Oldest Replication Slot Lag RDS alarm

alarms.tf

@@ -1,12 +1,13 @@
 locals {
   thresholds = {
-    BurstBalanceThreshold     = min(max(var.burst_balance_threshold, 0), 100)
-    CPUUtilizationThreshold   = min(max(var.cpu_utilization_threshold, 0), 100)
-    CPUCreditBalanceThreshold = max(var.cpu_credit_balance_threshold, 0)
-    DiskQueueDepthThreshold   = max(var.disk_queue_depth_threshold, 0)
-    FreeableMemoryThreshold   = max(var.freeable_memory_threshold, 0)
-    FreeStorageSpaceThreshold = max(var.free_storage_space_threshold, 0)
-    SwapUsageThreshold        = max(var.swap_usage_threshold, 0)
+    BurstBalanceThreshold      = min(max(var.burst_balance_threshold, 0), 100)
+    CPUUtilizationThreshold    = min(max(var.cpu_utilization_threshold, 0), 100)
+    CPUCreditBalanceThreshold  = max(var.cpu_credit_balance_threshold, 0)
+    DiskQueueDepthThreshold    = max(var.disk_queue_depth_threshold, 0)
+    FreeableMemoryThreshold    = max(var.freeable_memory_threshold, 0)
+    FreeStorageSpaceThreshold  = max(var.free_storage_space_threshold, 0)
+    OldestReplicationThreshold = max(var.oldest_replication_threshold, 0)
+    SwapUsageThreshold         = max(var.swap_usage_threshold, 0)
   }
 
   alarm_names = toset([
@@ -139,6 +140,25 @@ resource "aws_cloudwatch_metric_alarm" "free_storage_space_too_low" {
   }
 }
 
+resource "aws_cloudwatch_metric_alarm" "oldest_replication_too_high" {
+  alarm_name          = "oldest_replication_too_high"
+  comparison_operator = "GreaterThanThreshold"
+  evaluation_periods  = "1"
+  metric_name         = "OldestReplicationSlotLag"
+  namespace           = "AWS/RDS"
+  period              = "600"
+  statistic           = "Average"
+  threshold           = local.thresholds["OldestReplicationThreshold"]
+  alarm_description   = "Average database replication lag over last 10 minutes too high, disk may fill"
+  alarm_actions       = [aws_sns_topic.default.arn]
+  ok_actions          = [aws_sns_topic.default.arn]
+
+  dimensions = {
+    DBInstanceIdentifier = var.db_instance_id
+  }
+}
+
+
 resource "aws_cloudwatch_metric_alarm" "swap_usage_too_high" {
   alarm_name          = module.label["swap_usage_too_high"].id
   comparison_operator = "GreaterThanThreshold"

variables.tf

@@ -43,6 +43,13 @@ variable "free_storage_space_threshold" {
   # 2 Gigabyte in Byte
 }
 
+variable "oldest_replication_threshold" {
+  description = "The maximum amount of replication lag space in Megabyte."
+  type        = string
+  default     = 1000
+  # 1 Gigabyte in Megabyte
+}
+
 variable "swap_usage_threshold" {
   description = "The maximum amount of swap space used on the DB instance in Byte."
   type        = number