mount point of Cinder volume and VM root disk are swapped

I created a VM with 3 volumes using volumeattach. Firstly, the mount point property doesn't work. so I added "depends on " to make sure the mount point correct. But sometimes I found the first cinder volume and the VM root disk's mount point may swap. following is my heat template. It's supposed the data_vol_0 mount point is vdb, the VM root disk mount point is vda, but sometimes, the data_vol_0 mount pint is vda, the root disk mount point is vdb.

  data_vol_0:
    type: OS::Cinder::Volume
    properties:
      size: 10
      name: { list_join: [ "-", [ { get_param: prefix}, oam, {get_param: [ resources, {get_param: node_index}, _mappedIndex ]}, data_0 ] ] }
  data_vol_0_attachment:
    type: OS::Cinder::VolumeAttachment
    properties:
      instance_uuid: { get_resource: server }
      volume_id: { get_resource: data_vol_0 }
      mountpoint: /dev/vdb

  data_vol_1:
    type: OS::Cinder::Volume
    properties:
      size: 11
      name: { list_join: [ "-", [ { get_param: prefix}, oam, {get_param: [ resources, {get_param: node_index}, _mappedIndex ]}, data_1 ] ] }
  data_vol_1_attachment:
    type: OS::Cinder::VolumeAttachment
    depends_on: [ data_vol_0_attachment ]
    properties:
      instance_uuid: { get_resource: server }
      volume_id: { get_resource: data_vol_1 }
      mountpoint: /dev/vdc

  service_vol:
    type: OS::Cinder::Volume
    properties:
      size: 1
      name: { list_join: [ "-", [ { get_param: prefix}, oam, {get_param: [ resources, {get_param: node_index}, _mappedIndex ]}, data_2 ] ] }
  service_vol_attachment:
    type: OS::Cinder::VolumeAttachment
    depends_on: [ data_vol_1_attachment ]
    properties:
      instance_uuid: { get_resource: server }
      volume_id: { get_resource: service_vol }
      mountpoint: /dev/vdd

  server:
    type: OS::Nova::Server
    depends_on: [
                iport_0_ipv4,
                eport_1_dual,
                ]
    properties:
      flavor: { get_param: [ resources, {get_param: node_index}, server, flavorId ] }
      config_drive: true
      name: { list_join: [ "-", [ {get_param: prefix}, oam, {get_param: [ resources, {get_param: node_index}, _mappedIndex ]} ] ] }
      image: { get_param: [ resources, {get_param: node_index}, server, imageId ] }
      key_name: { get_param: key_name }
      availability_zone: { get_param: [ resources, {get_param: node_index}, server, azId ] }
      networks: [
            { "port": { get_resource: iport_0_ipv4 } },
            { "port": { get_resource: eport_1_dual } },
                ]