Iac

Terraform state is the source of truth for your infrastructure. Mess it up and you’ll be manually reconciling resources at 2 AM. Here’s how to manage state properly from day one. What Is State? Terraform state maps your configuration to real resources: 1 2 3 4 5 # main.tf resource "aws_instance" "web" { ami = "ami-12345" instance_type = "t3.micro" } 1 2 3 4 5 6 7 8 9 10 11 12 13 14 // terraform.tfstate (simplified) { "resources": [{ "type": "aws_instance", "name": "web", "instances": [{ "attributes": { "id": "i-0abc123def456", "ami": "ami-12345", "instance_type": "t3.micro" } }] }] } Without state, Terraform doesn’t know aws_instance.web corresponds to i-0abc123def456. It would try to create a new instance every time. ...

Terraform modules turn infrastructure code from scripts into libraries. Here’s how to design them well. Module Structure m └ o ─ d ─ u l v ├ ├ ├ ├ └ e p ─ ─ ─ ─ ─ s c ─ ─ ─ ─ ─ / / m v o v R a a u e E i r t r A n i p s D . a u i M t b t o E f l s n . e . s m s t . d . f t t f f # # # # # R I O P D e n u r o s p t o c o u p v u u t u i m r t d e c v e n e a v r t s r a a i l r t a u e i b e q o l s u n e i s r e m e n t s Basic Module variables.tf 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 variable "name" { description = "Name prefix for resources" type = string } variable "cidr_block" { description = "CIDR block for VPC" type = string default = "10.0.0.0/16" } variable "azs" { description = "Availability zones" type = list(string) } variable "private_subnets" { description = "Private subnet CIDR blocks" type = list(string) default = [] } variable "public_subnets" { description = "Public subnet CIDR blocks" type = list(string) default = [] } variable "tags" { description = "Tags to apply to resources" type = map(string) default = {} } main.tf 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 resource "aws_vpc" "this" { cidr_block = var.cidr_block enable_dns_hostnames = true enable_dns_support = true tags = merge(var.tags, { Name = var.name }) } resource "aws_subnet" "private" { count = length(var.private_subnets) vpc_id = aws_vpc.this.id cidr_block = var.private_subnets[count.index] availability_zone = var.azs[count.index % length(var.azs)] tags = merge(var.tags, { Name = "${var.name}-private-${count.index + 1}" Type = "private" }) } resource "aws_subnet" "public" { count = length(var.public_subnets) vpc_id = aws_vpc.this.id cidr_block = var.public_subnets[count.index] availability_zone = var.azs[count.index % length(var.azs)] map_public_ip_on_launch = true tags = merge(var.tags, { Name = "${var.name}-public-${count.index + 1}" Type = "public" }) } outputs.tf 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 output "vpc_id" { description = "VPC ID" value = aws_vpc.this.id } output "private_subnet_ids" { description = "Private subnet IDs" value = aws_subnet.private[*].id } output "public_subnet_ids" { description = "Public subnet IDs" value = aws_subnet.public[*].id } output "vpc_cidr_block" { description = "VPC CIDR block" value = aws_vpc.this.cidr_block } versions.tf 1 2 3 4 5 6 7 8 9 10 terraform { required_version = ">= 1.0" required_providers { aws = { source = "hashicorp/aws" version = ">= 4.0" } } } Using Modules Local Module 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 module "vpc" { source = "./modules/vpc" name = "production" cidr_block = "10.0.0.0/16" azs = ["us-east-1a", "us-east-1b", "us-east-1c"] private_subnets = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"] public_subnets = ["10.0.101.0/24", "10.0.102.0/24", "10.0.103.0/24"] tags = { Environment = "production" Project = "myapp" } } # Use outputs resource "aws_instance" "app" { subnet_id = module.vpc.private_subnet_ids[0] # ... } Registry Module 1 2 3 4 5 6 7 8 9 10 11 12 13 14 module "vpc" { source = "terraform-aws-modules/vpc/aws" version = "5.0.0" name = "my-vpc" cidr = "10.0.0.0/16" azs = ["us-east-1a", "us-east-1b"] private_subnets = ["10.0.1.0/24", "10.0.2.0/24"] public_subnets = ["10.0.101.0/24", "10.0.102.0/24"] enable_nat_gateway = true single_nat_gateway = true } Git Module 1 2 3 4 module "vpc" { source = "git::https://github.com/org/terraform-modules.git//vpc?ref=v1.2.0" # ... } Variable Validation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 