Variable Interpolation

[Matthew Ceroni]

Have a unique question, that I can't solve (maybe I can't do what I want).

Designing a VPC module, I have a variable availability_zones which defines which AZ to create subnets in. I want the default to be all availability zones in the region the VPC is being created in. Otherwise the user can supply the list of AZ to deploy into.

However I can't do variable interpolation within a variable.  I was trying to do something like

data "availabiilty_zones" "all" {}

variable "availability_zones" {

  type = "list"

  default = "${data.availability_zones.all.names}"

}

That fails since you can't do interpolation here.

Within the actual resource creation I use the variable availablity_zones in many places, as an example

resource "aws_subnet" "private" {

  count  = "${length(var.availability_zones)}"

  vpc_id = "${aws_vpc.environment.id}"

  cidr_block = "${cidrsubnet(aws_vpc.environment.cidr_block, var.cidr_block_bits, length(var.availability_zones) + count.index)}"

  /* load balance over all availability zones */

  availability_zone = "${element(var.availability_zones, count.index)}"

  /* private subnet, no public IPs */

  map_public_ip_on_launch = false

  /* merge all the tags together */

  tags = "${merge(var.tags, var.private_subnet_tags, map("Name", format("private-%d.%s", count.index, var.name)), map("builtWith", "terraform"))}"

}

I could replace all var.availablity_zones with a conditional CONDITIONAL ? TRUE : FALSE but that will get kind of ugly repeating that over and over. 

What I am basically looking for is a way to set that one place, and then just use it later on. 

[Anshu Prateek]

Some of the ways I can think of:

1) Always use data.aws_availability_zones.available.names instead of var.availability_zones

2) Use locals to convert data.aws_availability_zones.available.names to local.availability_zone

https://www.terraform.io/docs/configuration/locals.html

3) Use output to get availability_zone into you state file location and then use that with data_get (similar to 1 but you query your state store instead of querying AWS everytime)

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regards
Anshu Prateek
+91.991.610.2967