Terraform Advanced — count vs for_each

The definitive guide on when to use count and when to use for_each.

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Use count For: On/Off Toggles

# ✅ count is perfect for conditional creation
resource "aws_cloudwatch_metric_alarm" "cpu" {
  count = var.enable_alarms ? 1 : 0
  # ...
}

# Access with one()
output "alarm_arn" {
  value = one(aws_cloudwatch_metric_alarm.cpu[*].arn)
}

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Use for_each For: Named Collections

# ✅ for_each gives stable, name-based keys
resource "aws_iam_user" "this" {
  for_each = toset(["alice", "bob", "carol"])
  name     = each.value
}

# State: aws_iam_user.this["alice"], aws_iam_user.this["bob"]
# Removing "bob" only destroys bob — alice and carol are untouched

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The count Reorder Problem

# ❌ With count: removing item 0 shifts everything
variable "users" {
  default = ["alice", "bob", "carol"]
  # Remove "alice" → bob becomes [0], carol becomes [1]
  # Terraform destroys alice AND bob, recreates bob at [0]
}

resource "aws_iam_user" "this" {
  count = length(var.users)
  name  = var.users[count.index]
}

⚠️ Warning: Never use count with a list that may have items removed from the middle.

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Quick Rule

Need on/off?          → count
Named collection?     → for_each
List that may change? → for_each + toset()

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What’s Next?

Master the lifecycle block.

← Meta-Arguments lifecycle Block →