Support the count parameter for modules

[sinisterstumble]

First of all I wanted to thank all the contributors and especially Hashicorp for doing a great work. Thank you!

Currently the terraform modules do not support the count parameter. It would be a nice addition, encouraging a more DRY approach to writing modules.

Errors:
   * module root: module somemodule: count is not a valid parameter

[mitchellh]

Can you please share a use case for this?

[mitchellh]

Closing due to lack of response.

[blakeneyops]

I have a use case where this would be useful.

I am working on some templates to generically model an AWS VPC. I have created a module to model my default AZ configuration. As various regions support different numbers of AZs, I want the ability to specify the number of AZs to be provisioned for any given set of input parameters.

Example input parameters would be as follows:

variable "aws_vpc_tag" {
    default= "prod"
}
variable "vpc_cidr_block" {
    default = "10.120.0.0/22"
}
variable "region" {
    default = "us-east-1"
}
variable "az_count" {
    default = "4"
}
variable "az" {
    default = {
        "0" = "a"
        "1" = "b"
        "2" = "d"
        "3" = "e"
    }
}
variable "dmz_cidr_block" {
    default = {
        "0" = "10.120.0.0/25"
        "1" = "10.120.0.128/25"
        "2" = "10.120.1.0/25"
        "3" = "10.120.1.128/25"
    }
}
variable "lan_cidr_block" {
    default = {
        "0" = "10.120.2.0/25"
        "1" = "10.120.2.128/25"
        "2" = "10.120.3.0/25"
        "3" = "10.120.3.128/25"
    }
}
variable "instance_type" {}
variable "nat_ami" {}
variable "key_name" {}

The reason for the AZ map is that I have found that different AWS accounts will actually report different AZs as being VPC ready.

What I planned to do was the following:

module "az_conf" {
    source = "../templates/aws/vpc/az"
    count = "${var.az_count}"

    # Resource tags
    vpc_tag = "${var.aws_vpc_tag}"
    count_tag = "${count.index}"

    # VPC parameters
    vpc_id = "${module.vpc_conf.vpc_id}"
    igw_id = "${module.vpc_conf.igw_id}"

    region = "${var.region}"
    az = "${lookup(var.az, count.index)}"

    dmz_cidr_block = "${lookup(var.dmz_cidr_block, count.index)}"
    lan_cidr_block = "${lookup(var.lan_cidr_block, count.index)}"

    # EC2 parameters
    instance_type = "${var.instance_type}"
    nat_ami = "${var.nat_ami}"
    key_name = "${var.key_name}"
}

What I had to do instead for each AZ is more along the lines of:

module "az0" {
    source = "../templates/aws/vpc/az"

    # Resource tags
    vpc_tag = "${var.aws_vpc_tag}"
    count_tag = "0"

    # VPC parameters
    vpc_id = "${module.vpc_conf.vpc_id}"
    igw_id = "${module.vpc_conf.igw_id}"

    region = "${var.region}"
    az = "${lookup(var.az, 0)}"

    dmz_cidr_block = "${lookup(var.dmz_cidr_block, 0)}"
    lan_cidr_block = "${lookup(var.lan_cidr_block, 0)}"

    # EC2 parameters
    instance_type = "${var.instance_type}"
    nat_ami = "${var.nat_ami}"
    key_name = "${var.key_name}"
}

The alternative I reviewed was using count within the module itself, but it did not appear that there was a way to pass maps in as module input and I want to keep the modules generic with use case specific data in the calling template.

Let me know if you need any additional information or have any questions. Thanks.

[rcostanzo]

I hit this same problem today too. Not sure it's really related to this issue's title, so maybe move to its own. But I also would love to be able to pass a mapping into a module.

[johnrengelman]

I have the same use case for wanting to use count on a module. Basically creating the network topology and I want to wrap that up in a module and then iterate over AZs.

[jwthomp]

I am also hitting this as an issue. The use case is for using the docker provider to launch swarm agents across a list of nodes. I have put the swarm agent setup in a module to allow for being able to dynamically set up the docker provider based on a list of nodes.

module "agent" {
  source           = "./agent"
  docker_ip        = "${lookup(var.docker_ip, count.index)}"
  docker_port      = "${lookup(var.docker_port, count.index)}"
  swarm_version    = "${var.swarm_version}"
  swarm_token      = "${var.swarm_token}"
  count            = 3
}

[jjshoe]

#1471

[maartensl]

This is definitely useful.
I'm hitting a case where I have one core resource being used across every module. So in this case we have a resource encapsulated in a module, and the model then gets called numerous times to create the necessary resource (a VM) with variables to adjust memory, cpu, IP etc.

For scaling purposes we want to invoke the module in exactly the way explained here:
http://www.terraform.io/docs/configuration/resources.html#using-variables-with-count
Since the resource is encapsulated by the module, it wouldn't make sense to add the count at the resource level - it would break our abstraction model.