TfBuilder

TfBuilder is a small JavaScript library to generate Terraform code. It overcomes the limitations of HCL by using a well-known, general-purpose scripting language while maintaining concise syntax and simplicity. TfBuilder is thoroughly tested.

A Terraform module comprises a directory containing .tf files written in HCL (HashiCorp Configuration Language). But HCL is just an alternative syntax for JSON. Terraform also reads any .tf.json files and the two formats can be freely mixed. (https://www.terraform.io/docs/configuration/load.html) TfBuilder helps generate Terraform JSON.

Usage

npm install @vobarian/tfbuilder

const {Configuration} = require('@vobarian/tfbuilder')
const {resource, data} = config = Configuration()
// build config as described below
config.writeTo('myconfig.tf.json')

When it's time to invoke Terraform, first run your scripts to generate .tf.json files, then invoke normal terraform commands. (Use a shell script or makefile to do both.)

Configuration

Since we're ultimately building JSON, it really helps to understand the equivalence of HCL to JSON first: https://www.terraform.io/docs/configuration/syntax.html.

TfBuilder provides a single factory function, Configuration, which returns an object representing the top-level Terraform JSON object. Configuration exposes the following properties representing HCL blocks:

• resource
• data
• module
• variable
• locals
• output
• provider

Resources, Data Sources, and Modules

TfBuilder provides two shortcuts under the resource, data, and module properties:

1. Automatically adding objects when a new resource type, data source type, or module is referenced
2. Generating interpolation strings when an undefined property is read

Compare the following HCL and TfBuilder versions of a configuration that copies the file input.txt to output.txt while appending the word ok:

HCL:

data "local_file" "my_input" {
    filename = "input.txt"
}
resource "local_file" "my_output" {
    filename = "output.txt"
    content  = "${data.local_file.my_input.content} ok"
}

TfBuilder:

data.local_file.my_input = {
    filename: 'input.txt'
}
resource.local_file.my_output = {
    filename: 'output.txt',
    content: data.local_file.my_input.content + ' ok'
}

Notice:

1. You can assign the my_input property on the object data.local_file without initializing local_file to a new object first. (If data were a plain JS object, you'd get "Cannot set property 'my_input' of undefined".)
2. In the resource, the expression data.local_file.my_input.content evaluates to the required Terraform interpolation string, ${data.local_file.my_input.content}. When you read a non-existent property of a resource, data source, or module object, TfBuilder automatically builds an interpolation string for you.

You can further simplify with JavaScript variables:

const inputFile = data.local_file.my_input = {
    filename: 'input.txt'
}
assert(inputFile.content === "${data.local_file.my_input.content}")

Interpolation strings are generated for you only when you read a non-existent property. If you assign a value to a property and then read it, you just get back the value as you would with a normal object. This strategy usually works because the names of resource arguments and attributes rarely overlap. However, if you have a module, data source, or resource where the same name is used as both an input and an output, you can use the automatically provided $output method to generate the interpolation string:

// Suppose this module uses "special" as the name of
// both input and output variables
const m = config.module.example = {
    source: './example',
    special: 'something' // input variable named "special"
}
assert(m.special === "something")
assert(m.$output("special") === "${module.example.special}")

The objects you create correspond directly to the JSON that will be generated. Translating HCL documentation to JSON can be tricky because it's not always clear what the data type should be for nested blocks. Generally, any repeatable block must be in an array. If you use a map for a nested configuration block and get syntax errors when you run Terraform, try wrapping it in an array. Examples are cache_behavior blocks in aws_cloudfront_distribution or lifecycle_rule blocks in aws_s3_bucket.

Implementation note: When you attach an object to a Configuration as a resource, data source, or module, TfBuilder replaces its prototype. Do not try to use inheritance for your configuration objects. Also, you cannot assign the same object more than once as a resource, data source, or module, like this: resource.example.a = resource.example.b. Instead, use a factory function or deep clone. This design avoids unintentional side effects of aliasing.

Variables, Locals, and Outputs

Configuration has variable, locals, and output properties to facilitate interoperability with hand-written Terraform. These don't provide any magic properties or interpolation; they're just JavaScript objects. If you build everything with TfBuilder they should be unnecessary.

