A comparative guide to generate Terraform / HCL

Terraform end-users are programmers who would write .tf files. As such, if you are one, you would be familiar with a line like so:

resource "aws_vpc" "team_rocket" {
  cidr_block = "10.0.0.0/16"
}

Within the ecosystem, Terraform also has custom providers. Some programmers maintain and develop those for miscellaneous services. My employer is in this category, providing terraform-provider-firehydrant for customers.

Then, there is the third side that we rarely talk about, the inverse of Terraform end-users: maintaining a program to generate Terraform configuration. One such project is Terraformer from Waze, which scans existing infrastructure and outputs Terraform configuration that reflects it.

Writing program, to generate configuration, so that infrastructure is reflected as configuration and convention. Very Computer Thing going on here.

If it sounds slightly off, it kind of is! There are valid use cases for this, but certainly quite rare compared to the other two categories. Typically in migration / bootstrap situation for adopting new platform.

So you want to generate Terraform files ¶

I won’t advise you on whether you should do it or not. However, if you have decided to do it, the rest of this post may be helpful. We will be using two libraries:

• github.com/hashicorp/hcl/v2/hclwrite the primary package to generate HCL (HashiCorp Configuration Language) files
• github.com/zclconf/go-cty/cty the underlying type system used within HCL.

Start with the canvas. Think of it as a virtual file or buffer.

f := hclwrite.NewEmptyFile()
rootBody := f.Body()

Creating a resource block ¶

resource "some_resource" "for_some_team" {
  id   = 1
  name = "team rocket"
}

// Declare a new "block" for the resource
r := rootBody.AppendNewBlock(
  "resource",
  []string{"some_resource", "for_some_team"},
)
// Now write the content of the block
r.SetAttributeValue("id", cty.NumberIntVal(1))
r.SetAttributeValue("name", cty.StringVal("team rocket"))

Conversely, for “data” block, replace "resource" with "data".

You may also find blocks without extra attribute labels, like the terraform block.

terraform {
  required_providers {
    # ...cut...
  }
}

tf := rootBody.AppendNewBlock("terraform", nil)
p := p.AppendNewBlock("required_providers", nil)

Creating anonymous block / map ¶

musicians = {
  mozart = true
}

m := cty.ObjectVal(map[string]cty.Value{
  "mozart": cty.True,
})
rootBody.SetAttributeValue("musicians", m)

Referencing another object ¶

resource "acme_team" "team_rocket" {
  admin_id = acme_user.alice.id
}

r := rootBody.AppendNewBlock(
  "resource",
  []string{"acme_team", "team_rocket"},
}

r.SetAttributeTraversal(
  "member_ids",
  hcl.Traversal{
    hcl.TraverseRoot{Name: "acme_user"},
    hcl.TraverseAttr{Name: "alice"},
    hcl.TraverseAttr{Name: "id"},
  },
)

At the time of writing this post, writing a list of traversal involves SetAttributeRaw by providing Tokens value.

resource "acme_team" "team_rocket" {
  members = [
    acme_user.bob.id,
    acme_user.charlie.id,
  ]
}

bob := hcl.Traversal{ /* See previous example */ }
charlie := hcl.Traversal{ /* See previous example */ }

r.SetAttributeRaw(
  "members",
  hcl.TokensForTuple([]hcl.Tokens{
    hcl.TokensForTraversal(bob),
    hcl.TokensForTraversal(charlie),
  }),
)

Adding comments ¶

# This is
# a comment

rootBody.AppendUnstructuredTokens(
  hclwrite.Tokens{
    &hclwrite.Token{
      Type:  hclsyntax.TokenComment,
      Bytes: []byte("# This is\n# a comment"),

      SpacesBefore: 0,
    },
  },
)