Dynamically distribute EC2s in available Subnets via Terraform [duplicate] - amazon-web-services

Question

This question already has an answer here:
Terraform list element out of bounds?
(1 answer)
Closed 3 years ago.
The requirement is to create EC2s from the dynamically given list instance_names and distributed them evenly in the available subnets of the VPC.
I have tried looping and conditional statements with little luck.
Use Case 01 - (In a VPC with two subnets) If we are creating 2
servers, One EC2 should be in subnet 'a' and other in subnet 'b'
Use Case 02 - (In a VPC with two subnets) If we are creating 3
servers, two EC2s need to be in subnet 'a' and the other EC2 in subnet
'b'
Control Code
module "servers" {
source = "modules/aws-ec2"
instance_type = "t2.micro"
instance_names = ["server01", "server02", "server03"]
subnet_ids = module.prod.private_subnets
}
Module
resource "aws_instance" "instance" {
count = length(var.instance_names)
subnet_id = var.subnet_ids[count.index]
tags = {
Name = var.instance_names[count.index]
}
}

Answer 1

You can use element to loop around the subnet_ids list and get the correct id for each aws_instance.
In the docs you can see that element will give you the desired effect because:
If the given index is greater than the length of the list then the index is "wrapped around" by taking the index modulo the length of the list
Use Case 1
-> server01 - subnet 'a' <-> element(subnet_ids,0)
-> server02 - subnet 'b' <-> element(subnet_ids,1)
Use Case 2
-> server01 | subnet 'a' <-> element(subnet_ids,0)
-> server02 | subnet 'b' <-> element(subnet_ids,1)
# loop around the subnet id list the first id again
-> server03 | subnet 'a' <-> element(subnet_ids,2)
-> server04 | subnet 'b' <-> element(subnet_ids,3)
-> etc.
So the following update to the code should work:
resource "aws_instance" "instance" {
count = length(var.instance_names)
subnet_id = element(var.subnet_ids, count.index)
tags = {
Name = var.instance_names[count.index]
}
}

Answer 2

I found an interesting answer from Tom Lime for a similar question. I derived an answer from it to this scenario. For the Module, we provide the below logic for the subnet_id
subnet_id = "${var.subnet_ids[ count.index % length(var.subnet_ids) ]}"