Terraform创建Elastic Beanstalk环境时Subnets参数值类型错误求助

在VPC内使用Terraform创建Elastic Beanstalk环境时，因单个子网会触发"至少需要两个子网"的错误，所以配置了两个子网，但执行terraform apply时出现以下错误：

Inappropriate value for attribute "value": string required.

相关Terraform代码片段：

resource "aws_elastic_beanstalk_application" "elasticapp" {
  name = var.elasticapp
}

resource "aws_elastic_beanstalk_environment" "beanstalkappenv" {
  name                = var.beanstalkappenv
  application         = aws_elastic_beanstalk_application.elasticapp.name
  solution_stack_name = var.solution_stack_name
  tier                = var.tier

  setting {
    namespace = "aws:ec2:vpc"
    name      = "VPCId"
    value     = "${aws_vpc.prod-vpc.id}"
  }
  setting {
    namespace = "aws:ec2:vpc"
    name      = "Subnets"
    value     = ["${aws_subnet.prod-subnet-public-1.id}", "${aws_subnet.prod-subnet-public-2.id}"]
  }
}

错误根源在于Elastic Beanstalk的Subnets配置项要求逗号分隔的字符串，而非列表类型；同时Terraform的aws_elastic_beanstalk_environment资源中，setting块的value字段仅接受字符串值，不能直接传入列表。

修正代码的两种可行方式：

• 使用join函数将子网ID列表拼接为逗号分隔的字符串（推荐，灵活性更高）：

setting {
  namespace = "aws:ec2:vpc"
  name      = "Subnets"
  value     = join(",", [aws_subnet.prod-subnet-public-1.id, aws_subnet.prod-subnet-public-2.id])
}

• 手动拼接字符串（不推荐，扩展性差）：

setting {
  namespace = "aws:ec2:vpc"
  name      = "Subnets"
  value     = "${aws_subnet.prod-subnet-public-1.id},${aws_subnet.prod-subnet-public-2.id}"
}

另外需注意：你的代码中最后一个setting块缺少闭合的}，需补上避免语法错误。

内容的提问来源于stack exchange，提问作者Mike D Hovhannisyan