Terraform - Resource Behavior

 A resource block declares that you want a particular infrastructure object to exist with the given settings. If you are writing a new configuration for the first time, the resources it defines will exist only in the configuration, and will not yet represent real infrastructure objects in the target platform.

Applying a Terraform configuration is the process of creating, updating, and destroying real infrastructure objects in order to make their settings match the configuration.

How Terraform Applies a Configuration

When Terraform creates a new infrastructure object represented by a resource block, the identifier for that real object is saved in Terraform's state, allowing it to be updated and destroyed in response to future changes. For resource blocks that already have an associated infrastructure object in the state, Terraform compares the actual configuration of the object with the arguments given in the configuration and, if necessary, updates the object to match the configuration.

In summary, applying a Terraform configuration will:

• Create resources that exist in the configuration but are not associated with a real infrastructure object in the state.
• Destroy resources that exist in the state but no longer exist in the configuration.
• Update in-place resources whose arguments have changed.
• Destroy and re-create resources whose arguments have changed but which cannot be updated in-place due to remote API limitations.

This general behavior applies for all resources, regardless of type. The details of what it means to create, update, or destroy a resource are different for each resource type, but this standard set of verbs is common across them all.

The meta-arguments within resource blocks, documented in the sections below, allow some details of this standard resource behavior to be customized on a per-resource basis.

Accessing Resource Attributes

Expressions within a Terraform module can access information about resources in the same module, and you can use that information to help configure other resources. Use the <RESOURCE TYPE>.<NAME>.<ATTRIBUTE> syntax to reference a resource attribute in an expression.

In addition to arguments specified in the configuration, resources often provide read-only attributes with information obtained from the remote API; this often includes things that can't be known until the resource is created, like the resource's unique random ID.

Many providers also include data sources, which are a special type of resource used only for looking up information.

For a list of the attributes a resource or data source type provides, consult its documentation; these are generally included in a second list below its list of configurable arguments.

For more information about referencing resource attributes in expressions, see Expressions: References to Resource Attributes.

Resource Dependencies

Most resources in a configuration don't have any particular relationship, and Terraform can make changes to several unrelated resources in parallel.

However, some resources must be processed after other specific resources; sometimes this is because of how the resource works, and sometimes the resource's configuration just requires information generated by another resource.

Most resource dependencies are handled automatically. Terraform analyses any expressions within a resource block to find references to other objects, and treats those references as implicit ordering requirements when creating, updating, or destroying resources. Since most resources with behavioral dependencies on other resources also refer to those resources' data, it's usually not necessary to manually specify dependencies between resources.

However, some dependencies cannot be recognized implicitly in configuration. For example, if Terraform must manage access control policies and take actions that require those policies to be present, there is a hidden dependency between the access policy and a resource whose creation depends on it. In these rare cases, the depends_on meta-argument can explicitly specify a dependency.

You can also use the replace_triggered_by meta-argument to add dependencies between otherwise independent resources. It forces Terraform to replace the parent resource when there is a change to a referenced resource or resource attribute.

Local-only Resources

While most resource types correspond to an infrastructure object type that is managed via a remote network API, there are certain specialized resource types that operate only within Terraform itself, calculating some results and saving those results in the state for future use.

For example, local-only resource types exist for generating private keys, issuing self-signed TLS certificates, and even generating random ids. While these resource types often have a more marginal purpose than those managing "real" infrastructure objects, they can be useful as glue to help connect together other resources.

The behavior of local-only resources is the same as all other resources, but their result data exists only within the Terraform state. "Destroying" such a resource means only to remove it from the state, discarding its data.