Trigger random_id resource recreation on rds instance destroy and recreate - amazon-web-services

Question

Folks, am trying to find a way with terraform random_id resource to recreate and provide a new random value when the rds instance destroys and recreates due to a change that went in, say the username on rds has changed.
This random value am trying to attach to final_snapshot_identifier of the aws_db_instance resource so that the snapshot should have a unique value to its id everytime it gets created upon rds instance being destroyed.
Current code:
resource "random_id" "snap_id" {
byte_length = 8
}
locals {
inst_id = "test-rds-inst"
inst_snap_id = "${local.inst_id}-snap-${format("%.4s", random_id.snap_id.dec)}"
}
resource "aws_db_instance" "rds" {
.....
identifier = local.inst_id
final_snapshot_identifier = local.inst_snap_id
skip_final_snapshot = false
username = "foo"
apply_immediately = true
.....
}
output "snap_id" {
value = aws_db_instance.rds.final_snapshot_identifier
}
Output after terraform apply:
snap_id = "test-rds-inst-snap-5553"
Use case am trying out:
#1:
Modify value in rds instance to simulate a destroy & recreate:
Modify username to "foo-tmp"
terraform apply -auto-approve
Output:
snap_id = "test-rds-inst-snap-5553"
I was expecting the random_id to kick in and output a unique id, but it didn't.
Observation:
rds instance in deleting state
snapshot "test-rds-inst-snap-5553" in creating state
rds instance recreated and in available state
snapshot "test-rds-inst-snap-5553" in available state
#2:
Modify value again in rds instance to simulate a destroy & recreate:
Modify username to "foo-new"
terraform apply -auto-approve
Kind of expected below error, coz snap id didn't get a new value in prior attempt, but tired anyways..
Observation:
**Error:** error deleting DB Instance (test-rds-inst): DBSnapshotAlreadyExists: Cannot create the snapshot because a snapshot with the identifier test-rds-inst-snap-5553 already exists.
Am aware of the keepers{} map for random_id resource, but not sure on what from the rds_instance that I need to put in the map so that the random_id resource will be recreated and it ends up providing a new unique value to the snap_id suffix.
Also I feel using any attribute of rds instance in the random_id keepers, might cause a circular dependency issue. I may be wrong but haven't tried it though.
Any suggestions will be helpful. Thanks.

Answer 1

The easiest way to do this would be to use taint on the random_id resource, as per the documentation [1]:
To force a random result to be replaced, the taint command can be used to produce a new result on the next run.
Alternatively, looking at the example from the documentation, you could do something like:
resource "random_id" "snap_id" {
byte_length = 8
keepers {
snapshot_id = var.snapshot_id
}
}
resource "aws_db_instance" "rds" {
.....
identifier = local.inst_id
final_snapshot_identifier = random_id.snap_id.keepers.snapshot_id
skip_final_snapshot = false
username = "foo"
apply_immediately = true
.....
}
This means that until the value of the variable snapshot_id changes, the random_id will generate the same result. Not sure if that would work with locals, but you could try replacing var.snapshot_id with local.inst_snap_id. If that works, you could then name the snapshot using built-in functions like formatdate [2] and timestamp [3] to create a snapshot id which will be tied to the time when you were running apply, something like:
locals {
inst_id = "test-rds-inst"
snap_time = formatdate("YYYYMMDD", timestamp())
inst_snap_id = "${local.inst_id}-snap-${format("%.4s", random_id.snap_id.dec)}-${local.snap_time}"
}
[1] https://registry.terraform.io/providers/hashicorp/random/latest/docs#resource-keepers
[2] https://www.terraform.io/language/functions/formatdate
[3] https://www.terraform.io/language/functions/timestamp