Blocks and Arguments

# Block structure
resource "aws_instance" "example" {  # Block type, resource type, and name label
  ami           = "ami-0c55b159cbfafe1f0"  # Argument
  instance_type = "t2.micro"                # Another argument
  
  # Nested block
  ebs_block_device {
    device_name = "/dev/sdh"
    volume_size = 100
  }
}

Data Types

# String
variable "region" {
  type    = string
  default = "us-west-2"
}

# Number
variable "port" {
  type    = number
  default = 80
}

# Boolean
variable "enable_logging" {
  type    = bool
  default = true
}

# List
variable "availability_zones" {
  type    = list(string)
  default = ["us-west-2a", "us-west-2b", "us-west-2c"]
}

# Map
variable "tags" {
  type    = map(string)
  default = {
    Environment = "Development"
    Project     = "Example"
  }
}

# Complex types
variable "subnet_config" {
  type = object({
    cidr_block        = string
    availability_zone = string
    public            = bool
    tags              = map(string)
  })
  
  default = {
    cidr_block        = "10.0.1.0/24"
    availability_zone = "us-west-2a"
    public            = true
    tags              = {
      Name = "Public Subnet"
    }
  }
}

Expressions and Functions

# String interpolation
resource "aws_instance" "example" {
  tags = {
    Name = "web-server-${var.environment}"  # String interpolation
  }
}

# Conditional expression
resource "aws_instance" "example" {
  instance_type = var.environment == "production" ? "t2.medium" : "t2.micro"
}

# For expressions
resource "aws_subnet" "example" {
  count             = length(var.availability_zones)
  vpc_id            = aws_vpc.main.id
  cidr_block        = "10.0.${count.index + 1}.0/24"
  availability_zone = var.availability_zones[count.index]
}

# Built-in functions
locals {
  upper_name = upper(var.name)
  timestamp  = formatdate("YYYY-MM-DD", timestamp())
  subnets    = cidrsubnets(var.vpc_cidr, 8, 8, 8, 8)
}

Count and For Each

# Using count
resource "aws_instance" "server" {
  count         = 3
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  
  tags = {
    Name = "server-${count.index + 1}"
  }
}

# Using for_each with a map
resource "aws_instance" "server" {
  for_each      = {
    web  = "t2.micro"
    api  = "t2.small"
    db   = "t2.medium"
  }
  
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = each.value
  
  tags = {
    Name = "${each.key}-server"
  }
}

# Using for_each with a set
resource "aws_security_group_rule" "ingress" {
  for_each          = toset(["80", "443", "22"])
  
  type              = "ingress"
  security_group_id = aws_security_group.example.id
  from_port         = each.value
  to_port           = each.value
  protocol          = "tcp"
  cidr_blocks       = ["0.0.0.0/0"]
}

Dynamic Blocks

# Without dynamic blocks (repetitive)
resource "aws_security_group" "example" {
  name        = "example"
  description = "Example security group"
  
  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
  
  ingress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }
  
  ingress {
    from_port   = 22
    to_port     = 22
    protocol    = "tcp"
    cidr_blocks = ["10.0.0.0/8"]
  }
}

# With dynamic blocks (more concise)
variable "ingress_rules" {
  default = [
    {
      port        = 80
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
    },
    {
      port        = 443
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
    },
    {
      port        = 22
      protocol    = "tcp"
      cidr_blocks = ["10.0.0.0/8"]
    }
  ]
}

resource "aws_security_group" "example" {
  name        = "example"
  description = "Example security group"
  
  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
    }
  }
}

Locals

# Using locals for calculated values
locals {
  common_tags = {
    Environment = var.environment
    Project     = var.project_name
    Owner       = "Terraform"
    ManagedBy   = "Terraform"
  }
  
  is_production    = var.environment == "production"
  instance_type    = local.is_production ? "t2.medium" : "t2.micro"
  availability_zones = slice(data.aws_availability_zones.available.names, 0, var.az_count)
}

resource "aws_instance" "example" {
  ami           = var.ami_id
  instance_type = local.instance_type
  
  tags = merge(
    local.common_tags,
    {
      Name = "example-instance"
    }
  )
}

Lifecycle Rules

# Lifecycle rules modify how Terraform handles resource changes
resource "aws_instance" "example" {
  ami           = "ami-0c55b159cbfafe1f0"
  instance_type = "t2.micro"
  
  lifecycle {
    create_before_destroy = true  # Create replacement before destroying original
    prevent_destroy = true         # Prevent accidental destruction
    ignore_changes = [
      tags,                        # Ignore changes to tags
      user_data                    # Ignore changes to user_data
    ]
  }
}