Reader independent scaling (#256)

* add reader_independent_scaling input

allow to scale reader independently from primary using promotion_tier >= 2

https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless-v2.how-it-works.html#aurora-serverless-v2.how-it-works.scaling

* use promotion_tier

* Update variables.tf

Co-authored-by: RoseSecurity <72598486+RoseSecurity@users.noreply.github.com>

---------

Co-authored-by: RoseSecurity <72598486+RoseSecurity@users.noreply.github.com>

Author: jfcoz

README.md

@@ -446,6 +446,7 @@ Available targets:
 | <a name="input_performance_insights_enabled"></a> [performance\_insights\_enabled](#input\_performance\_insights\_enabled) | Whether to enable Performance Insights | `bool` | `false` | no |
 | <a name="input_performance_insights_kms_key_id"></a> [performance\_insights\_kms\_key\_id](#input\_performance\_insights\_kms\_key\_id) | The ARN for the KMS key to encrypt Performance Insights data. When specifying `performance_insights_kms_key_id`, `performance_insights_enabled` needs to be set to true | `string` | `""` | no |
 | <a name="input_performance_insights_retention_period"></a> [performance\_insights\_retention\_period](#input\_performance\_insights\_retention\_period) | Amount of time in days to retain Performance Insights data. Either 7 (7 days) or 731 (2 years) | `number` | `null` | no |
+| <a name="input_promotion_tier"></a> [reader\_independent\_scaling](#input\_reader\_independent\_scaling) | Failover priority. 2 or more allow independent scaling | `number` | `0` | no |
 | <a name="input_publicly_accessible"></a> [publicly\_accessible](#input\_publicly\_accessible) | Set to true if you want your cluster to be publicly accessible (such as via QuickSight) | `bool` | `false` | no |
 | <a name="input_rds_cluster_parameter_group_name"></a> [rds\_cluster\_parameter\_group\_name](#input\_rds\_cluster\_parameter\_group\_name) | The name to give to the created `aws_rds_cluster_parameter_group` resource.<br/>If omitted, the module will generate a name. | `string` | `""` | no |
 | <a name="input_rds_monitoring_interval"></a> [rds\_monitoring\_interval](#input\_rds\_monitoring\_interval) | The interval, in seconds, between points when enhanced monitoring metrics are collected for the DB instance. To disable collecting Enhanced Monitoring metrics, specify 0. The default is 0. Valid Values: 0, 1, 5, 10, 15, 30, 60 | `number` | `0` | no |

docs/terraform.md

@@ -132,6 +132,7 @@
 | <a name="input_performance_insights_enabled"></a> [performance\_insights\_enabled](#input\_performance\_insights\_enabled) | Whether to enable Performance Insights | `bool` | `false` | no |
 | <a name="input_performance_insights_kms_key_id"></a> [performance\_insights\_kms\_key\_id](#input\_performance\_insights\_kms\_key\_id) | The ARN for the KMS key to encrypt Performance Insights data. When specifying `performance_insights_kms_key_id`, `performance_insights_enabled` needs to be set to true | `string` | `""` | no |
 | <a name="input_performance_insights_retention_period"></a> [performance\_insights\_retention\_period](#input\_performance\_insights\_retention\_period) | Amount of time in days to retain Performance Insights data. Either 7 (7 days) or 731 (2 years) | `number` | `null` | no |
+| <a name="input_promotion_tier"></a> [reader\_independent\_scaling](#input\_reader\_independent\_scaling) | Failover priority. 2 or more allow independent scaling | `number` | `0` | no |
 | <a name="input_publicly_accessible"></a> [publicly\_accessible](#input\_publicly\_accessible) | Set to true if you want your cluster to be publicly accessible (such as via QuickSight) | `bool` | `false` | no |
 | <a name="input_rds_cluster_parameter_group_name"></a> [rds\_cluster\_parameter\_group\_name](#input\_rds\_cluster\_parameter\_group\_name) | The name to give to the created `aws_rds_cluster_parameter_group` resource.<br>If omitted, the module will generate a name. | `string` | `""` | no |
 | <a name="input_rds_monitoring_interval"></a> [rds\_monitoring\_interval](#input\_rds\_monitoring\_interval) | The interval, in seconds, between points when enhanced monitoring metrics are collected for the DB instance. To disable collecting Enhanced Monitoring metrics, specify 0. The default is 0. Valid Values: 0, 1, 5, 10, 15, 30, 60 | `number` | `0` | no |

main.tf

@@ -368,6 +368,7 @@ resource "aws_rds_cluster_instance" "default" {
   preferred_maintenance_window          = var.maintenance_window
   copy_tags_to_snapshot                 = var.copy_tags_to_snapshot
   ca_cert_identifier                    = var.ca_cert_identifier
+  promotion_tier                        = var.promotion_tier
 
   dynamic "timeouts" {
     for_each = var.timeouts_configuration

variables.tf

@@ -378,6 +378,15 @@ variable "performance_insights_retention_period" {
   default     = null
 }
 
+variable "promotion_tier" {
+  type        = number
+  default     = 0
+  description = <<-EOT
+Failover Priority setting on instance level. The reader who has lower tier has higher priority to get promoted to writer.
+  
+Readers in promotion tiers 0 and 1 scale at the same time as the writer. Readers in promotion tiers 2–15 scale independently from the writer. For more information, see: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless-v2.how-it-works.html#aurora-serverless-v2.how-it-works.scaling
+EOT
+}
 variable "autoscaling_enabled" {
   type        = bool
   default     = false