variable "environment" { description = "Environment name" type = string validation { condition = contains(["dev", "staging", "production"], var.environment) error_message = "Environment must be dev, staging, or production." } } variable "instance_type" { description = "EC2 instance type" type = string default = "t3.micro" validation { condition = can(regex("^t3\\.", var.instance_type)) error_message = "Instance type must be t3 family." } } variable "cidr_block" { description = "CIDR block" type = string validation { condition = can(cidrhost(var.cidr_block, 0)) error_message = "Must be a valid CIDR block." } } Complex Types 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 variable "services" { description = "Service configurations" type = list(object({ name = string port = number health_path = optional(string, "/health") replicas = optional(number, 1) environment = optional(map(string), {}) })) } # Usage services = [ { name = "api" port = 8080 replicas = 3 environment = { LOG_LEVEL = "info" } }, { name = "worker" port = 9090 } ] Conditional Resources 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 variable "create_nat_gateway" { description = "Create NAT gateway" type = bool default = true } resource "aws_nat_gateway" "this" { count = var.create_nat_gateway ? 1 : 0 allocation_id = aws_eip.nat[0].id subnet_id = aws_subnet.public[0].id } resource "aws_eip" "nat" { count = var.create_nat_gateway ? 1 : 0 domain = "vpc" } Dynamic Blocks 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 variable "ingress_rules" { description = "Ingress rules" type = list(object({ port = number protocol = string cidr_blocks = list(string) })) default = [] } resource "aws_security_group" "this" { name = var.name description = var.description vpc_id = var.vpc_id dynamic "ingress" { for_each = var.ingress_rules content { from_port = ingress.value.port to_port = ingress.value.port protocol = ingress.value.protocol cidr_blocks = ingress.value.cidr_blocks } } egress { from_port = 0 to_port = 0 protocol = "-1" cidr_blocks = ["0.0.0.0/0"] } } Module Composition Root Module 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 # main.tf module "vpc" { source = "./modules/vpc" # ... } module "security_groups" { source = "./modules/security-groups" vpc_id = module.vpc.vpc_id # ... } module "ecs_cluster" { source = "./modules/ecs-cluster" vpc_id = module.vpc.vpc_id private_subnet_ids = module.vpc.private_subnet_ids security_group_ids = [module.security_groups.app_sg_id] # ... } Shared Data 1 2 3 4 5 6 7 8 9 10 11 12 13 14 # data.tf - Shared data sources data "aws_caller_identity" "current" {} data "aws_region" "current" {} locals { account_id = data.aws_caller_identity.current.account_id region = data.aws_region.current.name common_tags = { Environment = var.environment Project = var.project ManagedBy = "terraform" } } For_each vs Count Count (Index-based) 1 2 3 4 5 # Problematic: Adding/removing items shifts indices resource "aws_subnet" "private" { count = length(var.private_subnets) cidr_block = var.private_subnets[count.index] } For_each (Key-based) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 # Better: Keys are stable variable "subnets" { type = map(object({ cidr_block = string az = string })) } resource "aws_subnet" "private" { for_each = var.subnets cidr_block = each.value.cidr_block availability_zone = each.value.az tags = { Name = each.key } } # Usage subnets = { "private-1a" = { cidr_block = "10.0.1.0/24", az = "us-east-1a" } "private-1b" = { cidr_block = "10.0.2.0/24", az = "us-east-1b" } } Sensitive Values 1 2 3 4 5 6 7 8 9 10 11 variable "database_password" { description = "Database password" type = string sensitive = true } output "connection_string" { description = "Database connection string" value = "postgres://user:${var.database_password}@${aws_db_instance.this.endpoint}/db" sensitive = true } Module Testing Terraform Test (1.6+) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 # tests/vpc_test.tftest.hcl run "vpc_creates_successfully" { command = plan variables { name = "test-vpc" cidr_block = "10.0.0.0/16" azs = ["us-east-1a"] } assert { condition = aws_vpc.this.cidr_block == "10.0.0.0/16" error_message = "VPC CIDR block incorrect" } } Run tests: ...

