Terraform Concepts

This page describes Terraform concepts as they relate to handling data within framework-based provider code. The What is Terraform, Terraform language, and Plugin Development documentation covers more general concepts behind Terraform's workflow, its configuration, and how it interacts with providers.

Schemas

Schemas specify the data structure and types of a provider, resource, data source, or ephemeral resource that is exposed to Terraform. This includes the configuration written by practitioners, any planning data, and the state stored by Terraform which can be referenced in other configuration. Providers, resources, data sources, and ephemeral resources have their own concept-specific types and available functionality.

Each part of the data within a schema is defined as either an attribute or block. In general, attributes set values and blocks are containers for other attributes and blocks. Each have differing configuration syntax and behaviors.

Discover more about framework implementations of this concept in the schema documentation.

Attributes

Attributes set values in a schema, such as a string or list. The Terraform type system documentation provides a general overview of the available kinds of data within Terraform, which is similar with type system concepts available in many programming languages. However, there are subtle differences with values discussed in the type system section.

Discover more about the framework implementations of this concept in the attribute documentation.

Blocks

A block is a container for other attributes and blocks. Terraform implements many top level blocks, such as provider and resource, while a schema supports nested blocks.

Discover more about the framework implementations of this concept in the block documentation.

Schema Based Data

Schemas provide the structures and types for representing data with Terraform. This section discusses the concepts behind the different types of data influenced by a schema.

Configuration

As part of the Terraform workflow, a practitioner writes configuration files, which are parsed into a graph of operations by Terraform commands. The structures and types of that configuration is the most visible aspect of a schema, since a schema has a direct effect on the expected configuration syntax.

In Terraform operations where the configuration data is available to providers, the framework typically represents this as a Config field in the request type.

Plan

As part of the Terraform workflow, any configuration changes are planned before they are applied into state so practitioners have an opportunity to review whether those anticipated changes have their intended effect. Conceptually, this data represents the merging of configuration data and any prior state data. Terraform refers to this data as the proposed new state, while the framework more generally refers to this data as the plan.

Plan data requires careful handling to prevent unexpected Terraform errors for practitioners. The framework implements various schema concepts and logic discussed in the resource plan modification documentation. In-depth documentation about Terraform requirements is available in the Terraform Resource Instance Change Lifecycle documentation.

In Terraform operations where the plan data is available to providers, the framework typically represents this as a Plan field in the request or response type.

State

As part of the Terraform workflow, any data that should be stored for configuration references or future Terraform executions must be written to the state. This data must exactly match any configuration data, and if applicable, any plan data with unknown values converted to known values.

In Terraform operations where the plan data is available to providers, the framework typically represents this as a State field in the request or response type.

Type System

The Terraform type system supports deeper functionality than the standard type systems built into programming languages. While the types in the Terraform type system are similar to what is found in most languages, values have extra metadata associated with them. The framework implements its own types to prevent the loss of this additional metadata that cannot be represented by Go built-in types.

Important value metadata concepts when implementing a provider include:

• Null values: Absence of a value.
• Unknown values: Value that is not yet known.

As Terraform exposes additional value metadata to providers, the framework type system will be updated. Therefore, it is always recommended to use the framework type system to ensure all Terraform value functionality is available in provider code.

Null Values

Null represents the absence of a Terraform value. It is usually encountered with optional attributes that the practitioner neglected to specify a value for, but can show up on any non-required attribute. Required attributes can never be null.

Unknown Values

Unknown represents a Terraform value that is not yet known. Terraform uses a graph of providers, resources, and data sources to do things in the right order, and when a provider, resource, or data source relies on a value from another provider, resource, or data source that has not been resolved yet, it represents that state by using the unknown value. For example:

resource "example_foo" "bar" {
  hello = "world"
  demo = true
}

resource "example_baz" "quux" {
  foo_id = example_foo.bar.id
}

In the example above, example_baz.quux is relying on the id attribute of example_foo.bar. The id attribute of example_foo.bar isn't known until after the apply. The plan would list it as (known after apply). During the plan phase, example_baz.quux would get an unknown value as the value for foo_id.

Because they can result from interpolations in the practitioner's config, you have no control over what attributes may contain an unknown value. However, by the time a resource is expected to be created, read, updated, or deleted, only its computed attributes can be unknown. The rest are guaranteed to have known values (or be null).

For concepts such as resource and data source configuration validation, this means that Terraform guarantees to call validation for a non-computed attribute eventually with a known value, so the provider should typically avoid returning error diagnostics when encountering an unknown value during validation.

Provider configuration values can also be unknown, and providers should handle that situation, even if that means just returning an error.