Terraform: Add iterator to create multiple similar resources

[bashtoni]

It's common to need to create multiple similar resources. It would be great if there were to be a way of iterating through an array or group of arrays, creating a resource for each item.

An example might be creating subnets in AWS VPC. Right now my declaration looks like this:

resource "aws_subnet" "igw-subnet-eu-west-1a" {
    vpc_id = "${aws_vpc.eu-west-1.id}"
    availability_zone = "eu-west-1a"
    cidr_block = "192.168.192.0/24"
}

resource "aws_subnet" "igw-subnet-eu-west-1b" {
    vpc_id = "${aws_vpc.eu-west-1.id}"
    availability_zone = "eu-west-1b"
    cidr_block = "192.168.193.0/24"
}

resource "aws_subnet" "igw-subnet-eu-west-1c" {
    vpc_id = "${aws_vpc.eu-west-1.id}"
    availability_zone = "eu-west-1c"
    cidr_block = "192.168.194.0/24"
}

With an iterator, maybe this could be compressed to something like:

variable subnets {
    zones = [ "eu-west-1a", "eu-west-1b", "eu-west-1c" ],
    blocks = [ "192.168.192.0/24", "192.168.193.0/24", "192.168.194.0/24" ]
}

resource "aws_subnet" "igw-subnet-${foreach(subnets,zones)}" {
    vpc_id = "${aws_vpc.eu-west-1.id}"
    availability_zone = "${foreach(subnets,zones)}"
    cidr_block = "${foreach(subnets,blocks)}"
}

[michaelmior]

I suggested something somewhat similar in #133. See some possible syntax below. This is more general as you can have more possible configurations without needing to rely on iteration. However, I can see both approaches as being useful as the iteration syntax is more compact for the scenario @bashtoni presented above.

resource "aws_instance" "base_instance" {
  instance_type = "m1.small"
  ami = "${lookup(var.aws_amis, var.aws_region)}"
  abstract = true
}

resource "base_instance" "web" {
  provisioner "remote-exec" {
    inline = [
      "setup web"
    ]
  }
}

[delitescere]

How about something like this?

variable subnets {
  az = ['a', 'c', 'd']
  base_net = "10.0.0.0/16"
}
variable vpc {}
variable region {}

for ${var.subnets.az} {
  resource "aws_subnet" "igw-subnet-${var.region}${forValue()}" {
    vpc_id = "${var.vpc}"
    availability_zone = "${var.region}${forVal()}"
    cidr_block = cidr("${var.subnet.base_net}", "0.0.${forNumber()}.0/24")
  }
}

New for "statement".
New forValue() and forNumber() builtin functions to access the iteration state.
New cidr() function to validate a string as a CIDR format, and optionally use a mask for addition (as in this case).

[mitchellh]

I think this is in the same realm of #353 so I'm going to close this to get all our thinking going there.

[jfromaniello]

I know this issue was closed long ago but I wanted to share the way I found to solve this specific scenario with the current interpolation features and the embedded functions:

variable "name" {
    name = "jose"
}

variable "region" {
    default = "us-west-1"
}

variable "vpc_cidr_block" {
    default = "172.31.0.0/16"
}

variable "azs" {
    default = {
        "us-west-1" = "us-west-1b,us-west-1c"
        "us-west-2" = "us-west-2a,us-west-2b,us-west-2c"
        "us-east-1" = "us-east-1c,us-west-1d,us-west-1e"
        # use "aws ec2 describe-availability-zones --region us-east-1"
        # to figure out the name of the AZs on every region
    }
}

resource "aws_vpc" "main" {
    cidr_block           = "${var.vpc_cidr_block}"
    enable_dns_hostnames = true
    enable_dns_support   = true
    instance_tenancy     = "default"

    tags {
        "Name" = "${var.name}"
    }
}


resource "aws_subnet" "private-subnet" {
    vpc_id            = "${aws_vpc.main.id}"
    count             = "${length(split(",", lookup(var.azs, var.region)))}"
    cidr_block        = "${cidrsubnet(var.vpc_cidr_block, 4, count.index)}"
    availability_zone = "${element(split(",", lookup(var.azs, var.region)), count.index)}"
    map_public_ip_on_launch = false

    tags {
        "Name" = "private-${element(split(",", lookup(var.azs, var.region)), count.index)}-sn"
    }
}

If I run plan for this model I get this:

+ module.vpc.aws_subnet.private-subnet.0
    availability_zone:       "" => "us-west-1b"
    cidr_block:              "" => "172.31.0.0/20"
    map_public_ip_on_launch: "" => "0"
    tags.#:                  "" => "1"
    tags.Name:               "" => "private-us-west-1b-sn"
    vpc_id:                  "" => "${aws_vpc.main.id}"

+ module.vpc.aws_subnet.private-subnet.1
    availability_zone:       "" => "us-west-1c"
    cidr_block:              "" => "172.31.16.0/20"
    map_public_ip_on_launch: "" => "0"
    tags.#:                  "" => "1"
    tags.Name:               "" => "private-us-west-1c-sn"
    vpc_id:                  "" => "${aws_vpc.main.id}"

+ module.vpc.aws_vpc.main
    cidr_block:                "" => "172.31.0.0/16"
    default_network_acl_id:    "" => "<computed>"
    default_security_group_id: "" => "<computed>"
    dhcp_options_id:           "" => "<computed>"
    enable_dns_hostnames:      "" => "1"
    enable_dns_support:        "" => "1"
    instance_tenancy:          "" => "default"
    main_route_table_id:       "" => "<computed>"
    tags.#:                    "" => "1"
    tags.Name:                 "" => "jose"

[scholzie]

@jfromaniello This is a great solution. I am in a position where I need to get these subnet IDs as a list (for instance, to create an ELB). Do you know of an easy way to retrieve a list of all subnets created?

[billyogat]

@scholzie I'm in the same boat. Did you ever figure this out?

[billyogat]

found the answer on https://www.terraform.io/docs/configuration/interpolation.html

To reference attributes of other resources, the syntax is TYPE.NAME.ATTRIBUTE. For example, ${aws_instance.web.id} will interpolate the ID attribute from the "aws_instance" resource named "web". If the resource has a count attribute set, you can access individual attributes with a zero-based index, such as ${aws_instance.web.0.id}. You can also use the splat syntax to get a list of all the attributes: ${aws_instance.web.*.id}. This is documented in more detail in the resource configuration page.