Creating valid JSON for container_definitions in Terraform - json

Question

I’m creating an aws_ecs_task_definition resource. Within that resource, I need a container_definitions, which needs to be a JSON string. I’d like to add multiple secrets to that definition from a list of strings; [“var1”, “var2”].
The output I need looks like:
“secrets”: [
{
“name” = “var1”,
“valueFrom” = “arn:somestuffvar1”
},
{
“name” = “var2”,
“valueFrom” = “arn:somestuffvar2”
}
],
I have tried string interpolation and templatefile, this is the section from my .tftpl
  "secrets": [
   %{ for myvar in myvars ~}
    {
      "name": "${myvar}",
     "valueFrom”: “arn:somestuff${myvar}"
    }
    %{ endfor }
  ],
The problem is the commas. the above gives me
[
{
“name” = “var1”,
“valueFrom” = “arn:somestuffvar1”
}
{
“name” = “var2”,
“valueFrom” = “arn:somestuffvar2”
}
],
with no commas between the braces, if I add a comma, then i get a trailing comma
[
{
},
{
},
],
I’ve tried a zillion syntax variations, I’ve tried jsonencode on the interpolated string, I’ve tried stripping the trailing comma. Nothing gives me valid JSON. What am I missing?

Answer 1

The templatefile function documentation has a section specifically about Generating JSON or YAML from a template, which explicitly discourages using string templating to try to build valid JSON from string fragments like this.
Instead, you should use the jsonencode function as the entire definition of your template, and thus let Terraform be the one to worry about generating valid JSON syntax. You then only need to worry about writing an expression that describes the data structure that remote system expects.
In your case, a template generating a JSON object with just this "secrets" property would look like this:
${jsonencode({
secrets = [
for myvar in myvars : {
name = myvar
valueFrom = "arn:somestuff${myvar}"
}
],
})}
Notice that the entire template consists of a single interpolation sequence ${ ... } and the expression inside it is one big call to the jsonencode function, with the argument describing the data structure to serialize. Therefore inside that argument we're using normal Terraform expression syntax (like you'd write in a resource argument in a .tf file) rather than the special template interpolation/repetition syntaxes. In particular, the value of "secrets" is defined using a for expression.
With your { myvars = ["var1", "var2"] } template variables, this will produce a minified version of the following JSON structure, which I'm showing with manually-added indentation and newlines just so you can read it:
{
"secrets": [
{
"name": "var1",
"valueFrom": "arn:somestuffvar1"
},
{
"name": "var2",
"valueFrom": "arn:somestuffvar2"
}
]
}
I understand that you're only showing a fragment of the template here and so the above won't include all of the other properties included in your template, but hopefully you can see how to use Terraform expression syntax to describe those properties as Terraform object attributes too, so that the overall result of this template will be a valid JSON serialization of the total data structure.