Terraform state is both essential and dangerous. It’s how Terraform knows what exists, what changed, and what to do. Mismanage it, and you’ll either destroy production or spend hours untangling drift. What State Actually Is State is Terraform’s record of reality. It maps your configuration to real resources: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 { "resources": [ { "type": "aws_instance", "name": "web", "instances": [ { "attributes": { "id": "i-0abc123def456", "ami": "ami-12345678", "instance_type": "t3.medium" } } ] } ] } Without state, Terraform would: ...

Infrastructure as Code (IaC) means your servers, networks, and services are defined in version-controlled files rather than clicked into existence through consoles. The benefits are obvious: reproducibility, auditability, collaboration. But IaC done poorly creates its own problems: state drift, copy-paste sprawl, untestable configurations. The principles matter more than the tools. Declarative Over Imperative Describe what you want, not how to get there: 1 2 3 4 5 6 7 8 9 # Declarative (Terraform) - what resource "aws_instance" "web" { ami = "ami-0c55b159cbfafe1f0" instance_type = "t3.micro" tags = { Name = "web-server" } } 1 2 3 4 5 # Imperative (script) - how aws ec2 run-instances \ --image-id ami-0c55b159cbfafe1f0 \ --instance-type t3.micro \ --tag-specifications 'ResourceType=instance,Tags=[{Key=Name,Value=web-server}]' Declarative code is idempotent — run it ten times, get the same result. Imperative scripts need guards against re-running. ...

Terraform state is where infrastructure-as-code meets reality. It’s also where most Terraform disasters originate. Here’s how to manage state without losing sleep. The Problem Terraform tracks what it’s created in a state file. This file maps your HCL resources to real infrastructure. Without it, Terraform can’t update or destroy anything — it doesn’t know what exists. The default is a local file called terraform.tfstate. This works fine until: Someone else needs to run Terraform Your laptop dies Two people run apply simultaneously You accidentally commit secrets to Git Rule 1: Remote State from Day One Never use local state for anything beyond experiments: ...

Ansible playbooks can quickly become unwieldy spaghetti. Here are battle-tested patterns for writing infrastructure code that scales with your team and your infrastructure. The Role Structure That Actually Works Forget the minimal examples. Real roles need this structure: r └ o ─ l ─ e s w ├ │ ├ │ ├ │ │ │ │ ├ │ ├ │ ├ │ └ / e ─ ─ ─ ─ ─ ─ ─ b ─ ─ ─ ─ ─ ─ ─ s e d └ v └ t ├ ├ ├ └ h └ t └ f └ m └ r e ─ a ─ a ─ ─ ─ ─ a ─ e ─ i ─ e ─ v f ─ r ─ s ─ ─ ─ ─ n ─ m ─ l ─ t ─ e a s k d p e a r u m / m s m i c s l m l n s s / m / l a a / a n o e e a a g / s a t i i i s n r r i t i l i s n n n t f v s n e n - n / . . . a i i / . s x p . y y y l g c y / . a y m m m l u e m c r m l l l . r . l o a l y e y n m m . m f s l y l . . m j c l 2 o n # # # # # # # f # D R E P C S R D e o n a o e e e f l t c n r s p a e r k f v t e u y a i i a n l v g g c r d t a p e u e t e r o r / n v i i i a m r c a a n n t a e i r b t s i n l e i l t o a o s a e - a n g a b s l e d l j l f m e ( u a i e h s h s t l n a i t i e t n ( g o s d l h i n l o e n e w r c r e l s s p u t r d e e p c s r e e d c e e n d c e e n ) c e ) The key insight: tasks/main.yml should only contain includes: ...

You test your application code. Why not your infrastructure code? Infrastructure as Code (IaC) has the same failure modes as any software: bugs, regressions, unintended side effects. Yet most teams treat Terraform and Ansible like configuration files rather than code that deserves tests. Why Infrastructure Testing Matters A Terraform plan looks correct until it: Creates a security group that’s too permissive Deploys to the wrong availability zone Sets instance types that exceed your budget Breaks networking in ways that only manifest at runtime Manual review catches some issues. Automated testing catches more. ...