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Terraform - Data Source

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• Terraform - Data Source
  • Data Source
    • Custom Condition Checks
  • Common Data Source

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Data Source

• Data sources

  • allow Terraform to use information defined outside of Terraform, defined by another separate Terraform configuration, or modified by functions.
• many data sources correspond to an infrastructure object type that is accessed via a remote network API

• some specialized data sources operate only within Terraform itself, calculating some results and exposing them for use elsewhere.

• How it works

  • Terraform reads data resources during the planning phase when possible, but announces in the plan when it must defer reading resources until the apply phase to preserve the order of operations.
• declared using a data block

data data_source ds_name {
    # query constraints
}

• cause Terraform to read from a given data source (“data_source”) and export the result under the given local name (“ds_name”).

  • The name is used to refer to this resource from elsewhere in the same Terraform module, but has no significance outside of the scope of a module.
  • data source + name = identifier for a given resource
  • must be unique within a module.

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Custom Condition Checks

• can use precondition and postcondition blocks to specify assumptions and guarantees about how the data source operates.

data "aws_ami" "example" {
  id = var.aws_ami_id

  lifecycle {
    # The AMI ID must refer to an existing AMI that has the tag "nomad-server".
    postcondition {
      condition     = self.tags["Component"] == "nomad-server"
      error_message = "tags[\"Component\"] must be \"nomad-server\"."
    }
  }
}

Common Data Source

• Example:

# caller identity details, such as account ID and ARN, for the current AWS account.
data "aws_caller_identity" "current" {}

# current AWS region
data "aws_region" "current" {}

# list of availability zones available in the current AWS region
data "aws_availability_zones" "current" {}

• get the terraform remote state

# get an existing remote s3 state
data "terraform_remote_state" "s3_remote_state"{
    backend = "s3"

    config = {
        bucket  = "bucket_name"
        key     = "key_name"
        region  = "region_name"
    }
}

# output vpc
output "remote_state_vpc_id" {
    value = data.terraform_remote_state.s3_remote_state.vpc_id
}