12 format. (Once you've done this, you'll need to delete the
`.terraform` directory if you wish to return to Terraform v0.11, but no
real infrastructure or persisted state will be upgraded yet.)

It is possible that your configuration may be using configuration constructs
that are not Terraform v0.12 compatible and thus require upgrade. In that case,
`terraform init` will produce the following message:

```
Terraform has initialized, but configuration upgrades may be needed.

Terraform found syntax errors in the configuration that prevented full
initialization. If you've recently upgraded to Terraform v0.12, this may be
because your configuration uses syntax constructs that are no longer valid,
and so must be updated before full initialization is possible.

Terraform has installed the required providers to support the configuration
upgrade process. To begin upgrading your configuration, run the following:
    terraform 0.12upgrade

To see the full set of errors that led to this message, run:
    terraform validate
```

As mentioned in the message, Terraform has partially initialized the directory
just enough to perform the configuration upgrade process, which is described
in the following section.

We recommend running the configuration upgrade tool even if you do not see
the above message, because it may detect and fix constructs that are
syntactically correct but still need some changes to work as expected with
Terraform v0.12.

## Upgrading Terraform configuration

Terraform v0.12 includes a new command `terraform 0.12upgrade` that will
read the configuration files for a module written for Terraform 0.11 and
update them in-place to use the cleaner Terraform 0.12 syntax and also
adjust for use of features that have changed behavior in the 0.12 Terraform
language.

Simple configuration files are likely to be understood by Terraform 0.12 as-is,
because the language is still broadly compatible, but we recommend that everyone
run the upgrade tool nonetheless. Even if your configuration is already
compatible, the tool will update your configuration to use the cleaner syntax
available in Terraform 0.12, which should improve readability.

To run the command, first make sure that your local working directory is synced
with your version control system so that there are no changes outstanding. This
will make it easier to review the changes that the upgrade tool is proposing,
using the diff feature of your version control system.

With a fully-initialized working directory (all necessary providers and child
modules installed), run `terraform 0.12upgrade` to begin the process. By default
it will print some information about what it is about to do and prompt for
confirmation:

```
This command will rewrite the configuration files in the given directory so
that they use the new syntax features from Terraform v0.12, and will identify
any constructs that may need to be adjusted for correct operation with
Terraform v0.12.

We recommend using this command in a clean version control work tree, so that
you can easily see the proposed changes as a diff against the latest commit.
If you have uncommitted changes already present, we recommend aborting this
command and dealing with them before running this command again.

Would you like to upgrade the module in the current directory?
```

If you answer yes, the `.tf` and `.tfvars` files in your current working
directory will be rewritten in-place.

The upgrade tool may also print out warnings about constructs it wasn't able to
migrate fully automatically; in that case, it will also emit comments into the
rewritten source files containing the special marker `TF-UPGRADE-TODO`, as
a prompt for a decision you'll need to make to complete the upgrade.

Once the upgrade tool has successfully completed and you've resolved any
`TF-UPGRADE-TODO` prompts, use your version control tool to review the proposed
changes and then run `terraform plan` to see the effect of those changes.

In most cases, `terraform plan` should report that no changes are required,
because the updated configuration is equivalent to before.

The remaining sections below describe both some common changes that the upgrade
tool is able to make automatically, and some other upgrade situations that
the configuration tool may not be able to fully resolve. If you encounter
any errors during the upgrade or during the subsequent `terraform plan`, the
sections below may give some additional context for how to proceed.

Once you're happy with the updated configuration, commit it to version control
in the usual way and apply it with Terraform 0.12.

### Remote state references

The `terraform_remote_state` data source has changed slightly for the v0.12
release to make all of the remote state outputs available as a single map
value, rather than as top-level attributes as in previous releases.

In previous releases, a reference to a `vpc_id` output exported by the remote
state data source might have looked like this:

```hcl
data.terraform_remote_state.vpc.vpc_id
```

This value must now be accessed via the new `outputs` attribute:

```hcl
data.terraform_remote_state.vpc.outputs.vpc_id
```

The upgrade tool will rewrite remote state references automatically to include
the additional `outputs` attribute.

Where appropriate, you can also access the outputs attribute directly to
work with the whole map as a single value:

```hcl
data.terraform_remote_state.vpc.outputs
```

Another consideration for `terraform_remote_state` is that this data source must
be able to parse the latest state snapshot for a separate Terraform
configuration that may have been updated by a newer version of Terraform.
To provide flexibility when upgrading decomposed environments that use
`terraform_remote_state`, Terraform v0.11.14 introduced support for reading
outputs from the Terraform v0.12 state format, so if you upgrade all of your
configurations to Terraform v0.11.14 first you can then perform v0.12 upgrades
of individual configurations in any order, without breaking
`terraform_remote_state` usage.

