Sharable "count" meta-argument between Terraform modules? - terraform

Question

I have a case in which I have two modules, one that creates a resource off of OS1, and another using OS2. A common network configuration is used for these two resources such that they will attach to the same network upon provisioning. What I'm hoping to achieve is when specifying the IP addresses of these machines, I can have subsequent IPs for machines attaching to a particular network, for example, given
module one{
source = /path/to/OS1_module
count = 2
network_config = var.networkConfig1
}
module two{
source = /path/to/OS2_module
count = 2
network_config = var.networkConfig1
}
is there some way to share a variable between the two such that the subsequent two VMs provisioned in module two would follow those of module one? For example if I wanted to hold a variable defining the third octet of a 192.168.x/24 network, is it possible to make such a variable using count or some other method and share it between the two such that
VM1-OS1 = 192.168.1.10
VM2-OS1 = 192.168.1.11
VM3-OS2 = 192.168.1.12
VM4-OS3 = 192.168.1.13

Answer 1

I'm not totally sure I understand your requirement, but it sounds like you want module "two" to start its numbering at the next available address after module "one".
One way to achieve that would be to add a new input variable to this module to specify the host number and then set that variable for each instance in a way that takes into account the "stacking" of the modules, like this:
module "one" {
source = "./modules/OS1"
count = 2
network_config = var.networkConfig1
host_number = count.index
}
module "two" {
source = "./modules/OS1"
count = 2
network_config = var.networkConfig1
host_number = length(module.one) + count.index
}
This makes use of the fact that a module block with count appears in expressions as a list of objects representing all of the instances, and therefore length(module.one) will always equal the count value for that module block.
This approach does have the big caveat that if you change the count of module "one" in future then you'll also renumber all of the module "two" instances, because length(module.one) will change. Unfortunately that's a typical problem with IP address range planning, and I typically advise addressing it by centralizing your address assignments in a single location and then passing the assigned addresses into the separate modules that need them, since then in future you can maintain the IP address table in one place -- making sure not to inadvertently renumber any existing networks -- rather than having to carefully coordinate changes across several different modules.