AWS Patterns

Terraform patterns for AWS infrastructure development.

Before Generating Code

ALWAYS use doc-researcher or Terraform MCP to verify current provider schemas.

Naming Convention

Use {project}-{environment}-{resource} pattern consistently:

locals {
  name_prefix = "${var.project}-${var.environment}"
}

VPC Pattern

Three-Tier VPC with terraform-aws-modules

module "vpc" {
  source  = "terraform-aws-modules/vpc/aws"
  version = "~> 5.0"

  name = "${local.name_prefix}-vpc"
  cidr = var.vpc_cidr  # e.g., "10.0.0.0/16"

  azs             = data.aws_availability_zones.available.names
  private_subnets = [for k, v in local.azs : cidrsubnet(var.vpc_cidr, 4, k)]
  public_subnets  = [for k, v in local.azs : cidrsubnet(var.vpc_cidr, 8, k + 48)]
  intra_subnets   = [for k, v in local.azs : cidrsubnet(var.vpc_cidr, 8, k + 52)]

  enable_nat_gateway   = true
  single_nat_gateway   = var.environment != "prod"
  enable_dns_hostnames = true
  enable_dns_support   = true

  # EKS requirements
  public_subnet_tags = {
    "kubernetes.io/role/elb" = 1
  }
  private_subnet_tags = {
    "kubernetes.io/role/internal-elb" = 1
  }

  tags = var.tags
}

data "aws_availability_zones" "available" {
  state = "available"
}

locals {
  azs = slice(data.aws_availability_zones.available.names, 0, 3)
}

VPC Endpoints for AWS Services

module "vpc_endpoints" {
  source  = "terraform-aws-modules/vpc/aws//modules/vpc-endpoints"
  version = "~> 5.0"

  vpc_id = module.vpc.vpc_id

  endpoints = {
    s3 = {
      service         = "s3"
      service_type    = "Gateway"
      route_table_ids = module.vpc.private_route_table_ids
      tags            = { Name = "${local.name_prefix}-s3-endpoint" }
    }
    ecr_api = {
      service             = "ecr.api"
      private_dns_enabled = true
      subnet_ids          = module.vpc.private_subnets
      security_group_ids  = [aws_security_group.vpc_endpoints.id]
    }
    ecr_dkr = {
      service             = "ecr.dkr"
      private_dns_enabled = true
      subnet_ids          = module.vpc.private_subnets
      security_group_ids  = [aws_security_group.vpc_endpoints.id]
    }
    sts = {
      service             = "sts"
      private_dns_enabled = true
      subnet_ids          = module.vpc.private_subnets
      security_group_ids  = [aws_security_group.vpc_endpoints.id]
    }
  }

  tags = var.tags
}

resource "aws_security_group" "vpc_endpoints" {
  name_prefix = "${local.name_prefix}-vpc-endpoints-"
  vpc_id      = module.vpc.vpc_id

  ingress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = [module.vpc.vpc_cidr_block]
  }

  tags = merge(var.tags, { Name = "${local.name_prefix}-vpc-endpoints" })
}

IAM Patterns

Service Role with Assume Policy

data "aws_iam_policy_document" "assume_role" {
  statement {
    effect = "Allow"
    principals {
      type        = "Service"
      identifiers = ["ec2.amazonaws.com"]
    }
    actions = ["sts:AssumeRole"]
  }
}

resource "aws_iam_role" "service" {
  name               = "${local.name_prefix}-service-role"
  assume_role_policy = data.aws_iam_policy_document.assume_role.json
  tags               = var.tags
}

resource "aws_iam_instance_profile" "service" {
  name = "${local.name_prefix}-service-profile"
  role = aws_iam_role.service.name
}

Least Privilege Policy

data "aws_iam_policy_document" "permissions" {
  statement {
    sid    = "S3ReadAccess"
    effect = "Allow"
    actions = [
      "s3:GetObject",
      "s3:ListBucket"
    ]
    resources = [
      aws_s3_bucket.data.arn,
      "${aws_s3_bucket.data.arn}/*"
    ]
  }

  statement {
    sid    = "SecretsManagerAccess"
    effect = "Allow"
    actions = [
      "secretsmanager:GetSecretValue"
    ]
    resources = [
      "arn:aws:secretsmanager:${data.aws_region.current.name}:${data.aws_caller_identity.current.account_id}:secret:${var.project}/*"
    ]
  }
}

resource "aws_iam_role_policy" "service" {
  name   = "${local.name_prefix}-service-policy"
  role   = aws_iam_role.service.id
  policy = data.aws_iam_policy_document.permissions.json
}