Note that the `config` block should now be in the form of an assignment with the `=` sign:

```hcl
data "terraform_remote_state" "default" {
  backend = "gcs"
  config = {
    bucket = "..."
  }
}
```

### Attributes vs. blocks

Terraform resource configurations consist of both arguments that set
individual properties of the main object being described, and nested blocks
which declare zero or more other objects that are modeled as being part of
their parent. For example:

```hcl
resource "aws_instance" "example" {
  instance_type = "t2.micro"
  ami           = "ami-abcd1234"

  tags = {
    Name = "example instance"
  }

  ebs_block_device {
    device_name = "sda2"
    volume_type = "gp2"
    volume_size = 24
  }
}
```

In the above resource, `instance_type`, `ami`, and `tags` are both direct
arguments of the `aws_instance` resource, while `ebs_block_device` describes
a separate EBS block device object that is connected to the parent instance.

Due to the design of the configuration language decoder in Terraform v0.11 and
earlier, it was in many cases possible to interchange the argument syntax
(with `=`) and the block syntax (with just braces) when dealing with map
arguments vs. nested blocks. However, this led to some subtle bugs and
limitations, so Terraform v0.12 now requires consistent usage of argument
syntax for arguments and nested block syntax for nested blocks.

In return for this new strictness, Terraform v0.12 now allows map keys to be
set dynamically from expressions, which is a long-requested feature. The
main difference between a map attribute and a nested block is that a map
attribute will usually have user-defined keys, like we see in the `tags`
example above, while a nested block always has a fixed set of supported
arguments defined by the resource type schema, which Terraform will validate.

The configuration upgrade tool uses the provider's schema to recognize the
nature of each construct and will select the right syntax automatically. For
most simple usage, this will just involve adding or removing the equals sign
as appropriate.

A more complicated scenario is where users found that they could exploit this
flexibility to -- with some caveats -- dynamically generate nested blocks even
though this wasn't intentionally allowed:

```hcl
  # Example of no-longer-supported workaround from 0.11 and earlier
  ebs_block_device = "${concat(map("device_name", "sda4"), var.extra_block_devices)}"
```

Terraform v0.12 now includes a first-class feature for dynamically generating
nested blocks using expressions, using the special `dynamic` block type. The
above can now be written like this, separating the static block device from
the dynamic ones:

```hcl
  ebs_block_device {
    device_name = "sda4"
  }
  dynamic "ebs_block_device" {
    for_each = var.extra_block_devices
    content {
      device_name = ebs_block_device.value.device_name
      volume_type = ebs_block_device.value.volume_type
      volume_size = ebs_block_device.value.volume_size
    }
  }
```

The configuration upgrade tool will detect use of the above workaround and
rewrite it as a `dynamic` block, but it may make non-ideal decisions for how to
flatten your expression down into static vs. dynamic blocks, so we recommend
reviewing the generated `dynamic` blocks to see if any simplifications are
possible.

Terraform v0.12 now also requires that each argument be set only once within
a particular block, whereas before Terraform would either take the last
definition or, in some cases, attempt to merge together multiple definitions
into a list. The upgrade tool does not remove or attempt to consolidate
any existing duplicate arguments, but other commands like `terraform validate`
will detect and report these after upgrading.

### Integer vs. Float Number Types

From Terraform v0.12, the Terraform language no longer distinguishes between
integer and float types, instead just having a single "number" type that can
represent high-precision floating point numbers. This new type can represent
any value that could be represented before, plus many new values due to the
expanded precision.

In most cases this change should not cause any significant behavior change, but
please note that in particular the behavior of the division operator is now
different: it _always_ performs floating point division, whereas before it
would sometimes perform integer division by attempting to infer intent from
the argument types.

If you are relying on integer division behavior in your configuration, please
use the `floor` function to obtain the previous result. A common place this
would arise is in index operations, where the index is computed by division:

```hcl
  example = var.items[floor(count.index / var.any_number)]
```

Using a fractional number to index a list will produce an error telling you
that this is not allowed, serving as a prompt to add `floor`:

```
Error: Invalid index

The given key does not identify an element in this collection value: indexing a
sequence requires a whole number, but the given index (0.5) has a fractional
part.
```

Unfortunately the automatic upgrade tool cannot apply a fix for this case
because it does not have enough information to know if floating point or integer
division was intended by the configuration author, so this change must be made
manually where needed.

