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Bartłomiej Danek
Bartłomiej Danek

Posted on • Originally published at bard.sh

count vs for_each in Terraform

#terraform #hcl

count - simple but fragile

count creates N copies of a resource indexed by number.

resource "aws_iam_user" "devs" {
  count = length(var.usernames)
  name  = var.usernames[count.index]
}
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The problem: removing alice from ["alice", "bob", "carol"] shifts all indexes - Terraform destroys and recreates bob and carol.

# removing alice causes:
aws_iam_user.devs[0]  # destroyed (was alice)
aws_iam_user.devs[1]  # destroyed + recreated (was bob, now bob)
aws_iam_user.devs[2]  # destroyed + recreated (was carol, now carol)
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for_each - stable and explicit

for_each keys resources by a string - removing one item never touches the others.

resource "aws_iam_user" "devs" {
  for_each = toset(var.usernames)
  name     = each.key
}
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# removing alice causes only:
aws_iam_user.devs["alice"]  # destroyed
# bob and carol are untouched
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when to use count

  • identical resources with no meaningful name (e.g. 3 identical worker nodes)
  • toggling a resource on/off: count = var.enabled ? 1 : 0

when to use for_each

  • any named resource (users, buckets, DNS records, security groups)
  • when the set of items will change over time

Originally published at https://bard.sh/posts/count_vs_for_each/

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