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HCL Code Generation

Terramate supports the generation of arbitrary HCL code such as Terraform, OpenTofu and other HCL configurations, referencing data such as Variables and Metadata.

The generate_hcl block

HCL code generation is done using generate_hcl blocks in Terramate configuration files. References to Terramate globals and metadata are evaluated, but any other reference is just transported to the generated code (For details, please see partial evaluation).

hcl
# example.tm.hcl
generate_hcl "backend.tf" {
  content {
    backend "local" {}
  }
}

The label of the generate_hcl block names the file that will be generated within a stack. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported when defining labels. For more details about how code generation uses labels check the Labels Overview docs.

Argument reference of the generate_hcl block

  • content (required block) The content block defines the HCL code that will be generated as file content. It supports block definitions, attributes and expressions. Terramate Variables and Terramate Functions can be used and will be interpolated during code generation.

    The following variable namespaces are available within the content block:

    hcl
    content {
      backend "local" {}
    }

    In addition, the special block tm_dynamic is available to generate dynamic content. Any references to functions, variables or blocks that Terramate is unaware of will be rendered as-is. See partial code generation for details.

  • lets (optional block) One or more lets blocks can be used to define Lets variables that can be used in other arguments within the generate_hcl block and in the content block and are only available inside the current generate_hcl block.

    hcl
    lets {
      temp_a_plus_b = global.a + global.b
    }

    TIP

    Use Lets over Global variables whenever you want to provide computed variables available inside the current generate_hcl block only.

  • stack_filter (optional block) Stack filter allow to filter stacks where the code generation should be executed. Currently, only path-based filters are available but tag-based filters are coming soon. Stack filters support neither Terramate Functions nor Terramate Variables. For advanced filtering of stacks based on additional conditions and complex expressions please use condition argument. stack_filter blocks have precedence over conditions and will be executed first for performance reasons. A stack will only be selected for code generation if any stack_filter is true and the condition is true too.

    Each stack_filter block supports one or more of the following arguments. When specifying more attributes, all need to be true to mark the stack_filter block as true.

    • project_paths (optional list of strings) A list of patterns matched against the absolute project path of the stack. The patterns support globbing but no regular expressions. Any matched path in the list will mark the project path filter as true.
    • repository_paths (optional list of strings) A list of patterns matched against the absolute repository path of the stack. The patterns support globbing but no regular expressions. Any matched path in the list will mark the repository path filter as true.
    hcl
    stack_filter {
      project_paths = [
        "/path/to/specific/stack", # match exact path
        "/path/to/some/stacks/*",  # match stacks in a directory
        "/path/to/many/stacks/**", # match all stacks within a tree
      ]
    }
  • condition (optional boolean) The condition attribute supports any expression that renders to a boolean. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported. Variables are evaluated with the stack context. For details, please see Lazy Evaluation. If the condition is true and any stack_filter (if defined) is true the stack is selected for generating the code. As evaluating the condition for multiple stacks can be slow, using stack_filter for path-based generation is recommended.

    hcl
    condition = tm_anytrue([
       tm_contains(terramate.stack.tags, "my-tag"), # only render if tag is set
       tm_try(global.render_stack, false),          # only render if `render_stack` is `true`
    ])
  • assert (optional block) One or more assert blocks can be used to prevent wrong configurations in code generation assertion can be set to guarantee all preconditions for generating code are satisfied. Each assert block supports the following arguments:

    • assertion (required boolean) When the boolean expression is false the assertion is triggered and the message is printed to the user. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported.
    • message (required string) A descriptive message to present to the user to inform about the causes that made an assertion fail. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported.
      • warning (optional boolean) When set to true the code generation will not fail, but a warning is issued to the user. Default is false. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported.
    hcl
    assert {
      assertion = tm_can(global.is_enabled)
      message   = "'global.is_enabled' needs to be set to either true or false"
    }

The tm_dynamic block

INFO

The tm_dynamic block is only supported within the content block of a generate_hcl block.