### Referring to List Variables

In early versions of Terraform, before list support became first-class, we
required using seemingly-redundant list brackets around a single expression
in order to hint to the language interpreter that a list interpretation was
desired:

```hcl
  # Example for older versions of Terraform; not valid for v0.12
  example = ["${var.any_list}"]
```

This strange requirement was subsequently lifted after the introduction of
first-class list support, but we retained compatibility with this older usage
for a transitional period by including some fixup logic that would detect when
list brackets contain list expressions and automatically flatten to a single
list.

As part of implementing the first-class expressions support for v0.12, we needed
to finally remove that backward-compatibility mechanism to avoid ambiguity
in the language, so an expression like the above will now produce a list of
lists and thus produce a type checking error for any argument that was expecting
a list of some other type.

The upgrade tool is able to recognize most simple usage of this pattern and
rewrite automatically to just refer to the list directly:

```hcl
  example = var.any_list
```

However, an unintended side-effect of this compatibility mechanism was to
also flatten mixed lists of single-value and list expressions into a single
list automatically. We didn't intend for this to be a part of the language, but
in retrospect it was an obvious consequence of how the compatibility mechanism
was implemented. If you have expressions in your modules that produce a list
of strings by using list brackets with a mixture of string and list-of-string
sub-expressions, you will need to rewrite this to explicitly use
[the `flatten` function](/docs/language/functions/flatten.html)
to make the special treatment more obvious to the reader:

```hcl
  example = flatten([
    "single string",
    var.any_list,
  ])
```

The configuration upgrade tool unfortunately cannot make this change
automatically, because it doesn't have enough information to know for certain
which interpretation was intended for a given list.

For complex examples that the upgrade tool is not able to adjust automatically,
subsequent Terraform operations may produce an error message like the following:

```
Error: Incorrect attribute value type

  on redundant-list-brackets.tf line 9, in resource "aws_security_group" "foo":
   9:     cidr_blocks = ["${var.cidr_blocks}"]

Inappropriate value for attribute "cidr_blocks": element 0: string required.
```

This message is reporting that Terraform has understood this expression as a
list of lists, and therefore element zero is a list rather than a string. To
fix the error, remove the redundant list brackets and possibly add a
`flatten` function call as described above, for more complex cases.

### Reserved Variable Names

In preparation for new features planned for future releases, Terraform 0.12
reserves some additional names that can no longer be used as input variable
names for modules. These reserved names are:

* `count`
* `depends_on`
* `for_each`
* `lifecycle`
* `providers`
* `source`

When any of these names is used as the label of a `variable` block, Terraform
will now generate the following error:

```
Error: Invalid variable name

  on reserved-variable-names.tf line 2, in variable "count":
   2: variable "count" {

The variable name "count" is reserved due to its special meaning inside module
blocks.
```

The upgrade tool cannot automatically adjust for these reserved names, because
it does not know what new name would be more appropriate. To proceed, you must
unfortunately rename these input variables and make a new major release of
the module in question, since renaming input variables is a breaking change.

### Type Constraints on Variables

In Terraform v0.11, variables were documented as accepting only strings, lists
of strings, and maps of strings. However, in practice Terraform permitted
lists of lists and lists of maps and other nested structures in some cases,
even though it was then generally inconvenient to work with those values
elsewhere in the module due to limitations of the index syntax, `element`
function, and `lookup` function.

Terraform now allows various [type constraints](/docs/language/expressions/type-constraints.html)
to be specified, as part of the language's new type system and generalized
functions and operators. However, because lists and maps of non-string values
were not officially supported in 0.11, existing configurations do not have
enough information for the upgrade tool to know what element type was intended.
It will therefore assume that lists and maps are of strings as documented,
which will be incorrect for configurations using more complex structures. The
result will be one of the following error messages:

```
Error: Invalid default value for variable

  on child_module/example.tf line 4, in variable "example":
   4:   default = [
   5:     {
   6:       "foo" = "bar"
   7:     },
   8:   ]

This default value is not compatible with the variable's type constraint:
element 0: string required.
```

```
Error: Invalid value for module argument

  on variables-incorrect-elem-type.tf line 4, in module "child":
   4:   example = [
   5:     {
   6:       "foo" = "bar"
   7:     },
   8:   ]

The given value is not suitable for child module variable "example" defined at
child/child.tf:1,1-19: element 0: string required.
```

To fix this, change the `type` argument from `list(string)` or `map(string)`
to a more appropriate [type constraint](/docs/language/expressions/type-constraints.html).

If you're not sure what type constraint to use yet, another option is to
use the type constraint `any`, which will effectively disable validation and
allow any value. We recommend using specific types where possible, but selecting
`any` during upgrade may be preferable, so that the work to select and define
a more precise type can be saved for a later change at your leisure, once
upgrading is complete.

