» Upgrading to Terraform v0.12

Terraform v0.12 will be a major release focused on configuration language improvements and thus will include 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. Please read on for more information and recommendations on the upgrade process.

» Upgrade to Terraform 0.11 first

If you are currently using Terraform v0.10 or earlier, we strongly recommend first 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 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.

» Upgrading Terraform providers

The new language features in Terraform v0.12 required some changes to the protocol Terraform uses to interact with provider plugins. These changes give Terraform CLI access to the resource type schemas of each provider, allowing for more helpful validation-related error messages and more predictable behavior.

However, this means that provider releases built before v0.12 cannot be used. We have updated the provider SDK to support both the old and new protocols at once, to allow upgrading to newer provider versions while remaining on Terraform v0.11.

We recommend upgrading to the latest versions of all providers you use and ensuring that terraform plan is working with them before upgrading to Terraform v0.12, since this allows you to reduce risk by changing only one component at a time, particularly if you will be adopting a new major version of a provider which may have breaking changes of its own.

» Third-party Providers

The Terraform team at HashiCorp is working with the maintainers of the HashiCorp-distributed providers to produce v0.12-compatible releases, which will appear gradually before the v0.12.0 final release.

Third-party providers that are not distributed by HashiCorp will also require updates. We will share more information on the upgrade procedure as we get closer to final release. In the mean time, the first step is to upgrade the vendored github.com/hashicorp/terraform packages to a v0.12 release tag and verify that the acceptance tests are still working. Because acceptance tests contain configuration snippets, you may need to perform some of the configuration upgrade steps described in the following sections to make the acceptance tests compatible with the v0.12 configuration language.

» Upgrading Terraform configuration

Some users with simple configurations may find that no changes are required at all, and most configurations that do require updates can be upgraded automatically using the automatic upgrade tool.

Some users have written configurations that include workarounds for limitations in previous versions of the Terraform language, such as:

  • Treating block types like attributes in an attempt to work around Terraform not supporting generating nested blocks dynamically. (#7034)

  • Wrapping redundant list brackets ([ and ]) around splat expressions in order to force them to be interpreted as lists even when there are unknown items in the list.

These workarounds were clever solutions offered by community members, and have been partial inspiration for new language features. These workarounds should no longer be necessary in Terraform v0.12, but the same results may now need to be achieved using new language constructs.

The upgrade tool can replace many of these workarounds with the new solutions automatically. In rarer cases, the intent of the original configuration may be ambiguous, in which case the tool will add to your configuration a comment containing the marker TF-UPGRADE-TODO to indicate a situation where your human intuition is required to decide how to proceed.

We recommend running the upgrade tool in a clean version control work tree so that you can use the VCS diffing tools to easily see and review all of the proposed updates. Search the upgraded module for TF-UPGRADE-TODO to find the situations where human attention is required.

Even if your existing configuration works without upgrading, we still recommend to run the upgrade tool to update to the more readable syntax conventions supported in this release, and to draw attention to any potential issues.

The following sections describe in more detail some of the situations that will be detected and upgraded by the upgrade tool, both to help understand the purpose of certain proposed changes and to help users who may not wish to use the automatic upgrade tool. However, the following sections are not completely comprehensive so we still recommend using the upgrade tool to review its output, even if you then discard the proposed changes and make your updates manually.

» 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:


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


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


» 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:

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, in some sense, a part of 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:

  # 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:

  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.

» 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:


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:


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:

  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:

  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.

» 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 0.12-only features. However, for any module using a undocumented workarounds for 0.11 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 Sentinel policies

Terraform Enterprise users of Sentinel will need to complete the below steps to upgrade Sentinel to work with Terraform 0.12.

  1. Update Terraform configurations to 0.12
  2. Update Sentinel policies

Because Sentinel is applied across all workspaces in Terraform Enterprise, all workspaces must be upgraded to Terraform 0.12 otherwise Sentinel policies will fail on versions below 0.12.

More details on this upgrade process will be added prior to the final release.