» Schema Behaviors

Schema fields that can have an effect at plan or apply time are collectively referred to as "Behavioral fields", or an elements behaviors. These fields are often combined in several ways to create different behaviors, depending on the need of the element in question, typically customized to match the behavior of a cloud service API. For example, at time of writing AWS Launch Configurations cannot be updated through the AWS API. As a result all of the schema elements in the corresponding Terraform Provider resource aws_launch_configuration are marked as ForceNew: true. This behavior instructs Terraform to first destroy and then recreate the resource if any of the attributes change in the configuration, as opposed to trying to update the existing resource.

» Primitive Behaviors

» Optional

Data structure: bool
Values: true or false
Restrictions:

  • Cannot be used if Required is true
  • Must be set if Required is omitted and element is not Computed

Indicates that this element is optional to include in the configuration. Note that Optional does not itself establish a default value. See Default below.

Schema example:

"encrypted": {
  Type:     schema.TypeBool,
  Optional: true,
},

Configuration example:

resource "example_volume" "ex" {
  encrypted = true
}

» Required

Data structure: bool
Values: true or false
Restrictions:

  • Cannot be used if Optional is true
  • Cannot be used if Computed is true
  • Must be set if Optional is omitted and element is not Computed

Indicates that this element must be provided in the configuration. Omitting this attribute from configuration, or later removing it, will result in a plan-time error.

Schema example:

"name": {
  Type:     schema.TypeString,
  Required: true,
},

Configuration example:

resource "example_volume" "ex" {
  name = "swap volume"
}

» Default

Data structure: interface
Value: any value of an elements Type for primitive types, or the type defined by Elem for complex types.
Restrictions:

  • Cannot be used if Required is true
  • Cannot be used with DefaultFunc

If Default is specified, that value that is used when this item is not set in the configuration.

Schema example:

"encrypted": {
  Type:     schema.TypeBool,
  Optional: true,
  Default: false,
},

Configuration example (specified):

resource "example_volume" "ex" {
  name = "swap volume"
  encrypted = true
}

Configuration example (omitted):

resource "example_volume" "ex" {
  name = "swap volume"
  # encrypted receives its default value, false
}

» Computed

Data structure: bool
Value: true or false
Restrictions:

  • Cannot be used when Required is true
  • Cannot be used when Default is specified
  • Cannot be used with DefaultFunc

Computed is often used to represent values that are not user configurable or can not be known at time of terraform plan or apply, such as date of creation or a service specific UUID. Computed can be combined with other attributes to achieve specific behaviors, and can be used as output for interpolation into other resources

Schema example:

"uuid": {
  Type:     schema.TypeString,
  Computed: true,
},

Configuration example:

resource "example_volume" "ex" {
  name = "swap volume"
  encrypted = true
}

output "volume_uuid" {
  value = "${example_volume.ex.uuid}"
}

» ForceNew

Data structure: bool
Value: true or false

ForceNew indicates that any change in this field requires the resource to be destroyed and recreated.

Schema example:

"base_image": {
  Type:     schema.TypeString,
  Required: true,
  ForceNew: true,
},

Configuration example:

resource "example_instance" "ex" {
  name = "bastion host"
  base_image = "ubuntu_17.10" 
}

» Function Behaviors

» DiffSuppressFunc

Data structure: SchemaDiffSuppressFunc

When provided DiffSuppressFunc will be used by Terraform to calculate the diff of this field. Common use cases are capitalization differences in string names, or logical equivalences in JSON values.

Schema example:

"base_image": {
  Type:     schema.TypeString,
  Required: true,
  ForceNew: true,
  # Suppress the diff shown if the base_image name are equal when both compared in lower        #  case.
  DiffSuppressFunc: func(k, old, new string, d *schema.ResourceData) bool {
    if strings.ToLower(old) == strings.ToLower(new) {
      return true 
    }
    return false
  },
},

Configuration example:

Here we assume the service API accepts capitalizations of the base_image name and converts it to a lowercase string. The API then returns the lower case value in it’s responses.

resource "example_instance" "ex" {
  name = "bastion host"
  base_image = "UBunTu_17.10" 
}

» DefaultFunc

Data structure: SchemaDefaultFunc
Restrictions:

  • Cannot be used if Default is specified

When DefaultFunc will be used to compute a dynamic default for this element. The return value of this function should be "stable", such that it is uncommon to return different values in subsequent plans without any other changes being made, to avoid unnecessary diffs in terraform plan.

DefaultFunc is most commonly used in Provider schemas, allows elements to have a default read from the environment.

Schema example:

In this example, Terraform will attempt to read region from the environment if it is omitted from configuration. If it’s not found in the environment, a default value of us-west is given.

"region": {
  Type:     schema.TypeString,
  Required: true,
  DefaultFunc: func() (interface{}, error) {
    if v := os.Getenv("PROVIDER_REGION"); v != "" {
      return v, nil
    }

    return "us-west", nil
  },
},

Configuration example (provided):

provider "example" {
  api_key = "somesecretkey"
  region = "us-est"
}

Configuration example (default func with PROVIDER_REGION set to us-west in the environment):

provider "example" {
  api_key = "somesecretkey"
  # region is "us-west"
}

Configuration example (default func with PROVIDER_REGION unset in the environment):

provider "example" {
  api_key = "somesecretkey"
  # region is "us-east" 
}