Controlling order of resource/module replacement

Terraform
1

Hello,

I’m struggling with trying to control the replacement order/cadence for my project:

I have two EC2 instances that I need to be replaced on change (such as AMI change, etc) one at a time in order to avoid downtime. From my understanding and observation, Terraform will only destroy everything first before it creates anything.

I’d like to destroy + create Instance1 before destroying + creating Instance2. I have tried using depends_on but this will only implement the order of the destruction or creation but both instances will still be destroyed before they are then both created in whichever order.

Options I am aware of but are not currently viable:

  • For my project, I am unable to utilize terraform apply -replace to target a resource or any other customization of terraform apply
  • changing to using AutoScalingGroups
  • create_before_destroy (instances have network interfaces so this isn’t very feasible)

Any tips or help would be greatly appreciated! Thanks very much in advance for your time and consideration.

2

Terraform isn’t an orchestration tool.

To do orchestration involving Terraform you either need to be creating resources which do it (e.g. creating ASGs instead of EC2 directly) or having a tool that calls Terraform in different ways (e.g. passing in different variable values across multiple applies).

As you seem to have discounted both options I don’t believe there is anything else to suggest.