From 3787c795ed49063eaff0db4ed4ff2c0b93ab8775 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jo=C3=A3o=20Taveira=20Ara=C3=BAjo?= <joao@observeinc.com>
Date: Mon, 13 Mar 2023 11:44:35 -0700
Subject: [PATCH] fix(cloudwatch_metrics): revert to cron expression

PR #62 fixed our schedule expression to fire at the correct cadence, but
subtly changed the time at which the rule would trigger. The use of the
`rate` function ensures a rule will trigger at regular intervals from
rule creation. Given the nature of the data we are collecting, we want
rules to trigger aligned to fixed time boundaries (e.g. every minute,
every hour). This commit reintroduces a correct cron expression.
---
 modules/cloudwatch_metrics/main.tf      | 12 +++++++++++-
 modules/cloudwatch_metrics/variables.tf |  4 ++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/modules/cloudwatch_metrics/main.tf b/modules/cloudwatch_metrics/main.tf
index fb64fda..a90ebd3 100644
--- a/modules/cloudwatch_metrics/main.tf
+++ b/modules/cloudwatch_metrics/main.tf
@@ -36,10 +36,20 @@ resource "aws_iam_role_policy_attachment" "this" {
   policy_arn = aws_iam_policy.this.arn
 }
 
+
+
+locals {
+  cron_expression = coalesce(
+    var.interval >= 60 && var.interval < 60 * 60 ? "*/${var.interval / 60} * * * ? *" : "",                # minutely
+    var.interval >= 60 * 60 && var.interval < 24 * 60 * 60 ? "0 */${var.interval / 60 / 60} * * ? *" : "", # hourly
+    var.interval >= 24 * 60 * 60 ? "0 0 */${var.interval / 24 / 60 / 60} * ? *" : "",                      # daily
+  )
+}
+
 resource "aws_cloudwatch_event_rule" "trigger" {
   name_prefix         = var.eventbridge_name_prefix
   description         = "Periodically trigger Observe Lambda to collect CloudWatch metrics"
-  schedule_expression = var.interval == 60 ? "rate(1 minute)" : "rate(${var.interval / 60} minutes)"
+  schedule_expression = "cron(${local.cron_expression})"
   event_bus_name      = var.eventbridge_schedule_event_bus_name
 }
 
diff --git a/modules/cloudwatch_metrics/variables.tf b/modules/cloudwatch_metrics/variables.tf
index 4fee4ed..50fb6dc 100644
--- a/modules/cloudwatch_metrics/variables.tf
+++ b/modules/cloudwatch_metrics/variables.tf
@@ -35,8 +35,8 @@ variable "interval" {
   nullable    = false
   default     = 300
   validation {
-    condition     = var.interval >= 60 && var.interval <= 10800
-    error_message = "interval must be in [60, 10800] (1 minute to 3 hours)"
+    condition     = (var.interval >= 60 && var.interval <= 60 * 60) || (var.interval >= 60 * 60 && var.interval <= 24 * 60 * 60 && var.interval % (60 * 60) == 0)
+    error_message = "interval must be minutely, up to an hour, or hourly, up to a day"
   }
 }