Amazon web services 跨4个AZ部署多个实例时，在匹配AZ中选择subnet_id

我有一个资源集团，在现有专有网络中创建4个子网：

# Create a subnet to launch instances into resource 

"aws_subnet" "mesos_subnet" { 
   vpc_id = "${var.dcos_vpc}" 
   count = 4 
   cidr_block = "${cidrsubnet(var.subnet_range, 4,   count.index)}"
   availability_zone = "${element(split(",", lookup(var.azs, var.region)), count.index)}"     
  map_public_ip_on_launch = true 
  tags { "Name" = "${var.prefix}-${element(split(",", lookup(var.azs,   var.region)), 
 count.index)}-sn" } 

}

我正在使用这样的资源组创建实例，该资源组将实例分布在该区域的可用AZs 4中：

resource "aws_instance" "agents" {
instance_type = "m4.2xlarge"
ami = "${lookup(var.amis, var.region)}"
count = "${var.agent_instance_count}"
key_name = "${var.key_name}"
vpc_security_group_ids = ["${aws_security_group.internal_sg.id}","${aws_security_group.admin_sg.id   }"]
   associate_public_ip_address = true
   iam_instance_profile =    "${aws_iam_instance_profile.agent_profile.name}"
    availability_zone = "${element(split(",", lookup(var.azs,     var.region)), count.index)}"
   subnet_id = "${element(aws_subnet.dcos_subnet.*.id, count.index)}"
    tags {
      Name = "${var.prefix}-mesos-agent-${count.index}"
      owner = "${var.owner}"
    expiration = "${var.expiration}"
  }
  }

但是，在用于创建实例的子网选择器中，我似乎无法使实例在具有匹配AZ的子网中创建，即， terraform尝试在子网中创建实例，但与正确的AZ不匹配。因此，我在应用时出现以下错误：

 * aws_instance.agents.3: Error launching source instance:    InvalidParameterValue: Value (eu-west-1a) for parameter availabilityZone   is invalid. Subnet 'subnet-xxxxxx' is in the availability zone eu-west-1b
         status code: 400, request id: 26665eee-071b-4ca1-a96b-  19b22c2fff94
 * aws_instance.agents.4: Error launching source instance:    InvalidParameterValue: Value (eu-west-1b) for parameter availabilityZone   is invalid. Subnet 'subnet-yyyyyyyy' is in the availability zone eu-west-  1a

我想我需要一个select语句，它可以以某种方式过滤并选择与AZ相对应的已创建子网的子网ID 这是在：

 "availability_zone = "${element(split(",", lookup(var.azs, var.region)), count.index)}"".

有人知道我怎么做吗？

如果您只分配子网id，实例将被放置到该子网的适当可用性区域中。无需为aws_实例资源同时分配子网_id和可用性_区域属性。这两个属性都是可选的。只使用一个子网就足够了