### Working with `count` on resources

The `count` feature allows declaration of multiple instances of a particular
resource constructed from the same configuration. In Terraform v0.11, any
use of `count` would generally lead to referring to the resource in question
using the "splat expression" syntax elsewhere in the configuration:

```
aws_instance.example.*.id[0]
```

Because `aws_instance.example` itself was not directly referencable in
Terraform v0.11, the expression system allowed some flexibility in how such
expressions were resolved. For example, Terraform would treat
`aws_instance.example.id` as an alias for `aws_instance.example.*.id[0]`.

Terraform v0.12 allows referring to an entire resource as an object value,
but that required making a decision on what type of value is returned by
`aws_instance.example`. The new rules are as follows:

* For resources where `count` is _not_ set, a reference like
  `aws_instance.example` returns a single object, whose attributes can be
  accessed in the usual way, like `aws_instance.example.id`.

* For resources where `count` _is_ set -- even if the expression evaluates to
  `1` -- `aws_instance.example` returns a list of objects whose length is
  decided by the count. In this case `aws_instance.example.id` is an error,
  and must instead be written as `aws_instance.example[0].id` to access
  one of the objects before retrieving its `id` attribute value.

The splat syntax is still available and will still be useful in situations
where a list result is needed, but we recommend updating expressions like
`aws_instance.example.*.id[count.index]` to instead be
`aws_instance.example[count.index].id`, which should be easier to read and
understand for those who are familiar with other languages.

Another consequence of the new handling of `count` is that you can use the
`length` function directly with references to resources that have `count` set:

```
length(aws_instance.example)
```

This replaces the v0.11 special case of `aws_instance.example.count`, which
can no longer be supported due to `aws_instance.example` being a list.

The upgrade tool will automatically detect references that are inconsistent
with the `count` setting on the target resource and rewrite them to use the
new syntax. The upgrade tool will _not_ rewrite usage of splat syntax to
direct index syntax, because the old splat syntax form is still compatible.

Another `count`-related change is that Terraform now requires `count` to be
assigned a numeric value, and will not automatically convert a boolean value
to a number in the interests of clarity. If you wish to use a boolean value
to activate or deactivate a particular resource, use the conditional operator
to show clearly how the boolean value maps to a number value:

```hcl
  count = var.enabled ? 1 : 0
```

### First-class expressions

Terraform v0.11 and earlier allowed expressions only within interpolation
sequences, like `"${var.example}"`. Because expressions are such an important
part of Terraform -- they are the means by which we connect the attributes of
one resource to the configuration of another -- Terraform v0.12 now allows
you to use expressions directly when defining most attributes.

```
  ami = var.ami
```

The generalization of expression handling also has some other benefits. For
example, it's now possible to directly construct lists and maps within
expressions using the normal syntax, whereas in Terraform v0.11 we required
using the `list` and `map` functions:

```
  # Old 0.11 example
  tags = "${merge(map("Name", "example"), var.common_tags)}"

  # Updated 0.12 example
  tags = merge({ Name = "example" }, var.common_tags)
```

The automatic upgrade tool will perform rewrites like these automatically,
making expressions easier to read and understand.

### Default settings in `connection` blocks

Terraform v0.11 and earlier allowed providers to pre-populate certain arguments
in a `connection` block for use with remote provisioners. Several resource
type implementations use this to pre-populate `type` as `"ssh"` and `host`
as one of the IP addresses of the compute instance being created.

While that feature was convenient in some cases, we found that in practice it
was hard for users to predict how it would behave, since each provider had its
own rules for whether to prefer public vs. private IP addresses, which network
interface to use, whether to use IPv4 or IPv6, etc.

It also violated our design principle of "explicit is better than implicit": we
think it's important that someone who is unfamiliar with a particular Terraform
configuration (or with Terraform itself) to be able to read the configuration
and make a good guess as to what it will achieve, and the default connection
settings feature left an important detail unstated: how do the provisioners
access the host?

With this in mind, Terraform v0.12 no longer performs any automatic population
of `connection` blocks. Instead, if you are using any remote provisioners you
should explicitly set the connection type and the hostname to connect to:

```hcl
  connection {
    type = "ssh"
    host = self.public_ip
    # ...
  }
```

The automatic upgrade tool will detect existing `connection` blocks that are
lacking these settings within resource types that are known to have previously
set defaults, and it will write out an expression that approximates whatever
selection logic the provider was previously doing in its own implementation.

