Terraform 💻

Linting

The team uses TFLint for automated code checks.

Formatting

• Always use terraform fmt to rewrite Terraform configuration files to a canonical format and style.

Naming

• Use snake_case in all: resource names, data source names, variable names, outputs.
• Only use lowercase letters and numbers.
• Always use singular noun for names.
• Always use double quotes.

Resource and data source arguments

• Do not repeat the resource type in the resource name (not partially, nor completely)

  resource "aws_route_table" "public" {}
  

  resource "aws_route_table" "public_route_table" {}
  

  resource "aws_route_table" "public_aws_route_table" {}
  

• Resource name should be named this if there is not a more descriptive and general name available, or if the resource module creates a single resource of this type (e.g., there is a single resource of type aws_nat_gateway and multiple resources of type aws_route_table, so aws_nat_gateway should be named this and aws_route_table should have more descriptive names - like private, public, database). Normally, the team uses this to delegate the output of a module to the resource itself.

  # modules/nimble_alb/main.tf
  resource "aws_lb" "nimble" {
    name               = var.load_balancer_name
    load_balancer_type = "application"
    security_groups    = [var.vpc_security_group_ids]
    subnets            = [var.subnets]
  }
  
  # modules/nimble_alb/outputs.tf
  output "this_dns" {
    description = "Nimble Application load balancer DNS"
    value       = aws_lb.nimble.dns
  }
  
  # main.tf
  module "nimble_alb" {
    source = "./modules/nimble_alb"
    #...
  }
  
  # Get the DNS by `module.nimble_alb.this_dns`
  

• Include tags argument, if supported by resource as the last real argument, following by depends_on and lifecycle, if necessary. All of these should be separated by a single empty line.

  resource "aws_nat_gateway" "nimble" {
    count = "1"
  
    allocation_id = "..."
    subnet_id     = "..."
  
    tags = {
      Name = "..."
    }
  
    depends_on = ["aws_internet_gateway.this"]
  
    lifecycle {
      create_before_destroy = true
    }
  }
  

• Add count as the first argument at the top of each resource block, separated from the other arguments by a newline.

  resource "aws_route_table" "public" {
    count  = 2
  
    vpc_id = "vpc-12345678"
    #...
  }
  

• When using count to create the resource conditionally, use a boolean value if it is appropriate. Otherwise, use the length method.

  # create_public_subnets: true
  resource "aws_subnet" "public" {
    count = var.create_public_subnets
    #...
  }
  

  or

  resource "aws_subnet" "public" {
    count = length(var.public_subnets) > 0 ? 1 : 0
    #...
  }
  

• To make inverted conditions, use 1 - {boolean value}.

  # create_public_subnets: true
  # 1 - true => false
  count = 1 - var.create_public_subnets
  

Variables

• Do not reinvent the wheel in resource modules - use the same variable names, description, and default as defined in the “Argument Reference” section (example) for the resource in use.
• Omit type = "string" declaration if there is default = "" also
• Omit type = "list" declaration if there is default = [] also.
• Omit type = "map" declaration if there is default = {} also.
• Use plural form in name of variables of type list and map.
• When defining variables order the keys: description , type, default.

Outputs

Name for the outputs is important to make them consistent and understandable outside of its scope (when the user is using a module it should be obvious what type and attribute of the value is returned).

• The names of outputs should be descriptive and reflect the value they contain, avoid using vague or ambiguous names.

  output "alb_log_bucket_name" {
    description = "S3 bucket name for ALB logging"
    value       = aws_s3_bucket.alb_log.bucket
  }
  

  output "alb_s3" {
    description = "S3 for ALB"
    value       = aws_s3_bucket.alb_log.bucket
  }
  

• Always include description on all outputs even if they are obvious.

