155 lines
5.9 KiB
Markdown
155 lines
5.9 KiB
Markdown
---
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layout: "language"
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page_title: "Override Files - Configuration Language"
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sidebar_current: "docs-config-override"
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description: |-
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Override files allow additional settings to be merged into existing
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configuration objects.
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---
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# Override Files
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Terraform normally loads all of the `.tf` and `.tf.json` files within a
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directory and expects each one to define a distinct set of configuration
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objects. If two files attempt to define the same object, Terraform returns
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an error.
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In some rare cases, it is convenient to be able to override specific portions
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of an existing configuration object in a separate file. For example, a
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human-edited configuration file in the Terraform language native syntax
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could be partially overridden using a programmatically-generated file
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in JSON syntax.
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For these rare situations, Terraform has special handling of any configuration
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file whose name ends in `_override.tf` or `_override.tf.json`. This special
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handling also applies to a file named literally `override.tf` or
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`override.tf.json`.
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Terraform initially skips these _override files_ when loading configuration,
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and then afterwards processes each one in turn (in lexicographical order). For
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each top-level block defined in an override file, Terraform attempts to find
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an already-defined object corresponding to that block and then merges the
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override block contents into the existing object.
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Use override files only in special circumstances. Over-use of override files
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hurts readability, since a reader looking only at the original files cannot
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easily see that some portions of those files have been overridden without
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consulting all of the override files that are present. When using override
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files, use comments in the original files to warn future readers about which
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override files apply changes to each block.
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## Example
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If you have a Terraform configuration `example.tf` with the following contents:
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```hcl
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resource "aws_instance" "web" {
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instance_type = "t2.micro"
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ami = "ami-408c7f28"
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}
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```
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...and you created a file `override.tf` containing the following:
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```hcl
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resource "aws_instance" "web" {
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ami = "foo"
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}
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```
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Terraform will merge the latter into the former, behaving as if the original
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configuration had been as follows:
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```hcl
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resource "aws_instance" "web" {
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instance_type = "t2.micro"
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ami = "foo"
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}
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```
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## Merging Behavior
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The merging behavior is slightly different for each block type, and some
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special constructs within certain blocks are merged in a special way.
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The general rule, which applies in most cases, is:
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* A top-level block in an override file merges with a block in a normal
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configuration file that has the same block header. The block _header_ is the
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block type and any quoted labels that follow it.
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* Within a top-level block, an attribute argument within an override block
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replaces any argument of the same name in the original block.
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* Within a top-level block, any nested blocks within an override block replace
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_all_ blocks of the same type in the original block. Any block types that
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do not appear in the override block remain from the original block.
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* The contents of nested configuration blocks are not merged.
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* The resulting _merged block_ must still comply with any validation rules
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that apply to the given block type.
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If more than one override file defines the same top-level block, the overriding
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effect is compounded, with later blocks taking precedence over earlier blocks.
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Overrides are processed in order first by filename (in lexicographical order)
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and then by position in each file.
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The following sections describe the special merging behaviors that apply to
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specific arguments within certain top-level block types.
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### Merging `resource` and `data` blocks
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Within a `resource` block, the contents of any `lifecycle` nested block are
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merged on an argument-by-argument basis. For example, if an override block
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sets only the `create_before_destroy` argument then any `ignore_changes`
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argument in the original block will be preserved.
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If an overriding `resource` block contains one or more `provisioner` blocks
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then any `provisioner` blocks in the original block are ignored.
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If an overriding `resource` block contains a `connection` block then it
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completely overrides any `connection` block present in the original block.
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The `depends_on` meta-argument may not be used in override blocks, and will
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produce an error.
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### Merging `variable` blocks
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The arguments within a `variable` block are merged in the standard way
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described above, but some special considerations apply due to the interactions
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between the `type` and `default` arguments.
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If the original block defines a `default` value and an override block changes
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the variable's `type`, Terraform attempts to convert the default value to
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the overridden type, producing an error if this conversion is not possible.
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Conversely, if the original block defines a `type` and an override block changes
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the `default`, the overridden default value must be compatible with the
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original type specification.
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### Merging `output` blocks
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The `depends_on` meta-argument may not be used in override blocks, and will
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produce an error.
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### Merging `locals` blocks
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Each `locals` block defines a number of named values. Overrides are applied
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on a value-by-value basis, ignoring which `locals` block they are defined in.
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### Merging `terraform` blocks
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The settings within `terraform` blocks are considered individually when
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merging.
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If the `required_providers` argument is set, its value is merged on an
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element-by-element basis, which allows an override block to adjust the
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constraint for a single provider without affecting the constraints for
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other providers.
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In both the `required_version` and `required_providers` settings, each override
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constraint entirely replaces the constraints for the same component in the
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original block. If both the base block and the override block both set
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`required_version` then the constraints in the base block are entirely ignored.
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