Terraform-RDS Aurora-VPC-不同的 VPC 错误

Question

我正在尝试使用 Terraform 和其他 EC2 实例创建 AWS RDS Aurora。EC2 实例被定向到 VPC，没有任何问题，但由于数据库实例和 EC2 安全组位于不同的 VPC 中，RDS 无法加入 VPC。

我在这里遗漏了什么？

我创建了以下内容：

variable "vpc_cidr" {
  description = "CIDR for the VPC"
  default = "10.0.0.0/16"
}

resource "aws_vpc" "vpc" {
  cidr_block = "${var.vpc_cidr}"
  tags = {
    Name = "${var.env}_vpc"
  }
}

resource "aws_subnet" "vpc_subnet" {
  vpc_id = "${aws_vpc.vpc.id}"
  cidr_block = "${var.vpc_cidr}"
  availability_zone = "eu-west-1a"

  tags = {
    Name = "${var.env}_vpc"
  }
}

resource "aws_db_subnet_group" "subnet_group" {
  name        = "${var.env}-subnet-group"
  subnet_ids  = ["${aws_subnet.vpc_subnet.id}"]
}

resource "aws_security_group" "RDS_DB_SG" {
    name = "${var.env}-rds-sg"
    vpc_id = "${aws_vpc.vpc.id}"
    ingress {
        from_port = 3396
        to_port = 3396
        protocol = "tcp"
        security_groups = ["${aws_security_group.BE_SG.id}"]
    }
}

resource "aws_security_group" "BE_SG" {
    name = "${var.env}_BE_SG"
    vpc_id = "${aws_vpc.vpc.id}"

    ingress {
        from_port = 80
        to_port = 80
        protocol = "tcp"
        cidr_blocks = ["0.0.0.0/0"]
    }
}

resource "aws_rds_cluster" "rds-cluster" {
    cluster_identifier = "${var.env}-cluster"
    database_name = "${var.env}-rds"
    master_username = "${var.env}"
    master_password = "PASSWORD"
    backup_retention_period = 5
    vpc_security_group_ids = ["${aws_security_group.RDS_DB_SG.id}"]
}

resource "aws_rds_cluster_instance" "rds-instance" {
    count = 1
    cluster_identifier = "${aws_rds_cluster.rds-cluster.id}"
    instance_class = "db.r4.large"
    engine_version = "5.7.12"
    engine = "aurora-mysql"
    preferred_backup_window = "04:00-22:00"
}