cloudwatch_metric_math.tf

resource "aws_cloudwatch_metric_alarm" "error_rate" {
  # ... rest of configuration
    
  metric_query {
    id = "errorCount"

    metric {
      metric_name = "Errors"
      namespace   = "AWS/Lambda"
      period      = 900
      stat        = "Sum"

      dimensions = {
        FunctionName = "${var.environment}-lambda"
      }
    }
  }

  metric_query {
    id = "invocations"

    metric {
      metric_name = "Invocations"
      namespace   = "AWS/Lambda"
      period      = 900
      stat        = "Sum"

      dimensions = {
        FunctionName = "${var.environment}-lambda"
      }
    }
  }

  metric_query {
    id = "errorRate"

    expression  = " ( errorCount / invocations ) * 100"
    label       = "Lambda error rate percentage"
    return_data = "true"
  }
}