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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
and plans to update the remote object to match configuration.
In some rare cases, settings of a remote object are modified by processes
outside of Terraform, which Terraform would then attempt to "fix" on the
next run. In order to make 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 `ignore_chages` feature is intended to be used when a resource is
created with references to data that may change in the future, but should
not effect said resource after its creation. In some rare cases, settings
of a remote object are modified by processes outside of Terraform, which
Terraform would then attempt to "fix" on the next run. In order to make
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
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
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
Terraform to ignore _all_ attributes, which means that Terraform can
create and destroy the remote object but will never propose updates to it.