SDI Notes

Appearance

24 - Multiple Servers ​

Working Code: terraform/exercise-24-multi-server/

The Problem: Deploying 5 servers means copy-pasting the hcloud_server block 5 times. Tedious and hard to maintain.

The Solution: Use count to tell Terraform "Make X copies of this".

Objective ​

Deploy configurable number of servers (e.g., 2) with DNS records (work-1, work-2) and SSH wrappers.

How-to ​

1. Variables ​

hcl
variable "server_count" {
  default = 2
}

2. Servers with count ​

hcl
resource "hcloud_server" "workers" {
  count = var.server_count
  name = "work-${count.index + 1}"  # work-1, work-2...
  # ...
}

3. DNS Records with count ​

hcl
resource "dns_a_record_set" "workers" {
  count = var.server_count
  name = "work-${count.index + 1}"
  addresses = [hcloud_server.workers[count.index].ipv4_address]
}

4. SSH Modules with count ​

hcl
module "ssh_config" {
  count = var.server_count
  source = "./modules/ssh_utils"

  name = hcloud_server.workers[count.index].name
  ip   = hcloud_server.workers[count.index].ipv4_address
}

Verification ​

bash
terraform apply
ls bin/  # See ssh-work-1, ssh-work-2
# Change server_count to 3 and apply again → one new server

Problems & Learnings ​

Common Issues

  • DNS propagation delay: ssh-keyscan on the FQDN (work-1.g2.sdi.hdm-stuttgart.cloud) times out because DNS hasn't propagated yet when Terraform runs null_resource.known_hosts. Fix: scan by IP, then rewrite the entry with sed to use the FQDN.
  • macOS DNS resolution: Even after propagation, macOS's system resolver (getaddrinfo) may not resolve the Hetzner DNS entries that dig can resolve. The SSH wrapper connects by IP with -o HostKeyAlias=<fqdn> so the correct known_hosts entry is matched.

Key Takeaways

  • Always scan SSH host keys by IP during provisioning — DNS is not yet reliable at that stage.
  • Use HostKeyAlias to verify against a FQDN known_hosts entry while connecting by IP.