• If the output is returning a value with interpolation functions and multiple resources, the {name} and {type} there should be as generic as possible (this is often the most generic and should be preferred).

  output "this_security_group_id" {
    description = "The ID of the security group"
    value       = element(concat(coalescelist(aws_security_group.this.*.id, aws_security_group.this_name_prefix.*.id), list("")), 0)
  }
  

  output "security_group_id" {
    description = "The ID of the security group"
    value       = element(concat(coalescelist(aws_security_group.this.*.id, aws_security_group.web.*.id), list("")), 0)
  }
  
  output "another_security_group_id" {
    description = "The ID of web security group"
    value       = element(concat(aws_security_group.web.*.id, list("")), 0)
  }
  

• Use the plural form for the name if the returned value is a list.

  output "this_rds_cluster_instance_endpoints" {
    description = "A list of all cluster instance endpoints"
    value       = [aws_rds_cluster_instance.this.*.endpoint]
  }
  

Project Structure

A typical project for a cloud provider should follow this structure::

  terraform-project
  ├── README.md
  ├── modules
  │   ├── module_1
  │   │   ├── locals.tf
  │   │   ├── main.tf
  │   │   ├── outputs.tf
  │   │   └── variables.tf
  │   └── module_2
  │       ├── locals.tf
  │       ├── main.tf
  │       ├── outputs.tf
  │       └── variables.tf
  ├── core
  │   ├── locals.tf
  │   ├── main.tf
  │   ├── outputs.tf
  │   ├── providers.tf
  │   └── variables.tf
  └── shared
      ├── locals.tf
      ├── main.tf
      ├── outputs.tf.
      ├── providers.tf
      └── variables.tf

The project is organized into 3 directories:

• modules: contains shared modules, where each module is a collection of connected resources that together perform a common action. For example, aws_vpc creates VPC, subnets, NAT, etc.
• shared: uses the shared modules defined under /modules to create shared resources for all environments, e.g. AWS ECR, IAM groups and users. These resources must be created first before creating any environment-specific resources.
• core: uses the shared modules defined under /modules to create resources for each environment, e.g. staging DB cluster, production DB cluster.

Each directory share the same file structure

• main.tf: use the shared Terraform modules or resources in combination with the variables and local values to create the infrastructure resources.

  • The main.tf files under /modules typically use public modules or resources (example) from the Terraform registry.
  • The main.tf files under /core or /shared consumes the modules defined under /modules.
• locals.tf: contains declarations of local values used in main.tf which are the same between environments, e.g. database engine.
• variables.tf: contains declarations of variables used in main.tf which can be different by each environment.
• outputs.tf: contains outputs from the resources created in main.tf.

Under the core and shared directories, there is an additional providers.tf file that stores the configuration for the providers used in the project, such as Terraform, AWS, GCP, etc.

Main File Structure

Follow the below structure to organize the code in main.tf files.

• Group each component in the main file together:

  # data source arguments
  
  # resource/module
  
  # resource `null_resource`
  

• Order by top to down in term of execution

  # Create VPC first
  resource "aws_vpc" "nimble" {}
  
  # Create IAM role before the EC2 instance
  resource "aws_iam_role" "nimble_web" {}
  
  # Create EC2 instance
  resource "aws_instance" "nimble_web" {}
  

Variables vs Local Values

👉 Use local values to:

• Store setting values that do not change between application environments, such as the database engine.
• Store application configuration values by environment (./core/locals.tf). Changes to these values should be made through code and require code review.

👉 Use variables to store setting values that may change depending on the application environment, such as the database instance type. The values of the variables will be provided or overridden (if there is a default value) by the consumer of the modules, i.e., when calling the module from /core or /share.

Typically, variables are used when defining shared modules (under /modules). This allows each application to define custom setting values when using those modules to create infrastructure resources.

Here is a simple demonstration of how to use local values and variables.

  # .
  # ├── modules
  # │   └── rds
  # │       ├── locals.tf
  # │       ├── main.tf
  # │       └── variables.tf
  # └── core
  #     ├── locals.tf
  #     ├── main.tf
  #     └── variables.tf

  # ./modules/rds/locals.tf
  locals {
    db_engine = "aurora-postgresql"
  }

  # ./modules/rds/variables.tf
  variable "instance_class" {
    description = "The Aurora DB instance class"
    type        = string
  }

  # ./module/rds/main.tf
  module "db" {
    source  = "terraform-aws-modules/rds-aurora/aws"

    engine         = local.db_engine
    instance_class = var.instance_class
    #...
  }

  # ./base/variables.tf
  variable "environment" {
    description = "Application environment"
    type        = string
  }

