count in Terraform

The count meta-argument creates multiple instances of a resource from a single block. It’s the simplest way to provision N identical (or nearly identical) resources — subnets across availability zones, IAM users from a list, replicas of a server.

Basic count Usage

# Create 3 identical EC2 instances
resource "aws_instance" "workers" {
  count         = 3
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t3.micro"

  tags = {
    Name = "worker-${count.index}"   # worker-0, worker-1, worker-2
  }
}

count.index is the zero-based index of the current instance (0, 1, 2, …).

count.index for Unique Naming

# Subnets across availability zones
resource "aws_subnet" "public" {
  count             = length(var.availability_zones)
  vpc_id            = aws_vpc.main.id
  cidr_block        = cidrsubnet(var.vpc_cidr, 8, count.index)
  availability_zone = var.availability_zones[count.index]

  tags = {
    Name = "public-${var.availability_zones[count.index]}"
    Tier = "public"
  }
}

# Reference by index
output "public_subnet_ids" {
  value = aws_subnet.public[*].id   # All subnet IDs as a list
}

# Access a specific instance
output "first_public_subnet_id" {
  value = aws_subnet.public[0].id
}

count from Variable

variable "instance_count" {
  type    = number
  default = 2
}

resource "aws_instance" "app" {
  count         = var.instance_count
  ami           = data.aws_ami.ubuntu.id
  instance_type = var.instance_type

  tags = {
    Name  = "${var.environment}-app-${count.index + 1}"   # 1-based for display
    Index = count.index
  }
}

# Scale to 0 to create no instances
# var.instance_count = 0 → creates nothing

count from a List

variable "user_names" {
  type    = list(string)
  default = ["alice", "bob", "charlie"]
}

resource "aws_iam_user" "team" {
  count = length(var.user_names)
  name  = var.user_names[count.index]

  tags = {
    CreatedBy = "terraform"
    Index     = count.index
  }
}

output "user_arns" {
  value = aws_iam_user.team[*].arn   # All ARNs as a list
}

Conditional Resource Creation

Set count = 0 to create no instances, count = 1 to create one. Useful for optionally creating a resource:

variable "enable_bastion" {
  type    = bool
  default = false
}

resource "aws_instance" "bastion" {
  count         = var.enable_bastion ? 1 : 0
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t3.micro"
}

# Access the conditionally-created resource
output "bastion_ip" {
  value = var.enable_bastion ? aws_instance.bastion[0].public_ip : null
}

# Conditionally create an ElasticSearch domain in production only
resource "aws_elasticsearch_domain" "search" {
  count         = var.environment == "production" ? 1 : 0
  domain_name   = "search-production"
  elasticsearch_version = "8.10"
}

Splat Expressions: Accessing All Instances

# Collect all instance IDs
output "instance_ids" {
  value = aws_instance.app[*].id
}

# Pass all subnet IDs to a load balancer
resource "aws_lb" "main" {
  subnets = aws_subnet.public[*].id
}

# All security group IDs
resource "aws_db_instance" "main" {
  vpc_security_group_ids = [aws_security_group.database[*].id]
}

count vs for_each: When to Use Which

# Use count when:
# - Creating N identical or index-differentiated resources
# - You only care about the position, not the identity

# Creating 5 identical workers
resource "aws_instance" "workers" {
  count = 5
  # ...
}

# Use for_each when:
# - Each instance has a unique, stable identity
# - You want to reference instances by name, not index

variable "users" {
  type = set(string)
  default = ["alice", "bob", "charlie"]
}

resource "aws_iam_user" "team" {
  for_each = var.users
  name     = each.key   # Stable identity: "alice", "bob", "charlie"
}

The critical count limitation: if you remove an element from the middle of a list, all subsequent indices shift — Terraform destroys and recreates resources unexpectedly.

# If users = ["alice", "bob", "charlie"] and you remove "alice":
# count = 3 → count = 2
# aws_iam_user.team[0] was "alice", becomes "bob"
# aws_iam_user.team[1] was "bob", becomes "charlie"
# aws_iam_user.team[2] ("charlie") is deleted
# Result: Terraform destroys "bob" and "charlie" and recreates them as "bob" and "charlie"
# But their history/permissions are lost during recreation

# With for_each: removing "alice" only deletes "alice" — "bob" and "charlie" are untouched

Use count for homogeneous resources where identity doesn’t matter (replicas, subnets). Use for_each when each instance is distinct.