How to make Terraform Modules wait for resources to be created in place of using computed values

Today I am working on building out several custom Terraform Modules. The issue that I am trying to figure out how to work around dependencies within modules. Until TF 0.12 is released we cannot declare a module to be dependent upon another module. So, in this root level main.tf:

# ROOT level main.tf
# -------------------------------------------------------------------
# Create NAT Gateway - Associates EIP as well
# -------------------------------------------------------------------
module "vpc_nat_gateway" {
  source            = "./vpc_nat_gateway"
  vpc_id            = "${ module.vpc.id }"
  public_subnet_ids = "${ module.vpc_subnets.public_subnet_ids }"
  private_cidr      = "${ var.private_cidr }"
  common_tags       = "${ local.common_tags }"
}

# -------------------------------------------------------------------
# Create Private Routes
# -------------------------------------------------------------------
module "vpc_private_route" {
  source         = "./vpc_private_route"
  vpc_id.        = "${ module.vpc.id }"
  nat_gateway_id = "${ module.vpc_nat_gateway.nat_gateway_id }"
  common_tags    = "${ local.common_tags }"
}

# vpc_private_route module - main.tf
data "aws_nat_gateway" "az1" {
  vpc_id = "${ var.vpc_id }"

  tags {
    Name = "*NAT GW AZ 1"
  }
}

data "aws_nat_gateway" "az2" {
  vpc_id = "${ var.vpc_id }"

  tags {
    Name = "*NAT GW AZ 2"
  }
}

The result output is:

```
------ SNIP -----
module.vpc_nat_gateway.aws_nat_gateway.nat[1]: Creation complete after 1m50s (ID: nat-02a7f4279cec3a6c8)
module.vpc_nat_gateway.aws_nat_gateway.nat.0: Still creating... (2m0s elapsed)
module.vpc_nat_gateway.aws_nat_gateway.nat[0]: Creation complete after 2m0s (ID: nat-0695a12d9c0305e4c)

Error: Error applying plan:

3 error(s) occurred:

* module.vpc_private_route.data.aws_subnet_ids.private: data.aws_subnet_ids.private: no matching subnet found for vpc with id vpc-0b530d1885e74067b
* module.vpc_private_route.data.aws_nat_gateway.az2:

Update: As of TF 0.13x, depends_on is supported to create dependencies between modules. Using depends_on should be preferable over using the method detailed below.

The previous most popular answer is out of date with Terraform 0.12.24.

depends_on is a protected variable, and cannot be used in a module.
In addition there are a few syntax differences.

I've updated the example below.

# ROOT level main.tf
# -------------------------------------------------------------------
# Create NAT Gateway - Associates EIP as well
# -------------------------------------------------------------------
module "vpc_nat_gateway" {
  source            = "./vpc_nat_gateway"
  vpc_id            = module.vpc.id
  public_subnet_ids = module.vpc_subnets.public_subnet_ids
  private_cidr      = var.private_cidr
  common_tags       = local.common_tags
}

# -------------------------------------------------------------------
# Create Private Routes
# -------------------------------------------------------------------
module "vpc_private_route" {
  source         = "./vpc_private_route"
  vpc_id.        = module.vpc.id
  nat_gateway_id = module.vpc_nat_gateway.nat_gateway_id
  common_tags    = local.common_tags

  # Depends is a custom variable, depends_on is a reserved keyword.
  depends = [module.vpc_nat_gateway.nat_gateway_id]
}

# vpc_private_route module - main.tf
variable "depends" {
  default = []
}

resource "null_resource" "depends_on" {
  triggers = {
    depends_on = "${join("", var.depends)}"
  }
}

data "aws_nat_gateway" "az1" {
  vpc_id = var.vpc_id

  depends_on = [
    null_resource.depends_on
  ]
}

data "aws_nat_gateway" "az2" {
  vpc_id = var.vpc_id

  depends_on = [
    null_resource.depends_on
  ]
}

StackExchange DevOps Q#6163, answer score: 7