Intro

I’ve been doing a lot with terraform lately, and I’ve been looking for ways to make my terraform configurations a lot simpler and have less repetition. Like a lot of people, I’ve found myself repeating the same code over and over. An example is where I repeat the same resource over and over but with different configuration parameters. It’s essentially the same resource. Why should I do this? There has to be a better way.

What is a map?

In looking for a way to make my codebase cleaner and simpler, I started looking at using maps. I’ve not delved into them in depth before, so it was a fun thing to do on a rainy afternoon.

The terraform docs do a better job than I can of telling you what a map is - see here

Why are maps cool?

Maps are cool because they allow you to have groups of key value pairs that can be accessed in a neat way.

A simple example

A very simple example of a map is as follows:

variable "tcp_lb" {
  type = map
  default = {
    unit-config-origin = "8888"
    unit-git-origin = "8080"
  }
}

You will notice that the map above is actually a variable. That’s right you can use a map as a variable.

In this case, I am using a map to assign different ports to origin servers within a volterra resource. In this way, I don’t need to declare the same resource over and over, I can use a loop within my resource to access all of the items within my map.

The resource

The resource that uses the map above looks like this:

resource "volterra_tcp_loadbalancer" "unit-config" {
  for_each  = var.tcp_lb
  name      = "${each.key}"
  namespace = var.ns

  listen_port = "${each.value}"
  dns_volterra_managed = true
  domains = ["${var.domain_host}.${var.domain}"]
  advertise_on_public_default_vip = true

  retract_cluster = true

  origin_pools_weights {
    pool {
      name = "${each.key}"
      namespace = var.ns
    }
  }

  hash_policy_choice_round_robin = true
}

There are two pieces to this resource that I need to point out.

The for_each loop
Accessing map keys and values

The for_each loop

The for_each loop is used to loop through the map within the context of the resource. The for_each loop in terraofrm is documented here. The interesting thing is that a for_each loop can accept a map as an input!

  for_each  = var.tcp_lb

Note that the for_each loop references the variable that I defined above. The variable is actually a map, which the for_each loop can accept as an input. The for_each loop will loop through each key within the variable tcp_lb. In my case, the variable has two values. The for_each loop will run twice as it iterates over my map variable.

Accessing map keys and values

The map that I have defined has two keys and two values. Each key and each value can be acess separately within the context of the for_each loop.

In order to access each of the map values or names, I can use the following syntax:

  name      = "${each.key}"

and

  listen_port = "${each.value}"

The each.key and each.value keywords are used to access either the key or the value within the map.

In my case, on each iteration, the following will be true:

variable "tcp_lb" {
  type = map
  default = {
    unit-config-origin = "8888"
    unit-git-origin = "8080"
  }
}

One first iteration:

each.key = unit-config-origin
each.value = 8888

On the second iteration:

each.key = unit-git-origin
each.value = 8080

Conclusion

Using terraform maps simplifies your code by reducing the number of resources that you need to duplicate. It also makes your code a lot more readable.

There are traps and pitfalls using this method if you have dependant resources, but I’ll cover this in another post. :)

Terraform Maps