09_TF_LANG_CHECKS.md

CHECKS

• The check block can validate your infrastructure outside the usual resource lifecycle. check blocks address a gap between post-apply and functional validation of infrastructure
• check blocks allow you to define custom conditions that execute on every Terraform plan or apply operation without affecting the overall status of an operation. dheck blocks execute as the last step of a plan or apply after Terraform has planned or provisioned your infrastructure

Syntax

• You can declare a check block with a local name, zero-to-one scoped data sources, and one-to-many assertions.
• The following example loads the Terraform website and validates that it returns the expected status code 200.

check "health_check" {
  data "http" "terraform_io" {
    url = "https://www.terraform.io"
  }
  assert {
    condition = data.http.terraform_io.status_code == 200
    error_message = "${data.http.terraform_io.url} returned an unhealthy status code"
  }
}

• You can use any data source from any provider as a scoped data source within a check block
• A check block can optionally contain a nested (a.k.a. scoped) data source. This data block behaves like an external data source, except you can not reference it outside its enclosing check block
• Scoped data sources support the depends_on and provider meta-arguments. Scoped data sources do not support the count or for_each meta-arguments.
• The first time Terraform runs this check, it always throws a potentially distracting error message. You can fix this by adding depends_on to your scoped data source, ensuring it depends on an essential piece of your site's infrastructure. The check returns known after apply until that crucial piece of your website is ready
• Check blocks validate your custom assertions using assert blocks. Each check block must have at least one, but potentially many, assert blocks. Each assert block has a condition attribute and an error_message attribute.
• Check blocks offer the most flexible validation solution within Terraform. You can reference outputs, variables, resources, and data sources within check assertions. You can also use checks to model every alternate Custom Condition.