Preventing destroy of resources when refactoring Terraform to use indices - amazon-web-services

Question

When I was just starting to use Terraform, I more or less naively declared resources individually, like this:
resource "aws_cloudwatch_log_group" "image1_log" {
name = "${var.image1}-log-group"
tags = module.tagging.tags
}
resource "aws_cloudwatch_log_group" "image2_log" {
name = "${var.image2}-log-group"
tags = module.tagging.tags
}
resource "aws_cloudwatch_log_stream" "image1_stream" {
name = "${var.image1}-log-stream"
log_group_name = aws_cloudwatch_log_group.image1_log.name
}
resource "aws_cloudwatch_log_stream" "image2_stream" {
name = "${var.image2}-log-stream"
log_group_name = aws_cloudwatch_log_group.image2_log.name
}
Then, 10-20 different log groups later, I realized this wasn't going to work well as infrastructure grew. I decided to define a variable list:
variable "image_names" {
type = list(string)
default = [
"image1",
"image2"
]
}
Then I replaced the resources using indices:
resource "aws_cloudwatch_log_group" "service-log-groups" {
name = "${element(var.image_names, count.index)}-log-group"
count = length(var.image_names)
tags = module.tagging.tags
}
resource "aws_cloudwatch_log_stream" "service-log-streams" {
name = "${element(var.image_names, count.index)}-log-stream"
log_group_name = aws_cloudwatch_log_group.service-log-groups[count.index].name
count = length(var.image_names)
}
The problem here is that when I run terraform apply, I get 4 resources to add, 4 resources to destroy. I tested this with an old log group, and saw that all my logs were wiped (obviously, since the log was destroyed).
The names and other attributes of the log groups/streams are identical- I'm simply refactoring the infrastructure code to be more maintainable. How can I maintain my existing log groups without deleting them yet still refactor my code to use lists?

Answer 1

You'll need to move the existing resources within the Terraform state.
Try running terraform show to get the strings under which the resources are stored, this will be something like [module.xyz.]aws_cloudwatch_log_group.image1_log ...
You can move it with terraform state mv [module.xyz.]aws_cloudwatch_log_group.image1_log '[module.xyz.]aws_cloudwatch_log_group.service-log-groups[0]'.
You can choose which index to assign to each resource by changing [0] accordingly.
Delete the old resource definition for each moved resource, as Terraform would otherwise try to create a new group/stream.
Try it with the first import and check with terraform plan if the resource was moved correctly...
Also check if you need to choose some index for the image_names list jsut to be sure, but I think that won't be necessary.