使用Terraform创建Aurora PostgreSQL只读副本遇参数组合错误

问题 #

尝试使用Terraform为us-east-1区域的现有Aurora PostgreSQL集群创建同区域只读副本，使用的Terraform脚本如下：

resource "aws_rds_cluster" "test_db_cluster" {
  cluster_identifier            = "${var.base_name}-read-replica"
  engine                        = "aurora-postgresql"
  engine_mode                   = "provisioned"
  engine_version                = "16.1"
  skip_final_snapshot           = true
  storage_encrypted             = true
  replication_source_identifier = "arn:aws:rds:us-east-1:<xxx>:cluster:testdev"
  source_region                 = "us-east-1"
}

执行后出现错误：

InvalidParameterCombination: The DB engine aurora-postgresql doesn't support cross-region read replicas.

即使移除source_region参数，错误依然存在。原集群同样通过Terraform创建，脚本如下：

resource "aws_rds_cluster" "test_db_cluster" {
  cluster_identifier = var.base_name
  engine             = "aurora-postgresql"
  engine_mode        = "provisioned"
  engine_version     = "16.1"
  database_name      = var.database_name
  master_username    = var.master_username
  db_subnet_group_name = var.subnet_group
  vpc_security_group_ids = [var.security_group_id]
  availability_zones     = [var.az]
  skip_final_snapshot    = true
  storage_encrypted = true
  manage_master_user_password   = true
  master_user_secret_kms_key_id = aws_kms_key.scopp_db_kms.key_id

  serverlessv2_scaling_configuration {
    max_capacity = var.max_capacity
    min_capacity = var.min_capacity
  }
}

resource "aws_rds_cluster_instance" "test_db_instance" {
  identifier = "${var.base_name}-instance-1"
  cluster_identifier = aws_rds_cluster.test_db_cluster.id
  instance_class     = "db.serverless"
  engine             = aws_rds_cluster.test_db_cluster.engine
  engine_version     = aws_rds_cluster.test_db_cluster.engine_version
  performance_insights_enabled = true
}

resource "aws_kms_key" "test_db_kms" {
  description = "Test DB KMS Key"
}

原因分析 #

1. 原集群为Serverless v2模式，不支持独立只读副本集群
   你的原Aurora PostgreSQL集群配置了serverlessv2_scaling_configuration，且实例类为db.serverless，属于Serverless v2架构。而Aurora PostgreSQL的Serverless v2集群不支持创建单独的只读副本集群——无论同区域还是跨区域都不允许这种操作。

2. 错误提示存在误导性
   虽然你是在同区域创建副本，但AWS API的错误提示会误报为"不支持跨区域副本"，本质原因是Serverless v2集群本身不支持这种副本创建方式。

3. 正确的只读访问实现方式
   对于Serverless v2的Aurora PostgreSQL集群，无需创建独立副本集群，直接在同一个集群内添加只读实例即可实现只读访问。可以通过新增aws_rds_cluster_instance资源并指定promotion_tier参数标记为只读实例：

   resource "aws_rds_cluster_instance" "test_db_read_replica" {
     identifier         = "${var.base_name}-read-instance-1"
     cluster_identifier = aws_rds_cluster.test_db_cluster.id
     instance_class     = "db.serverless"
     engine             = aws_rds_cluster.test_db_cluster.engine
     engine_version     = aws_rds_cluster.test_db_cluster.engine_version
     promotion_tier     = 1 # 标记为只读实例，无法被提升为主实例
     performance_insights_enabled = true
   }
   

内容的提问来源于stack exchange，提问作者Kstackr