Additionally, a labels attribute can be provided for generating the block's labels. Example:

hcl
globals {
  values = ["a", "b", "c"]
}

generate_hcl "file.tf" {
  content {
    tm_dynamic "block" {
      for_each = global.values
      iterator = value
      labels   = ["some", "labels", value.value]

      content {
        key   = value.key
        value = value.value
      }
    }
  }
}

which generates:

hcl
block "some" "labels" "a" {
  key   = 0
  value = "a"
}
block "some" "labels" "b" {
  key   = 1
  value = "b"
}
block "some" "labels" "c" {
  key   = 2
  value = "c"
}

The labels must evaluate to a list of strings, otherwise it fails.

The tm_dynamic content block only evaluates the Terramate variables/functions, everything else is just copied as is to the final generated code.

The same goes when using attributes:

hcl
globals {
  values = ["a", "b", "c"]
}

generate_hcl "file.tf" {
  content {
    tm_dynamic "block" {
      for_each = global.values
      iterator = value

      attributes = {
        attr  = "index: ${value.key}, value: ${value.value}"
        attr2 = not_evaluated.attr
      }
    }
  }
}

Also generates a file.tf file like this:

hcl
block {
  attr = "index: 0, value: a"
  attr2 = not_evaluated.attr
}

block {
  attr = "index: 1, value: b"
  attr2 = not_evaluated.attr
}

block {
  attr = "index: 2, value: c"
  attr2 = not_evaluated.attr
}

The for_each attribute is optional. If it is not defined then only a single block will be generated and no iterator will be available on block generation.

The tm_dynamic block also supports an optional condition attribute that must evaluate to a boolean. When not defined it is assumed to be true. If the condition is false the tm_dynamic block is ignored, including any of its nested tm_dynamic blocks. No other attribute of the tm_dynamic block is evaluated if the condition is false, so it is safe to use it like this:

hcl
generate_hcl "file.tf" {
  content {
    tm_dynamic "block" {
      for_each  = global.values
      condition = tm_can(global.values)
      iterator  = value

      attributes = {
        attr  = "index: ${value.key}, value: ${value.value}"
        attr2 = not_evaluated.attr
      }
    }
  }
}

And if global.values is undefined the block is just ignored.

Filter-based Code Generation

To only generate HCL code for stacks matching specific criteria, a stack_filter block can be added within a generate_hcl block.

The following filter attributes for path filtering are supported:

  • project_paths: Match any of the given stack paths relative to the project root.
  • repository_paths: Match any of the given stack paths relative to the repository root.

Stack paths support glob-style wildcards:

  • * matches any sequence of characters until the next directory separator (/).
  • ** matches any sequence of characters.

Unless a path starts with * or /, it is implicitly prefixed with **/.

If multiple attributes are set per stack_filter, all of them must match.

If multiple stack_filter blocks are added, at least one must match.

Here's an example:

hcl
generate_hcl "file" {
  stack_filter {
    project_paths = ["networking/**"]
  }
  content {
    resource "networking_resource" "name" {
      # ...
    }
  }
}

This generates a file containing a networking resource only for stacks located within a directory named networking. The implicitly added prefix **/ means that this directory can be located anywhere in our project. The suffix /** means that the stack can be in any nested sub-directory of a networking directory.

If we change the attribute to project_paths = ["/networking/*"], it only matches stacks that are directly in a networking directory located at the project root level.

If more complex logic is required to decide if a file should be generated, see the condition attribute described in the next section.

Conditional Code Generation

Conditional code generation is achieved by the use of the condition attribute. The condition attribute should always evaluate to a boolean. The file will be generated only if it evaluates to true.

If the condition attribute is absent then it is assumed to be true.