Dynamic ARN Construction

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
}

S3 Patterns

Secure Bucket with Encryption and Versioning

resource "aws_s3_bucket" "data" {
  bucket = "${local.name_prefix}-data-${local.account_id}"
  tags   = var.tags
}

resource "aws_s3_bucket_versioning" "data" {
  bucket = aws_s3_bucket.data.id
  versioning_configuration {
    status = "Enabled"
  }
}

resource "aws_s3_bucket_server_side_encryption_configuration" "data" {
  bucket = aws_s3_bucket.data.id
  rule {
    apply_server_side_encryption_by_default {
      sse_algorithm     = "aws:kms"
      kms_master_key_id = aws_kms_key.s3.arn
    }
    bucket_key_enabled = true
  }
}

resource "aws_s3_bucket_public_access_block" "data" {
  bucket = aws_s3_bucket.data.id

  block_public_acls       = true
  block_public_policy     = true
  ignore_public_acls      = true
  restrict_public_buckets = true
}

resource "aws_s3_bucket_lifecycle_configuration" "data" {
  bucket = aws_s3_bucket.data.id

  rule {
    id     = "transition-to-ia"
    status = "Enabled"

    transition {
      days          = 30
      storage_class = "STANDARD_IA"
    }

    transition {
      days          = 90
      storage_class = "GLACIER"
    }

    noncurrent_version_expiration {
      noncurrent_days = 30
    }
  }
}

Security Group Patterns

Security Group Chaining

# ALB Security Group - Public facing
resource "aws_security_group" "alb" {
  name_prefix = "${local.name_prefix}-alb-"
  vpc_id      = module.vpc.vpc_id

  ingress {
    description = "HTTPS from internet"
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["0.0.0.0/0"]
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }

  lifecycle {
    create_before_destroy = true
  }

  tags = merge(var.tags, { Name = "${local.name_prefix}-alb" })
}

# Application Security Group - Only from ALB
resource "aws_security_group" "app" {
  name_prefix = "${local.name_prefix}-app-"
  vpc_id      = module.vpc.vpc_id

  ingress {
    description     = "From ALB"
    from_port       = 8080
    to_port         = 8080
    protocol        = "tcp"
    security_groups = [aws_security_group.alb.id]
  }

  egress {
    from_port   = 0
    to_port     = 0
    protocol    = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }

  lifecycle {
    create_before_destroy = true
  }

  tags = merge(var.tags, { Name = "${local.name_prefix}-app" })
}

# Database Security Group - Only from App
resource "aws_security_group" "db" {
  name_prefix = "${local.name_prefix}-db-"
  vpc_id      = module.vpc.vpc_id

  ingress {
    description     = "PostgreSQL from app"
    from_port       = 5432
    to_port         = 5432
    protocol        = "tcp"
    security_groups = [aws_security_group.app.id]
  }

  lifecycle {
    create_before_destroy = true
  }

  tags = merge(var.tags, { Name = "${local.name_prefix}-db" })
}

KMS Pattern

resource "aws_kms_key" "main" {
  description             = "KMS key for ${local.name_prefix}"
  deletion_window_in_days = 7
  enable_key_rotation     = true

  policy = jsonencode({
    Version = "2012-10-17"
    Statement = [
      {
        Sid    = "Enable IAM User Permissions"
        Effect = "Allow"
        Principal = {
          AWS = "arn:aws:iam::${local.account_id}:root"
        }
        Action   = "kms:*"
        Resource = "*"
      }
    ]
  })

  tags = var.tags
}

resource "aws_kms_alias" "main" {
  name          = "alias/${local.name_prefix}-main"
  target_key_id = aws_kms_key.main.key_id
}

Tagging Strategy

variable "project" {
  description = "Project name for resource naming and tagging"
  type        = string
}

variable "environment" {
  description = "Environment name (dev, staging, prod)"
  type        = string

  validation {
    condition     = contains(["dev", "staging", "prod"], var.environment)
    error_message = "Environment must be dev, staging, or prod."
  }
}

variable "additional_tags" {
  description = "Additional tags to apply to all resources"
  type        = map(string)
  default     = {}
}

locals {
  default_tags = {
    Project     = var.project
    Environment = var.environment
    Terraform   = "true"
    ManagedBy   = "terraform"
  }

  tags = merge(local.default_tags, var.additional_tags)
}

Common Data Sources

# Current AWS context
data "aws_caller_identity" "current" {}
data "aws_region" "current" {}
data "aws_partition" "current" {}

# Availability zones
data "aws_availability_zones" "available" {
  state = "available"
}

# Latest Amazon Linux 2 AMI
data "aws_ami" "amazon_linux_2" {
  most_recent = true
  owners      = ["amazon"]

  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }

  filter {
    name   = "virtualization-type"
    values = ["hvm"]
  }
}

# SSM Parameter for EKS AMI
data "aws_ssm_parameter" "eks_ami" {
  name = "/aws/service/eks/optimized-ami/${var.cluster_version}/amazon-linux-2/recommended/image_id"
}