What concept allows Terraform to integrate variables and resource attributes together?

The concept that allows Terraform to integrate variables and resource attributes together is local values. Local values in Terraform are a way to define reusable values within a configuration file, which can include both static data and computed expressions. When you define a local value, you can reference it throughout your configuration, which helps in reducing duplication and maintaining clarity.

By utilizing local values, you can effectively combine variables and attributes from different resources into a single value that can be reused multiple times. This not only simplifies your code but also ensures that updates to that value are uniformly applied wherever it is referenced. This encapsulation of logic helps in achieving modularity and ease of maintenance in Terraform configurations.

In contrast, while HCL (HashiCorp Configuration Language) is the syntax used to write Terraform configurations, it does not specifically address the integration of variables and resource attributes. Modules allow you to group resources and reuse them, but they do not inherently integrate variables and attributes in the way local values do. Providers connect Terraform with different infrastructure services but do not involve the manipulation of variables and attributes together. Local values uniquely fulfill the requirement of integrating these components within a Terraform configuration.