Interview › Infrastructure as Code (Terraform, Ansible)
What is the difference between count and for_each, and when do you use each?
Infrastructure as Code (Terraform, Ansible) · Basic level
Answer
count creates N instances addressed by numeric index; for_each creates instances keyed by map or set keys. I use count for simple on/off or identical numbered resources, and for_each when each instance has a stable identity such as subnet name, user name, or environment key.
Technical explanation
count instances are addressed like aws_instance.web[0]; for_each instances are addressed like aws_instance.web["blue"].
Stable addressing is important because address changes can force replacement or state moves.
for_each requires keys known during planning.
Keep Terraform's ownership boundary clear: one state should own a resource or field, and other tools should consume published outputs instead of modifying it.
Use fmt, validate, linting, policy checks, plan review, and state locking before production applies.
Design for small blast radius by splitting state around lifecycle, permissions, and recovery boundaries.
Hands-on example
1. Model repeated resources for: What is the difference between count and for_each, and when do you use each?
2. Prefer stable keys with for_each:
variable "subnets" {
type = map(object({ cidr = string, az = string }))
}
resource "aws_subnet" "private" {
for_each = var.subnets
vpc_id = aws_vpc.main.id
cidr_block = each.value.cidr
availability_zone = each.value.az
tags = { Name = "private-${each.key}" }
}
3. For nested blocks, use dynamic only when the input list genuinely drives repeated nested configuration:
dynamic "ingress" {
for_each = var.ingress_rules
content { from_port = ingress.value.port to_port = ingress.value.port protocol = "tcp" cidr_blocks = ingress.value.cidrs }
}
4. Remove one key and run plan; confirm only that keyed instance is affected rather than later list indexes shifting.
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