  # ./base/locals.tf
  locals {
    current_db_config = local.db_config[var.environment]
    db_config = {
      staging = {
        instance_class = "db.t3.small"
        #...
      }

      prod = {
        instance_class = "db.t3.large"
        #...
      }
  }

  #./base/main.tf
  module "db" {
    source = "../modules/db"

    instance_class = local.current_db_config.instance_class
    #...
  }

Shared Modules

• The team prefers using shared modules published on the registry instead of individual Terraform resources to manage infrastructure resources. This approach can reduce code, improve consistency, enable reuse, and implement best practices.

  module "db" {
    source  = "terraform-aws-modules/rds-aurora/aws"
    version = "7.6.0"
  
    name           = local.cluster_id
    engine         = local.engine
    engine_version = var.engine_version
  
    vpc_id   = var.vpc_id
    subnets  = var.subnet_ids
    #...
  }
  

  resource "aws_db_instance" "main" {
    #...
  }
  
  resource "aws_db_subnet_group" "db-subnet" {
    name       = "db_subnet_group"
    subnet_ids = var.subnet_ids
  }
  

• Always set the module version to ensure predictability, security, and stability and avoid breaking changes.

  module "rds" {
    source  = "terraform-aws-modules/rds/aws"
    version = "2.18.0"
    #...
  }
  

  module "rds" {
    source  = "terraform-aws-modules/rds/aws"
    #...
  }
  

Resource Tagging

Use the default_tags under Terraform provider to ensure that there are at least two default tags: environment and owner, to group all created resources by the project and its environment. These tags are useful to group all project resources (e.g. to filter on billing dashboard), in case there are other resources created by other teams or projects.

# ./shared/provider.tf
provider "aws" {
  region = local.region

  default_tags {
    tags = {
      Environment = var.environment # staging
      Owner       = local.owner # ACME
    }
  }
}

State storage

Always using Remote State

• Start the project with Terraform Cloud Remote State Management if it is possible.

  terraform {
    cloud {
      organization = "nimble"
  
      workspaces {
        name = "overblock"
      }
    }
  }
  

• Storing Terraform states in a remote bucket, such as AWS S3, is recommended as a second option.

  terraform {
    backend "s3" {
      region  = "aws_region"
      # This bucket needs to be created beforehand
      bucket  = "bucket_name"
      key     = "aws-setup/state.tfstate"
      encrypt = true # AES-256 encryption
    }
  }
  

  Enable version control on this bucket. And set IAM Policy to limit the access read/write to that bucket.

Security

• Prefer using tfsec and checkov for scanning security and misconfiguration issues.

• Do NOT skip an entire check of the scanning tool. Instead, disable the check for lines of code that require an exception.

  # .checkov.yaml
  skip-check:
    - CKV_AWS_20
  

  # module/s3/main.tf
  resource "aws_s3_bucket" "foo-bucket" {
    region        = var.region
    bucket        = local.bucket_name
    force_destroy = true
    #checkov:skip=CKV_AWS_20:The bucket is a public static content host
    acl           = "public-read"
  }
  

• Avoid hardcoding sensitive data such as passwords or SSH keys in Terraform code. Instead, use Terraform cloud variables to store these values and consume these values in the code.

  # variables.tf
  # the value will be stored using a Terraform Cloud sensitive variable.
  variable "db_master_password" {
    type      = string
    sensitive = true
  }
  
  # main.tf
  module "db" {
    source  = "terraform-aws-modules/rds-aurora/aws"
    version = "7.6.0"
      
    master_password = var.db_master_password
    #...
  }
  

Continuous Integration

While multiple services are available for Continuous Integration in a Terraform project, the team typically uses Terraform Cloud and follows the following conventions.

Workspaces structure

An infrastructure project usually consists of the following workspaces:

• <project-name>-shared (e.g. acme-shared): uses the ./shared directory as the working directory.
• <project-name>-<environment> (e.g. acme-staging, acme-production): uses the ./base directory as the working directory.

Terraform Runs

• To avoid potential side effects, do not keep a Terraform Cloud plan for an extended period before applying as the plan could become outdated. Instead, if it is unable to apply the plan immediately after planning, initialize a new plan and apply it instead.

  # In this example, the Terraform plan may become outdated 
  # as the task definition is changed during an application deployment.
  data "aws_ecs_task_definition" "latest" {
    task_definition = "acme-web-service"
  }
  

Reference Resources

• Terraform best practices
• Best practices when using terraform
• Terraform documentation
• Terraform Up & Running