Providers

In HCL, provider blocks are unusual because, unlike other repeated configuration blocks, they don't have unique names before the curly braces; e.g.:

provider "aws" {
    region = "us-east-1"
}
provider "aws" {
    alias  = "backup"
    region = "us-west-2"
}

Each provider block must have a unique alias field, except for one of each type, which becomes the "default". This is significant because it translates to some really quirky JSON: the provider property is actually an array where each element is an object with a single key (the provider name) and a value containing the rest of the config block. The above HCL in JSON is:

{
    "provider": [
        {
            "aws": {
                "region": "us-east-1"
            }
        },
        {
            "aws": {
                "alias": "backup",
                "region": "us-west-2"
            }
        }
    ]
}

To ease the pain, Configuration has a provider array with a special add method:

config.provider.add('aws', {
    region: 'us-east-1'
})
config.provider.add('aws', {
    alias:  'backup',
    region: 'us-west-2'
})

Terraform prohibits using interpolation in the alias argument but you can use a JavaScript variable or function since it's evaluated before the JSON is written out.

Building Larger Configurations

An advantage of using JavaScript is that functions can help modularize code and reduce duplication. Functions are more flexible and concise than Terraform modules. Also, functions can be used to build nested blocks, whereas Terraform modules can only create whole resources and data sources.

A function's parameters take the place of module input variables. Output variables generally aren't needed since any properties can be read directly from the returned object.

If a function only creates a single resource, data source, or module, you can just return it and assign it to an appropriate place in the configuration tree:

function secureS3Bucket(params) { /* return object with aws_s3_bucket args */ }

config.resource.aws_s3_bucket.my_bucket = secureS3Bucket(/* params */)

Configuration#merge combines two Configurations by adding all the configuration objects (resources, data sources, etc.) from the Configuration parameter. A function that creates multiple resources can return a new Configuration to be merged into the main one:

function loadBalancedCluster(options) {
    const {resource, data} = config = Configuration()
    // add resources
    return config
}

mainConfig.merge(loadBalancedCluster(/* params */))

The latter approach reduces the amount of code at the call site and relieves the caller of the responsibility for knowing the resource or data source type, so you may prefer it even for functions that only create one resource.

Every resource in Terraform needs a unique address. A normal Terraform module can be used multiple times as long as each instance has a unique name because Terraform creates unique addresses for resources in the module by prefixing them with module.NAME.

But Terraform is unaware of any JavaScript functions, so if you create resources in a function and want to use that function repeatedly, it's up to you to create unique names. You could pass a prefix as a parameter to the function. The function would use the prefix in all resource names it generates. Another option is to include a unique property of the resource itself in the Terraform name. For example, if a function creates AWS S3 buckets, the bucket names are already unique, so the bucket name could be used as part of the resource name to make it unique.

Overcome HCL Limitations

This section demonstrates cool things you can do more easily with JavaScript.

Local Variables

Local variables are awkward in HCL:

locals {
    app_name = "5250 Cloud Proxy"
}
resource "example" "abc" {
    some_arg = "${local.app_name}"
}

In JS, just use a constant anywhere you need to have a consistent value but don't want to have to find-and-replace later:

const appName = '5250 Cloud Proxy'
resource.example.abc = {
    some_arg: appName
}

Of course, you still have to use Terraform local variables for dynamic values which aren't known before Terraform runs. Also beware that you can't just use string constants if one resource depends on another; Terraform infers the dependency graph from interpolations, which affects the order in which resources are created. If one resource depends on another, interpolate the attributes of the first into the arguments of the second using Terraform interpolation syntax (via TfBuilder's magic properties shortcut).

