opentf/website/docs/language/resources/syntax.mdx

---
description: >-
  Resources correspond to infrastructure objects like virtual networks or
  compute instances. Learn about resource types, syntax, behavior, and
  arguments.
---

# Resource Blocks

_Resources_ are the most important element in the OpenTF 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.

```hcl
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 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](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/instance).

:::note
Resource names must start with a letter or underscore, and may
contain only letters, digits, underscores, and dashes.
:::

## 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](/docs/language/providers/requirements),
which is a plugin for OpenTF 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 OpenTF
itself, but OpenTF can automatically install most providers when initializing
a working directory.

In order to manage resources, a 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](/docs/language/providers/requirements), for declaring which
  providers a module uses.
- [Provider Configuration](/docs/language/providers/configuration), for configuring provider settings.

OpenTF 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](/docs/language/meta-arguments/resource-provider) 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](/docs/language/expressions) and other dynamic OpenTF
language features.

There are also some _meta-arguments_ that are defined by OpenTF itself
and apply across all resource types. (See [Meta-Arguments](#meta-arguments) below.)

### Documentation for Resource Types

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

Most publicly available providers are distributed on the
[Public Terraform Registry](https://registry.terraform.io/browse/providers), which also
hosts their documentation. When viewing a provider's page on the OpenTF
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
[Public Terraform Registry](https://registry.terraform.io/browse/providers).

:::note
Provider documentation previously existed as part of OpenTF's core documentation. Although some provider documentation
might still be hosted here, the Public Terraform Registry is now the main home for all
public provider docs.
:::

## Resource Behavior

For more information about how OpenTF manages resources when applying a
configuration, see
[Resource Behavior](/docs/language/resources/behavior).

## Meta-Arguments

The OpenTF 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](/docs/language/meta-arguments/depends_on)
- [`count`, for creating multiple resource instances according to a count](/docs/language/meta-arguments/count)
- [`for_each`, to create multiple instances according to a map, or set of strings](/docs/language/meta-arguments/for_each)
- [`provider`, for selecting a non-default provider configuration](/docs/language/meta-arguments/resource-provider)
- [`lifecycle`, for lifecycle customizations](/docs/language/meta-arguments/lifecycle)
- [`provisioner`, for taking extra actions after resource creation](/docs/language/resources/provisioners/syntax)

## 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.

```hcl
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](/docs/language/expressions/custom-conditions#preconditions-and-postconditions) 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`](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/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.

```hcl
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.