Data Sources
Data sources allow Terraform to use information defined outside of Terraform, defined by another separate Terraform configuration, or modified by functions.
Hands-on: Try the Query Data Sources tutorial.
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:
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
Terraform reads data resources during the planning phase when possible, but announces in the plan when it must defer reading resources until the apply phase to preserve the order of operations. Terraform defers reading data resources in the following situations:
- At least one of the given arguments is a managed resource attribute or other value that Terraform cannot predict until the apply step.
- The data resource depends directly on a managed resource that itself has planned changes in the current plan.
- The data resource has custom conditions and it depends directly or indirectly on a managed resource that itself has planned changes in the current plan.
Refer to Data Resource Dependencies for details on what it means for a data resource to depend on other objects. Any resulting attribute of such a data resource will be unknown during planning, so it cannot be used in situations where values must be fully known.
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-calculated each time a new plan is created.
Data Resource Dependencies
Data resources have the same dependency resolution behavior
as defined for managed resources.
Setting the depends_on
meta-argument within data
blocks defers reading of
the data source until after all changes to the dependencies have been applied.
In order to ensure that data sources are accessing the most up to date
information possible in a wide variety of use cases, arguments directly
referencing managed resources are treated the same as if the resource was
listed in depends_on
. This behavior can be avoided when desired by indirectly
referencing the managed resource values through a local
value, unless the
data resource itself has
custom conditions.
NOTE: In Terraform 0.12 and earlier, 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.
Custom Condition Checks
You can use precondition
and postcondition
blocks to specify assumptions and guarantees about how the data source operates. The following examples creates a postcondition that checks whether the AMI has the correct tags.
Custom conditions can help capture assumptions, helping future maintainers understand the configuration design and intent. They also return useful information about errors earlier and in context, helping consumers more easily diagnose issues in their configurations.
Refer to Custom Condition Checks for more details.
Multiple Resource Instances
Data resources support count
and for_each
meta-arguments as defined for managed resources, with the same syntax and behavior.
As with managed resources, when count
or for_each
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 provider
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:
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; as with
resources, each provider on the
Terraform Registry has its own
documentation for configuring and using the data types it provides.
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:
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.
Non-Default Provider Configurations
Similarly to resources, when
a module has multiple configurations for the same provider you can specify which
configuration to use with the provider
meta-argument:
See
The Resource provider
Meta-Argument
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.