Any way to conditionalize variable in jsonencoded data? - terraform

Question

Say I have the simplified following snippet to create a task definition as json.
...
task_container_definitions = jsonencode([{
name : var.name,
image : "${var.image}:${var.tag}",
cpu : var.cpu,
memory : var.memory,
}])
...
Say I want to add a variable to optionally create an additional definition so it looks something like this:
variable "another_definition" {
type = any
default = {}
}
...
task_container_definitions = jsonencode([{
name : var.name,
image : "${var.image}:${var.tag}",
cpu : var.cpu,
memory : var.memory,
},
var.another_definition
])
And define it as follows.
another_definition = {
name = "another_container"
image = "another_container"
cpu = 10
memory = 512
essential = true
}
I am able to get this to to output as expected as long as the variable is defined.
...
+ {
+ cpu = 10
+ essential = true
+ image = "another_container"
+ memory = 512
+ name = "another_container"
},
But if the variable is not defined, I see empty {} added to the output when I do a terraform plan, which is not what I expect. I have tried using null as well as the default but get an error.
...
+ {},
Is there a way to toggle this variable off so that if it is not defined then it doesn't show up in the outputted json definition? Is there a better approach than what I am attempting?

Answer 1

I was a little confused at first as to what you were asking, thinking that you were asking for the functionality of the merge function, and I mention that only in case I was right the first time, but I think I now understand your problem as that you want this task_container_definitions to have either one or two elements, depending on whether var.another_definition is set.
There's no single function for that particular situation, but I think we can combine some language features together to get that result.
First, let's decide that the variable being set means that it has a non-null value, and thus its default value should be null to represent the "unset" case:
variable "another_definition" {
type = any
default = null
validation {
# The time constraint above is looser than we really
# want, so this validation rule also enforces that
# the caller can't set this to something inappropriate,
# like a single string or a list.
condition = (
var.another_definition != null ?
can(keys(var.another_definition)) :
true
)
error_message = "Additional task container definition must be an object."
}
}
In Terraform it's a pretty common situation to need to convert between a value that might be null and a list that might have zero or one elements, or vice-versa, and so Terraform has some language features to help with that. In this case we can use a splat expression to concisely represent that. Let's see how that looks in terraform console first just to give a sense of what we're achieving with this:
$ terraform console
> null[*]
[]
> "hello"[*]
[
"hello",
]
> { object = "example" }[*]
[
{
"object" = "example"
},
]
Notice that when I applied the [*] operator to null it returned an empty list, but when I applied it to these other values it converted them to a single-element list. This is how the [*] operator behaves when you apply it to something that isn't a list; see the splat operator docs if you want to learn about the different behavior for lists, which isn't really relevant here because of the validation rule I added above which prevents the var.another_definition value from being a list.
Another tool we have in our Terraform toolbox here is the concat function, which takes one or more lists and returns a single list with the input elements all concatenated together in the given order. We can use this to combine your predefined list that's populated from var.name, var.cpu, etc with the zero-or-one element list created by [*], in order to create a list with their one or two elements:
locals {
task_container_definitions = concat(
[
name = var.name
image = "${var.image}:${var.tag}"
cpu = var.cpu
memory = var.memory
],
var.another_definition[*],
)
task_container_definitions_json = jsonencode(local.task_container_definitions)
}
If any of the arguments to concat are empty lists then they are effectively ignored altogether, because they contribute no elements to the result, and so this achieves (what I hope is) the desired result, by making the "other definition" appear in the result only when it's set to something other than null.