In Terraform, what is '${element(aws_subnet.public.*.id, count.index)}'?

Question

Can anyone explain this line.

subnet_id      = "${element(aws_subnet.public.*.id, count.index)}"

I used this in terraform code for creating one stack

Answer 1

aws_subnet.public.*.id return a list of subnet ids, e.g.:

["subnet-3232", "subnet-555", "subnet-6666"]

So element(aws_subnet.public.*.id, count.index) will pick a single subnet id from the list, depending on the value of count.index. For example:

element(["subnet-3232", "subnet-555", "subnet-6666"], 0) # pick first one
element(["subnet-3232", "subnet-555", "subnet-6666"], 1) # pick second one
element(["subnet-3232", "subnet-555", "subnet-6666"], 2) # pick last one
element(["subnet-3232", "subnet-555", "subnet-6666"], 3) # pick first one again (aka wrap-around)

Also if count.index is greater then the number of elements, it element will go back and start from the head of the list (aka "wrap-around").