Interpolation strings can get really long and hard to read. In JS you can easily alias an object to a local variable and then get properties, including interpolation expressions, off the local variable:

const oai = resource.aws_cloudfront_origin_access_identity.identity = {
    comment = 'what a long resource name'
}
// Now these 3 lines are equivalent, but one is much shorter:
// resource.aws_cloudfront_origin_access_identity.identity.cloudfront_access_identity_path
// "${aws_cloudfront_origin_access_identity.identity.cloudfront_access_identity_path}"
// oai.cloudfront_access_identity_path

If Statements

Using count for conditionals is hacky and doesn't work for some nested config blocks. For example, in HCL, it's currently impossible to make an S3 bucket module with a logging configuration conditionally enabled by input variables. In JavaScript it's trivial:

function customBucket({otherParams, accessLogsBucket = null}) {
    const bucket = { /*bunch of fancy config*/ }
    if (accessLogsBucket) {
        bucket.logging = {
            target_bucket: accessLogsBucket
        }
    }
    return bucket
}

And this train wreck (which includes "AU" in the whitelist only if the current level is beta):

locals {
  geo_whitelist_base = ["US","CA","AU"]
  geo_whitelist = "${slice(local.geo_whitelist_base, 0, length(local.geo_whitelist_base) - (var.level == "beta" ? 0 : 1))}"
}

becomes:

const countryWhitelist = ["US", "CA"]
if (level === "beta") countryWhitelist.push("AU")

Functions

Functions reduce duplication. One good use case is a CloudFront distribution with multiple cache behaviors that differ only in the path mapping and origin. For example, one project required 120 lines of HCL to define 5 cache behaviors; the TfBuilder equivalent is 37 lines.

Functions can also be much more convenient than Terraform modules:

HCL:

module "cloudfront_tags" {
    source = "./tags_validator"
    tags = {
        Name    = "Our CloudFront Distribution"
        Version = "1.0.0"
    }
}
resource "aws_cloudfront_distribution" "cloudfront" {
    tags = "${module.cloudfront_tags.tags}"
    // ...
}

TfBuilder:

const validateTags = require('validate-tags') // ONCE per file
resource.aws_cloudfront_distribution.cloudfront = {
    tags: validateTags({
        Name    : "Our CloudFront Distribution",
        Version : "1.0.0",
    })
    // ...
}

Arrays, Maps, and Loops

Terraform uses the count meta-parameter to create multiple resources. But modules and most nested blocks don't support count. In JavaScript you can use loops, forEach, map, etc. with functions to generate repeating blocks of any type at any nesting level.

HCL:

resource "aws_waf_ipset" "ipset" {
  name = "mainOffice"

  ip_set_descriptors {
    type  = "IPV4"
    value = "192.0.7.0/24"
  }

  ip_set_descriptors {
    type  = "IPV4"
    value = "10.16.0.0/16"
  }

  ip_set_descriptors {
    type  = "IPV4"
    value = "10.21.0.0/16"
  }
}

TfBuilder:

const ips = ["192.0.7.0/24", "10.16.0.0/16", "10.21.0.0/16"]
resource.aws_waf_ipset.ipset = {
    name: "mainOffice",
    ip_set_descriptors: ips.map(value => ({ value, type: "IPV4" }))
}

Another common problem in Terraform is that count.index becomes part of the resource address in the Terraform state. So, if you're generating resources from a list and you remove an item from the middle, all the resources after it get modified or even destroyed and re-created. Using TfBuilder you can completely control the resource address and decouple it from the index.

This HCL example creates three files with random contents:

locals {
    files = ["a", "b", "c"]
}

resource "random_string" "content" {
    count  = "${length(local.files)}"
    length = 10
}

resource "local_file" "file" {
    count    = "${length(local.files)}"
    filename = "${local.files[count.index]}.txt"
    content  = "${random_string.content.*.result[count.index]}"
}

If you delete "b", both b.txt and c.txt get destroyed and then c.txt is re-created. TfBuilder solution:

const files = ["a", "b", "c"]
files.forEach(file => {
    const rand = resource.random_string['content_' + file] = {
        length: 10
    }
    resource.local_file['file_' + file] = {
        filename: file + '.txt',
        content: rand.result
    }
})

In the TfBuilder example, the file name (which is already unique) is used as part of the resource address, so removing "b" just deletes b.txt.

However, JS arrays and loops are not a complete replacement for Terraform's count. Whereas count can be used with a list produced by a data source, JS can only work with data that is known before Terraform runs.