locals {
  lambda_alarm_matrix = {
    errors = {
      metric_name = "Errors"
      statistic   = "Sum"
      threshold   = 0
      comparison  = "GreaterThanThreshold"
      period      = 300
    }
    throttles = {
      metric_name = "Throttles"
      statistic   = "Sum"
      threshold   = 0
      comparison  = "GreaterThanThreshold"
      period      = 300
    }
  }

  lambda_alarms = {
    for pair in flatten([
      for fn_key, fn in local.functions : [
        for metric_key, metric in local.lambda_alarm_matrix : {
          key         = "${fn_key}-${metric_key}"
          function    = fn.function_name
          metric_key  = metric_key
          metric_name = metric.metric_name
          statistic   = metric.statistic
          threshold   = metric.threshold
          comparison  = metric.comparison
          period      = metric.period
          alarm_name  = "${fn.function_name}-${metric_key}"
          description = "${fn.function_name} ${metric_key}"
        }
      ]
    ]) : pair.key => pair
  }
}

resource "aws_cloudwatch_metric_alarm" "lambda_errors_throttles" {
  for_each = local.lambda_alarms

  alarm_name          = each.value.alarm_name
  alarm_description   = each.value.description
  namespace           = "AWS/Lambda"
  metric_name         = each.value.metric_name
  dimensions          = { FunctionName = each.value.function }
  statistic           = each.value.statistic
  period              = each.value.period
  evaluation_periods  = 1
  threshold           = each.value.threshold
  comparison_operator = each.value.comparison
  treat_missing_data  = "notBreaching"
  alarm_actions       = [local.site_alerts_arn]
}

resource "aws_cloudwatch_metric_alarm" "lambda_duration" {
  for_each = local.functions

  alarm_name          = "${each.value.function_name}-duration"
  alarm_description   = "${each.value.function_name} approaching timeout (~80% of ${each.value.timeout}s)"
  namespace           = "AWS/Lambda"
  metric_name         = "Duration"
  dimensions          = { FunctionName = each.value.function_name }
  statistic           = "Maximum"
  period              = 300
  evaluation_periods  = 1
  threshold           = each.value.duration_ms
  comparison_operator = "GreaterThanThreshold"
  treat_missing_data  = "notBreaching"
  alarm_actions       = [local.site_alerts_arn]
}

resource "aws_cloudwatch_metric_alarm" "ddb_read_throttle" {
  alarm_name          = "payments-dashboard-table-read-throttle"
  alarm_description   = "PaymentsDashboard table read requests throttled"
  namespace           = "AWS/DynamoDB"
  metric_name         = "ReadThrottleEvents"
  dimensions          = { TableName = aws_dynamodb_table.dashboard.name }
  statistic           = "Sum"
  period              = 300
  evaluation_periods  = 1
  threshold           = 0
  comparison_operator = "GreaterThanThreshold"
  treat_missing_data  = "notBreaching"
  alarm_actions       = [local.site_alerts_arn]
}

resource "aws_cloudwatch_metric_alarm" "ddb_write_throttle" {
  alarm_name          = "payments-dashboard-table-write-throttle"
  alarm_description   = "PaymentsDashboard table write requests throttled"
  namespace           = "AWS/DynamoDB"
  metric_name         = "WriteThrottleEvents"
  dimensions          = { TableName = aws_dynamodb_table.dashboard.name }
  statistic           = "Sum"
  period              = 300
  evaluation_periods  = 1
  threshold           = 0
  comparison_operator = "GreaterThanThreshold"
  treat_missing_data  = "notBreaching"
  alarm_actions       = [local.site_alerts_arn]
}

resource "aws_cloudwatch_metric_alarm" "api_5xx" {
  alarm_name          = "payments-dashboard-api-5xx"
  alarm_description   = "payments-dashboard HTTP API returned 5xx responses"
  namespace           = "AWS/ApiGateway"
  metric_name         = "5xx"
  dimensions          = { ApiId = aws_apigatewayv2_api.http.id }
  statistic           = "Sum"
  period              = 300
  evaluation_periods  = 1
  threshold           = 0
  comparison_operator = "GreaterThanThreshold"
  treat_missing_data  = "notBreaching"
  alarm_actions       = [local.site_alerts_arn]
}

resource "aws_cloudwatch_metric_alarm" "api_4xx" {
  alarm_name          = "payments-dashboard-api-4xx"
  alarm_description   = "payments-dashboard HTTP API elevated 4xx responses"
  namespace           = "AWS/ApiGateway"
  metric_name         = "4xx"
  dimensions          = { ApiId = aws_apigatewayv2_api.http.id }
  statistic           = "Sum"
  period              = 300
  evaluation_periods  = 1
  threshold           = 10
  comparison_operator = "GreaterThanThreshold"
  treat_missing_data  = "notBreaching"
  alarm_actions       = [local.site_alerts_arn]
}

resource "aws_cloudwatch_metric_alarm" "api_latency" {
  alarm_name          = "payments-dashboard-api-latency-p99"
  alarm_description   = "payments-dashboard HTTP API p99 latency elevated (>3s)"
  namespace           = "AWS/ApiGateway"
  metric_name         = "Latency"
  dimensions          = { ApiId = aws_apigatewayv2_api.http.id }
  extended_statistic  = "p99"
  period              = 300
  evaluation_periods  = 1
  threshold           = 3000
  comparison_operator = "GreaterThanThreshold"
  treat_missing_data  = "notBreaching"
  alarm_actions       = [local.site_alerts_arn]
}

resource "aws_cloudwatch_metric_alarm" "process_csv_dlq" {
  alarm_name          = "payments-processPaymentCsv-async-dlq-messages"
  alarm_description   = "Failed processPaymentCsv async invocations landed in the DLQ"
  namespace           = "AWS/SQS"
  metric_name         = "ApproximateNumberOfMessagesVisible"
  dimensions          = { QueueName = aws_sqs_queue.process_csv_dlq.name }
  statistic           = "Maximum"
  period              = 300
  evaluation_periods  = 1
  threshold           = 0
  comparison_operator = "GreaterThanThreshold"
  treat_missing_data  = "notBreaching"
  alarm_actions       = [local.site_alerts_arn]
}