Creating RDS Instances from Snapshot Using Terraform

Question

Working on a Terraform project in which I am creating an RDS cluster by grabbing and using the most recent production db snapshot:

# Get latest snapshot from production DB
data "aws_db_snapshot" "db_snapshot" {
    most_recent = true
    db_instance_identifier = "${var.db_instance_to_clone}"
}

#Create RDS instance from snapshot
resource "aws_db_instance" "primary" {
    identifier = "${var.app_name}-primary"
    snapshot_identifier = "${data.aws_db_snapshot.db_snapshot.id}"
    instance_class = "${var.instance_class}"
    vpc_security_group_ids = ["${var.security_group_id}"]
    skip_final_snapshot = true
    final_snapshot_identifier = "snapshot"
    parameter_group_name = "${var.parameter_group_name}"
    publicly_accessible = true
    timeouts {
      create = "2h"
    }
}

The issue with this approach is that following runs of the terraform code (once another snapshot has been taken) want to re-create the primary RDS instance (and subsequently, the read replicas) with the latest snapshot of the DB. I was thinking something along the lines of a boolean count parameters that specifies first run, but setting count = 0 on the snapshot resource causes issues with the snapshot_id parameters of the db resource. Likewise setting a count = 0 on the db resource would indicate that it would destroy the db.

Use case for this is to be able to make changes to other aspects of the production infrastructure that this terraform plan manages without having to re-create the entire RDS cluster, which is a very time consuming resource to destroy/create.

Answer 1

Try placing an ignore_changes lifecycle block within your aws_db_instance definition:

lifecycle {
    ignore_changes = [
      snapshot_identifier,
    ]
}

This will cause Terraform to only look for changes to the database's snapshot_identifier upon initial creation.

If the database already exists, Terraform will ignore any changes to the existing database's snapshot_identifier field -- even if a new snapshot has been created since then.