In the previous two recipes, we learned that it is possible to use either the data block or the terraform_remote_state block to retrieve external data. However, there are scenarios where the data block does not exist in the provider or terraform_remote_state cannot be used, such as when we need to process with an external API or need to use a local tool and process its output.
To meet this need, there is an external resource in Terraform that allows you to call an external program and retrieve its output data so that it can be used in the Terraform configuration.
Use of the external provider imposes prerequisites that may not be obvious (for example, in this case, we expect a particular version of PowerShell) or may be difficult to communicate other than through README files or documentation. Also, Terraform is generally designed to work the same cross-platform (operating system/architecture), but this essentially restricts the configuration to particular platforms that can (and do) run PowerShell – presumably just Windows. These requirements apply to both CI and local environments.
In this recipe, we will learn how to call an external program and retrieve its output so that we can reuse it.
