Client-side fields #

Client-side fields are most often used as flags to modify the behavior of a Terraform resource. Because they don’t correspond to an API field, there are some additional considerations in terms of how to implement them.

Common client-side fields include:

MMv1

Add to the schema #

Instead of adding the field in parameters or properties, use a section called virtual_fields.

Example:

virtual_fields:
  - name: 'deletion_protection'
    type: Boolean
    default_value: true
    description: |
      Whether Terraform will be prevented from destroying the CertificateAuthority.
      When the field is set to true or unset in Terraform state, a `terraform apply`
      or `terraform destroy` that would delete the CertificateAuthority will fail.
      When the field is set to false, deleting the CertificateAuthority is allowed.

This will automatically ensure that the field works as users expect.

Handwritten

Add to the schema #

Add the field to the schema as usual.

Example:

"deletion_protection": {
	Type:        schema.TypeBool,
	Optional:    true,
	Default:     true,
	Description: `Whether Terraform will be prevented from destroying the instance. When the field is set to true or unset in Terraform state, a terraform apply or terraform destroy that would delete the table will fail. When the field is set to false, deleting the table is allowed.`,
},

Set on read #

For fields with default values, you need to explicitly set client-side fields in the Read function to avoid a diff that “sets” the field to its default value when users upgrade to the version that contains the new field.

Example:

// Explicitly set client-side fields to default values if unset
if _, ok := d.GetOkExists("deletion_protection"); !ok {
	if err := d.Set("deletion_protection", true); err != nil {
		return fmt.Errorf("Error setting deletion_protection: %s", err)
	}
}

Short-circuit updates if only client-side fields were modified #

Client-side fields can always be updated in-place. However, if the resource is otherwise immutable, you will need to ensure that an Update function is present for the resource. Terraform automatically updates the state based on the plan; this does not need to happen in the Update function unless the value for a field might have changed (for example, based on an API response, which doesn’t apply to client-side fields).

If only client-side fields were modified, you can short-circuit the Update function to avoid sending an API request. This is important because the update request will be empty (which causes errors for some APIs.) This can go at the top of the Update function:

clientSideFields := map[string]bool{"deletion_protection": true}
clientSideOnly := true
for field := range ResourceSpannerInstance().Schema {
	if d.HasChange(field) && !clientSideFields[field] {
		clientSideOnly = false
		break
	}
}
if clientSideOnly {
	return nil
}

Replace ResourceSpannerInstance with the appropriate resource function.

Update data source #

If the resource has a corresponding data source that calls the resource’s Read function, you will need to make the following changes to the data source:

Add the client-side field to the data source’s Schema as an output-only field. (This will happen automatically for data sources that use tpgresource.DatasourceSchemaFromResourceSchema.)
```
"deletion_protection": {
  Type:        schema.TypeBool,
  Computed:    true,
},
```

Unset the field in the data source’s Read function.

if err := d.Set("deletion_protection", nil); err != nil {
  return fmt.Errorf("Error setting deletion_protection: %s", err)
}

Implement logic #

At this point, you should be ready to implement your logic! For example, a deletion_protection field short-ciruits the deletion process if it is not explicitly set to false.

MMv1

Add the following as pre_delete custom code.

if d.Get("deletion_protection").(bool) {
	return fmt.Errorf("cannot destroy folder without setting deletion_protection=false and running `terraform apply`")
}

Handwritten

Add the following at the beginning of the resource’s Delete function.

if d.Get("deletion_protection").(bool) {
	return fmt.Errorf("cannot destroy folder without setting deletion_protection=false and running `terraform apply`")
}