Terraform Dynamic Blocks

The problem

Some resources take repeated nested blocks, for example security group ingress rules. Writing them out by hand is repetitive, and you can't use for_each on a nested block directly.

dynamic blocks

variable "ingress_ports" {
  default = [80, 443]
}

resource "aws_security_group" "web" {
  name = "web"

  dynamic "ingress" {
    for_each = var.ingress_ports
    content {
      from_port   = ingress.value
      to_port     = ingress.value
      protocol    = "tcp"
      cidr_blocks = ["0.0.0.0/0"]
    }
  }
}

Inside content, the iterator is named after the block (ingress.value, ingress.key).

# The same data shaping a dynamic block does, shown as an output:
variable "ingress_ports" {
  default = [80, 443]
}

output "rules" {
  value = [for p in var.ingress_ports : {
    from_port = p
    to_port   = p
    protocol  = "tcp"
  }]
}

rules = [
  {
    "from_port" = 80
    "protocol" = "tcp"
    "to_port" = 80
  },
  {
    "from_port" = 443
    "protocol" = "tcp"
    "to_port" = 443
  },
]

Use dynamic blocks sparingly: they make configurations harder to read. They are best inside reusable modules.