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Override Files

OpenTF normally loads all of the .tf and .tf.json files within a directory and expects each one to define a distinct set of configuration objects. If two files attempt to define the same object, OpenTF returns an error.

In some rare cases, it is convenient to be able to override specific portions of an existing configuration object in a separate file. For example, a human-edited configuration file in the OpenTF language native syntax could be partially overridden using a programmatically-generated file in JSON syntax.

For these rare situations, OpenTF has special handling of any configuration file whose name ends in _override.tf or _override.tf.json. This special handling also applies to a file named literally override.tf or override.tf.json.

OpenTF initially skips these override files when loading configuration, and then afterwards processes each one in turn (in lexicographical order). For each top-level block defined in an override file, OpenTF attempts to find an already-defined object corresponding to that block and then merges the override block contents into the existing object.

Use override files only in special circumstances. Over-use of override files hurts readability, since a reader looking only at the original files cannot easily see that some portions of those files have been overridden without consulting all of the override files that are present. When using override files, use comments in the original files to warn future readers about which override files apply changes to each block.

Example​

If you have an OpenTF configuration example.tf with the following contents:

resource "aws_instance" "web" {
  instance_type = "t2.micro"
  ami           = "ami-408c7f28"
}

...and you created a file override.tf containing the following:

resource "aws_instance" "web" {
  ami = "foo"
}

OpenTF will merge the latter into the former, behaving as if the original configuration had been as follows:

resource "aws_instance" "web" {
  instance_type = "t2.micro"
  ami           = "foo"
}

Merging Behavior​

The merging behavior is slightly different for each block type, and some special constructs within certain blocks are merged in a special way.

The general rule, which applies in most cases, is:

  • A top-level block in an override file merges with a block in a normal configuration file that has the same block header. The block header is the block type and any quoted labels that follow it.

  • Within a top-level block, an attribute argument within an override block replaces any argument of the same name in the original block.

  • Within a top-level block, any nested blocks within an override block replace all blocks of the same type in the original block. Any block types that do not appear in the override block remain from the original block.

  • The contents of nested configuration blocks are not merged.

  • The resulting merged block must still comply with any validation rules that apply to the given block type.

If more than one override file defines the same top-level block, the overriding effect is compounded, with later blocks taking precedence over earlier blocks. Overrides are processed in order first by filename (in lexicographical order) and then by position in each file.

The following sections describe the special merging behaviors that apply to specific arguments within certain top-level block types.

Merging resource and data blocks​

Within a resource block, the contents of any lifecycle nested block are merged on an argument-by-argument basis. For example, if an override block sets only the create_before_destroy argument then any ignore_changes argument in the original block will be preserved.

If an overriding resource block contains one or more provisioner blocks then any provisioner blocks in the original block are ignored.

If an overriding resource block contains a connection block then it completely overrides any connection block present in the original block.

The depends_on meta-argument may not be used in override blocks, and will produce an error.

Merging variable blocks​

The arguments within a variable block are merged in the standard way described above, but some special considerations apply due to the interactions between the type and default arguments.

If the original block defines a default value and an override block changes the variable's type, OpenTF attempts to convert the default value to the overridden type, producing an error if this conversion is not possible.

Conversely, if the original block defines a type and an override block changes the default, the overridden default value must be compatible with the original type specification.

Merging output blocks​

The depends_on meta-argument may not be used in override blocks, and will produce an error.

Merging locals blocks​

Each locals block defines a number of named values. Overrides are applied on a value-by-value basis, ignoring which locals block they are defined in.

Merging terraform blocks​

The settings within terraform blocks are considered individually when merging.

If the required_providers argument is set, its value is merged on an element-by-element basis, which allows an override block to adjust the constraint for a single provider without affecting the constraints for other providers.

In both the required_version and required_providers settings, each override constraint entirely replaces the constraints for the same component in the original block. If both the base block and the override block both set required_version then the constraints in the base block are entirely ignored.

The presence of a block defining a backend (either cloud or backend) in an override file always takes precedence over a block defining a backend in the original configuration. That is, if a cloud block is set within the original configuration and a backend block is set in the override file, OpenTF will use the backend block specified in the override file upon merging. Similarly, if a backend block is set within the original configuration and a cloud block is set in the override file, OpenTF will use the cloud block specified in the override file upon merging.