backend.tf

provider "aws" {
  region = "eu-west-1"
}

terraform {
  backend "s3" { # will handle correctly multiple workspaces
    bucket         = "project"
    region         = "eu-west-1"
    key            = "project/terraform.tfstate"
    dynamodb_table = "project-terraform-lock" # to prevent race conditions, table needs to be created manually
  }
}

data "terraform_remote_state" "network" {
  backend = "s3" # bucket needs to be created manually

  config { 
    bucket = "tf-project"
    key    = "project/terraform.tfstate"
    region = "eu-west-1" # make sure terraform has permissions
  }
}

merge_vars.tf

variable "common_tags" {
  default = {
    ServiceName  = "platform"
    Team = "my@email.com"
  }
}

resource "aws_subnet" "private" {
  tags = "${merge(
    var.common_tags, 
    map("Name", "PriSubnet1"),
    map("Network", "Private"),
  )}"
}

workspace-map.tf

locals { # select a workspace using `terraform workspace select $NAME`
  domains { 
    dev     = "dev.mydomain.com"
    prod    = "prod.mydomain.com"
  }

  domain = "${local.domains[terraform.workspace]}"
}

lifecycle {
    prevent_destroy = true
}

terraform lifecycle methods may prevent you from breaking things accidentally

https://hackernoon.com/blue-green-infrastructure-with-terraform-d5f9e8f79ed4

https://github.com/skyscrapers/terraform-bluegreen