Any expression that produces a boolean can be used, including references to globals and function calls. For example:

hcl
generate_hcl "file" {
  condition = tm_length(global.list) > 0
  content {
    locals {
      list = global.list
    }
  }
}

Will only generate the file for stacks that the expression tm_length(global.list) > 0 evaluates to true.

When condition is false the content block won't be evaluated.

  • A single label is required to define the type of the block to be dynamically generated. If the block needs to specify any labels the labels argument can be used to populate any number of labels.
  • labels (optional list of string) Define any number of labels the block shall be generated with. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported when defining labels. In addition, the iterator namespace is available which defaults to the label of the block being generated but can be renamed by using the iterator argument.
  • content (optional block) The content block is optional when attributes are defined. It supports the same features as the generate_hcl.content block. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported when defining labels. In addition, the iterator namespace is available which defaults to the label of the block being generated but can be renamed by using the iterator argument.
  • attributes (optional map) The attributes argument specifies a map of attributes that shall be rendered inside the generated block. Those attributes are merged with attributes and blocks defined in the content block, but they can not conflict, meaning any given attribute can either defined in attributes or content but not in both. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported when defining labels. In addition, the iterator namespace is available which defaults to the label of the block being generated but can be renamed by using the iterator argument.
  • for_each (optional list or map of any type) The for_each argument provides the complex list of values to iterate over. In each iteration, the iterator will be populated with a value of the current element. The element is accessible using the iterator namespace and defaults to the label of the block being generated. The value can be accessed with the value field.
  • iterator (optional string) The iterator sets the name of a temporary variable namespace that represents the current element of the complex value defined in for_each. If omitted, the name of the variable defaults to the label of the dynamic block.
  • condition (optional boolean) Instead of using the for_each the condition argument can be used for triggering generating the block based on an expression. Terramate Variables (let, global, and terramate namespaces) and all Terramate Functions are supported when defining labels.

Partial Evaluation

A partial evaluation strategy is used when generating HCL code. This means that you can generate code with unknown references/function calls and those will be copied verbatim to the generated code.

Let's assume we have a single global as Terramate data:

hcl
globals {
  terramate_data = "terramate_data"
}

And we want to mix those Terramate references with Terraform references, like locals, vars, outputs, etc. All we have to do is define our generate_hcl block like this:

hcl
generate_hcl "main.tf" {
  content {
    resource "myresource" "name" {
      count = var.enabled ? 1 : 0
      data  = global.terramate_data
      path  = terramate.path
      name  = local.name
    }
  }
}

And it will generate the following main.tf file:

hcl
resource "myresource" "name" {
  count = var.enabled ? 1 : 0
  data  = "terramate_data"
  path  = "/path/to/stack"
  name  = local.name
}

The global.terramate_data and terramate.path references were evaluated, but the references to var.enabled and local.name were retained as is, hence the partial evaluation.

Function calls are also partially evaluated. Any unknown function call will be retained as is, but any function call starting with the prefix tm_ is considered a Terramate function and will be evaluated. Terramate function calls can only have as parameters Terramate references or literals.

For example, given:

hcl
generate_hcl "main.tf" {
  content {
    resource "myresource" "name" {
      data  = tm_upper(global.terramate_data)
      name  = upper(local.name)
    }
  }
}

This will be generated:

hcl
resource "myresource" "name" {
  data  = "TERRAMATE_DATA"
  name  = upper(local.name)
}

If one of the parameters of an unknown function call is a Terramate reference the value of the Terramate reference will be replaced on the function call.

This:

hcl
generate_hcl "main.tf" {
  content {
    resource "myresource" "name" {
      data = upper(global.terramate_data)
      name = upper(local.name)
    }
  }
}

Generates:

hcl
generate_hcl "main.tf" {
  content {
    resource "myresource" "name" {
      data  = upper("terramate_data")
      name  = upper(local.name)
    }
  }
}

Currently, there is no partial evaluation of for expressions. Referencing Terramate data inside a for expression will result in an error (for expressions with unknown references are copied as is).