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update ignore_changes doc 

We can remove the caveat about changing map elements.

Add a little more text about the intended use case for ignore_changes,
as the common case of fixing erroneous provider behavior should not be
the primary motivation for the maintenance of this feature.
This commit is contained in:
James Bardin 2020-09-30 09:46:10 -04:00
parent ad0b81de81
commit 82793f8158
1 changed files with 13 additions and 36 deletions
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49
website/docs/configuration/resources.html.md
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@ -638,16 +638,22 @@ meta-arguments are supported:
any difference in the current settings of a real infrastructure object any difference in the current settings of a real infrastructure object
and plans to update the remote object to match configuration. and plans to update the remote object to match configuration.
In some rare cases, settings of a remote object are modified by processes The `ignore_chages` feature is intended to be used when a resource is
outside of Terraform, which Terraform would then attempt to "fix" on the created with references to data that may change in the future, but should
next run. In order to make Terraform share management responsibilities not effect said resource after its creation. In some rare cases, settings
of a single object with a separate process, the `ignore_changes` of a remote object are modified by processes outside of Terraform, which
meta-argument specifies resource attributes that Terraform should ignore Terraform would then attempt to "fix" on the next run. In order to make
when planning updates to the associated remote object. Terraform share management responsibilities of a single object with a
separate process, the `ignore_changes` meta-argument specifies resource
attributes that Terraform should ignore when planning updates to the
associated remote object.
The arguments corresponding to the given attribute names are considered The arguments corresponding to the given attribute names are considered
when planning a _create_ operation, but are ignored when planning an when planning a _create_ operation, but are ignored when planning an
_update_. _update_. The arugments are the relative address of the attributes in the
resource. Map and list elements can be referenced using index notation,
like `tags["Name"]` and `list[0]` respectively.
```hcl ```hcl
resource "aws_instance" "example" { resource "aws_instance" "example" {
@ -663,35 +669,6 @@ meta-arguments are supported:
} }
``` ```
You can also ignore specific map elements by writing references like
`tags["Name"]` in the `ignore_changes` list, though with an important
caveat: the ignoring applies only to in-place updates to an existing
key. Adding or removing a key is treated by Terraform as a change to the
containing map itself rather than to the individual key, and so if you
wish to ignore changes to a particular tag made by an external system
you must ensure that the Terraform configuration creates a placeholder
element for that tag name so that the external system changes will be
understood as an in-place edit of that key:
```hcl
resource "aws_instance" "example" {
# ...
tags = {
# Initial value for Name is overridden by our automatic scheduled
# re-tagging process; changes to this are ignored by ignore_changes
# below.
Name = "placeholder"
}
lifecycle {
ignore_changes = [
tags["Name"],
]
}
}
```
Instead of a list, the special keyword `all` may be used to instruct Instead of a list, the special keyword `all` may be used to instruct
Terraform to ignore _all_ attributes, which means that Terraform can Terraform to ignore _all_ attributes, which means that Terraform can
create and destroy the remote object but will never propose updates to it. create and destroy the remote object but will never propose updates to it.