feat: support reserved instances

main.tf

@@ -7,6 +7,8 @@ locals {
   is_regional_cluster      = var.cluster_type == "regional"
   is_serverless            = var.engine_mode == "serverless"
   ignore_admin_credentials = var.replication_source_identifier != "" || var.snapshot_identifier != null
+  reserved_instance_engine = split("-", var.engine)[1]
+  use_reserved_instances   = var.use_reserved_instances && !local.is_serverless && contains(["mysql", "postgresql"], local.reserved_instance_engine)
 }
 
 data "aws_partition" "current" {
@@ -88,6 +90,28 @@ resource "aws_security_group_rule" "egress_ipv6" {
   security_group_id = join("", aws_security_group.default[*].id)
 }
 
+data "aws_rds_reserved_instance_offering" "default" {
+  count               = local.use_reserved_instances
+  db_instance_class   = var.db_cluster_instance_class
+  duration            = var.rds_ri_duration
+  multi_az            = local.is_regional_cluster
+  offering_type       = var.rds_ri_offering_type
+  product_description = local.reserved_instance_engine
+}
+
+# Note: I'm not sure what will happen when the db reservation expires, and this is not easy to test.
+# It will either be recreated or will require manual intervention to recreate.
+resource "aws_rds_reserved_instance" "default" {
+  count = local.use_reserved_instances
+
+  offering_id = data.aws_rds_reserved_instance_offering.default[0].id
+  instance_count = local.cluster_instance_count
+  lifecycle {
+    # Once created, we want to avoid any case of accidentally re-creating.
+    prevent_destroy = true
+  }
+}
+
 # The name "primary" is poorly chosen. We actually mean standalone or regional.
 # The primary cluster of a global database is actually created with the "secondary" cluster resource below.
 resource "aws_rds_cluster" "primary" {

variables.tf

@@ -542,4 +542,27 @@ variable "network_type" {
   type        = string
   default     = "IPV4"
   description = "The network type of the cluster. Valid values: IPV4, DUAL."
-}
+}
+
+variable "use_reserved_instances" {
+  description = <<-EOT
+    WARNING: Observe your plans and applies carefully when using this feature.
+    It has potential to be very expensive if not used correctly.
+
+    Whether to use reserved instances.
+  EOT
+  type        = bool
+  default     = false
+}
+
+variable "rds_ri_offering_type" {
+  description = "Offering type of reserved DB instances. Valid values are 'No Upfront', 'Partial Upfront', 'All Upfront'."
+  type        = string
+  default     = ""
+}
+
+variable "rds_ri_duration" {
+  description = "The number of years to reserve the instance. Values can be 1 or 3 (or in seconds, 31536000 or 94608000)"
+  type        = number
+  default     = 1
+}