Unfortunately in some cases the provider did not export the result of the
possibly-rather-complex host selection expression as a single attribute, and so
for some resource types the generated `host` expression will be quite
complicated. We recommend reviewing these and replacing them with a simpler
expression where possible, since you will often know better than Terraform does
which of the instance IP addresses are likely to be accessible from the host
where Terraform is running.

### Equality operations must be valid on value and type

In 0.11, `"1"` would compare truthfully against `1`, however, in 0.12,
values must be equal on both value and type in order to be true. That is, in 0.11
you would see:

```
> "1" == 1
true
```

and in 0.12:

```
> "1" == 1
false
```

This means special care should be taken if you have any conditionals comparing to say,
`count.index` where you were previously expecting it to be a string, when it is now a number.

This is a scenario where you would need to update existing 0.11 code to work as you expect in 0.12:

```
resource "server_instance" "app" {
    server_status = "${count.index == local.prod_index ? "production" : "standby"}"
  }
}

locals {
  # when migrating to 0.12, be sure to change this value to a number
  # to ensure expected behavior
  prod_index = "0"
}
```

Also take care that if you have a variable that is a number, but defined as a string,
the upgrade tool will not change it to a number, so take care to inspect your code:

```
locals {
  some_count = "3" # will not be changed to a number after config upgrade
}
```

## Upgrades for reusable modules

If you are making upgrades to a reusable module that is consumed by many
different configurations, you may need to take care with the timing of your
upgrade and of how you publish it.

We strongly recommend using module versioning, either via a Terraform registry
or via version control arguments in your module source addresses, to pin
existing references to the old version of the module and then publish the
upgraded version under a new version number. If you are using semantic
versioning, such as in a Terraform registry, the updates made by the upgrade
tool should be considered a breaking change and published as a new major
version.

The migration tool will automatically add a `>= 0.12.0` Terraform version
constraint to indicate that the module has been upgraded to use v0.12-only
features. By using version constraints, users can gradually update their callers
to use the newly-upgraded version as they begin to use Terraform v0.12 with
those modules.

For simpler modules it may be possible to carefully adapt them to be both
0.11 and 0.12 compatible at the same time, by following the upgrade notes in
earlier sections and avoiding any v0.12-only features. However, for any module
using a undocumented workarounds for v0.11 limitations it is unlikely to be
possible to both update it for Terraform v0.12 and retain v0.11 compatibility
at the same time, because those undocumented workarounds have been replaced
with new features in Terraform v0.12.

## Map variables no longer merge when overridden

In prior versions of Terraform, a variable of type `"map"` had a special
behavior where any value provided via mechanisms such as the `-var` command
line option would be keywise-merged with any default value associated with
the variable. This was useful in early versions of Terraform that lacked
mechanisms for doing such merging explicitly, but since Terraform v0.10
introduced the concept of local values we consider it preferable to perform
such merges manually so that they are explicit in configuration:

```
variable "example_map" {
  type = map(string)
  default = {}
}

locals {
  default_map_keys = {
    "a" = "b"
  }
  merged_map_keys = merge(local.default_map_keys, var.example_map)
}
```

In order to improve the consistency of variable handling across types, the
map variable merging behavior is removed in Terraform v0.12. Because this
mechanism was driven by command line options rather than configuration, the
automatic upgrade tool cannot automatically handle it. If you are relying on
the merging feature, you must reorganize your configuration to use explicit
merging like in the above example, or else your default map value will be
entirely overridden by any explicitly-set value.

## Upgrading `remote` Backend Configuration

Terraform Cloud and Terraform Enterprise users will need
to run `terraform init -reconfigure` to upgrade to Terraform 0.12.

Terraform provides a message stating that `terraform init` is required; while
there is no harm in running this command, the next error message will clarify
that `terraform init -reconfigure` is required.

## Upgrading Sentinel policies

The Terraform Sentinel imports have been updated to work with Terraform 0.12.
Care has been taken to ensure that the API is as backwards compatible as
possible and most policies will continue to work without modification. However,
there are some important changes and certain policies will need to modified.

More information on the changes can be found in our page on [using Sentinel with
Terraform 0.12](/docs/cloud/sentinel/sentinel-tf-012.html).

It's strongly advised that you test your Sentinel policies after upgrading to
Terraform 0.12 to ensure they continue to work as expected. [Mock
generation](/docs/cloud/sentinel/mock.html) has also been updated to
produce mock data for the Sentinel imports as they appear in Terraform 0.12.

For more information on testing a policy with 0.11 and 0.12 at the same time,
see the section on [testing a policy with 0.11 and 0.12
simultaneously](/docs/cloud/sentinel/sentinel-tf-012.html#testing-a-policy-with-0-11-and-0-12-simultaneously).