» Data Sources

Data sources allow data to be fetched or computed for use elsewhere in Terraform configuration. Use of data sources allows a Terraform configuration to make use of information defined outside of Terraform, or defined by another separate Terraform configuration.

Each provider may offer data sources alongside its set of resource types.

» Using Data Sources

A data source is accessed via a special kind of resource known as a data resource, declared using a data block:

data "aws_ami" "example" {
  most_recent = true

  owners = ["self"]
  tags = {
    Name   = "app-server"
    Tested = "true"
  }
}

A data block requests that Terraform read from a given data source ("aws_ami") and export the result under the given local name ("example"). The name is used to refer to this resource from elsewhere in the same Terraform module, but has no significance outside of the scope of a module.

The data source and name together serve as an identifier for a given resource and so must be unique within a module.

Within the block body (between { and }) are query constraints defined by the data source. Most arguments in this section depend on the data source, and indeed in this example most_recent, owners and tags are all arguments defined specifically for the aws_ami data source.

When distinguishing from data resources, the primary kind of resource (as declared by a resource block) is known as a managed resource. Both kinds of resources take arguments and export attributes for use in configuration, but while managed resources cause Terraform to create, update, and delete infrastructure objects, data resources cause Terraform only to read objects. For brevity, managed resources are often referred to just as "resources" when the meaning is clear from context.

» Data Source Arguments

Each data resource is associated with a single data source, which determines the kind of object (or objects) it reads and what query constraint arguments are available.

Each data source in turn belongs to a provider, which is a plugin for Terraform that offers a collection of resource types and data sources that most often belong to a single cloud or on-premises infrastructure platform.

Most of the items within the body of a data block are defined by and specific to the selected data source, and these arguments can make full use of expressions and other dynamic Terraform language features.

However, there are some "meta-arguments" that are defined by Terraform itself and apply across all data sources. These arguments often have additional restrictions on what language features can be used with them, and are described in more detail in the following sections.

» Data Resource Behavior

If the query constraint arguments for a data resource refer only to constant values or values that are already known, the data resource will be read and its state updated during Terraform's "refresh" phase, which runs prior to creating a plan. This ensures that the retrieved data is available for use during planning and so Terraform's plan will show the actual values obtained.

Query constraint arguments may refer to values that cannot be determined until after configuration is applied, such as the id of a managed resource that has not been created yet. In this case, reading from the data source is deferred until the apply phase, and any references to the results of the data resource elsewhere in configuration will themselves be unknown until after the configuration has been applied.

» Local-only Data Sources

While many data sources correspond to an infrastructure object type that is accessed via a remote network API, some specialized data sources operate only within Terraform itself, calculating some results and exposing them for use elsewhere.

For example, local-only data sources exist for rendering templates, reading local files, and rendering AWS IAM policies.

The behavior of local-only data sources is the same as all other data sources, but their result data exists only temporarily during a Terraform operation, and is re-calulated each time a new plan is created.

» Data Resource Dependencies

Data resources have the same dependency resolution behavior as defined for managed resources.

In particular, the depends_on meta-argument is also available within data blocks, with the same meaning and syntax as in resource blocks.

However, due to the data resource behavior of deferring the read until the apply phase when depending on values that are not yet known, using depends_on with data resources will force the read to always be deferred to the apply phase, and therefore a configuration that uses depends_on with a data resource can never converge.

Due to this behavior, we do not recommend using depends_on with data resources.

» Multiple Resource Instances

Data resources support the count meta-argument as defined for managed resources, with the same syntax and behavior.

As with managed resources, when count is present it is important to distinguish the resource itself from the multiple resource instances it creates. Each instance will separately read from its data source with its own variant of the constraint arguments, producing an indexed result.

» Selecting a Non-default Provider Configuration

Data resources support the providers meta-argument as defined for managed resources, with the same syntax and behavior.

» Lifecycle Customizations

Data resources do not currently have any customization settings available for their lifecycle, but the lifecycle nested block is reserved in case any are added in future versions.

» Example

A data source configuration looks like the following:

# Find the latest available AMI that is tagged with Component = web
data "aws_ami" "web" {
  filter {
    name   = "state"
    values = ["available"]
  }

  filter {
    name   = "tag:Component"
    values = ["web"]
  }

  most_recent = true
}

» Description

The data block creates a data instance of the given type (first block label) and name (second block label). The combination of the type and name must be unique.

Within the block (the { }) is configuration for the data instance. The configuration is dependent on the type, and is documented for each data source in the providers section.

Each data instance will export one or more attributes, which can be used in other resources as reference expressions of the form data.<TYPE>.<NAME>.<ATTRIBUTE>. For example:

resource "aws_instance" "web" {
  ami           = data.aws_ami.web.id
  instance_type = "t1.micro"
}

» Meta-Arguments

As data sources are essentially a read only subset of resources, they also support the same meta-arguments of resources with the exception of the lifecycle configuration block.

» Multiple Provider Instances

Similarly to resources, the provider meta-argument can be used where a configuration has multiple aliased instances of the same provider:

data "aws_ami" "web" {
  provider = "aws.west"

  # ...
}

See Resources: Multiple Provider Instances for more information.

» Data Source Lifecycle

If the arguments of a data instance contain no references to computed values, such as attributes of resources that have not yet been created, then the data instance will be read and its state updated during Terraform's "refresh" phase, which by default runs prior to creating a plan. This ensures that the retrieved data is available for use during planning and the diff will show the real values obtained.

Data instance arguments may refer to computed values, in which case the attributes of the instance itself cannot be resolved until all of its arguments are defined. In this case, refreshing the data instance will be deferred until the "apply" phase, and all interpolations of the data instance attributes will show as "computed" in the plan since the values are not yet known.