Resource Blocks

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Resources are the most important element in the Terraform language. Each resource block describes one or more infrastructure objects, such as virtual networks, compute instances, or higher-level components such as DNS records.

Resource Syntax

Resource declarations can include a number of advanced features, but only a small subset are required for initial use. More advanced syntax features, such as single resource declarations that produce multiple similar remote objects, are described later in this page.

resource "aws_instance" "web" {
  ami           = "ami-a1b2c3d4"
  instance_type = "t2.micro"
}

A resource block declares a resource of a given type ("aws_instance") with a given local name ("web"). The name is used to refer to this resource from elsewhere in the same Terraform module, but has no significance outside that module's scope.

The resource type 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 the configuration arguments for the resource itself. Most arguments in this section depend on the resource type, and indeed in this example both ami and instance_type are arguments defined specifically for the aws_instance resource type.

Resource Types

Each resource is associated with a single resource type, which determines the kind of infrastructure object it manages and what arguments and other attributes the resource supports.

Providers

Each resource type is implemented by a provider, which is a plugin for Terraform that offers a collection of resource types. A provider usually provides resources to manage a single cloud or on-premises infrastructure platform. Providers are distributed separately from Terraform itself, but Terraform can automatically install most providers when initializing a working directory.

In order to manage resources, a Terraform module must specify which providers it requires. Additionally, most providers need some configuration in order to access their remote APIs, and the root module must provide that configuration.

For more information, see:

• Provider Requirements, for declaring which providers a module uses.
• Provider Configuration, for configuring provider settings.

Terraform usually automatically determines which provider to use based on a resource type's name. (By convention, resource type names start with their provider's preferred local name.) When using multiple configurations of a provider (or non-preferred local provider names), you must use the provider meta-argument to manually choose an alternate provider configuration. See the provider meta-argument for more details.

Resource Arguments

Most of the arguments within the body of a resource block are specific to the selected resource type. The resource type's documentation lists which arguments are available and how their values should be formatted.

The values for resource arguments can make full use of expressions and other dynamic Terraform language features.

There are also some meta-arguments that are defined by Terraform itself and apply across all resource types. (See Meta-Arguments below.)

Documentation for Resource Types

Every Terraform provider has its own documentation, describing its resource types and their arguments.

Most publicly available providers are distributed on the Terraform Registry, which also hosts their documentation. When viewing a provider's page on the Terraform Registry, you can click the "Documentation" link in the header to browse its documentation. Provider documentation on the registry is versioned, and you can use the dropdown version menu in the header to switch which version's documentation you are viewing.

To browse the publicly available providers and their documentation, see the providers section of the Terraform Registry.

Resource Behavior

For more information about how Terraform manages resources when applying a configuration, see Resource Behavior.

Meta-Arguments

The Terraform language defines several meta-arguments, which can be used with any resource type to change the behavior of resources.

The following meta-arguments are documented on separate pages:

• depends_on, for specifying hidden dependencies
• count, for creating multiple resource instances according to a count
• for_each, to create multiple instances according to a map, or set of strings
• provider, for selecting a non-default provider configuration
• lifecycle, for lifecycle customizations
• provisioner, for taking extra actions after resource creation

Custom Condition Checks

You can use precondition and postcondition blocks to specify assumptions and guarantees about how the resource operates. The following example creates a precondition that checks whether the AMI is properly configured.

resource "aws_instance" "example" {
  instance_type = "t2.micro"
  ami           = "ami-abc123"

  lifecycle {
    # The AMI ID must refer to an AMI that contains an operating system
    # for the `x86_64` architecture.
    precondition {
      condition     = data.aws_ami.example.architecture == "x86_64"
      error_message = "The selected AMI must be for the x86_64 architecture."
    }
  }
}

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.

Operation Timeouts

Some resource types provide a special timeouts nested block argument that allows you to customize how long certain operations are allowed to take before being considered to have failed. For example, aws_db_instance allows configurable timeouts for create, update and delete operations.

Timeouts are handled entirely by the resource type implementation in the provider, but resource types offering these features follow the convention of defining a child block called timeouts that has a nested argument named after each operation that has a configurable timeout value. Each of these arguments takes a string representation of a duration, such as "60m" for 60 minutes, "10s" for ten seconds, or "2h" for two hours.

resource "aws_db_instance" "example" {
  # ...

  timeouts {
    create = "60m"
    delete = "2h"
  }
}

The set of configurable operations is chosen by each resource type. Most resource types do not support the timeouts block at all. Consult the documentation for each resource type to see which operations it offers for configuration, if any.