HOT template for cinder volume with or without volume_type - openstack

Question

I am trying to write a HOT template for Openstack volume, and need to have the volume_type as a parameter. I also need to support a case when the parameter is not given, and default to the Cinder default volume type.
First attempt was to pass null to the volume_type , hoping it would give the default volume type. However no matter what I pass (null, ~, default, "" ) , seems there is no way to get the default volume type.
type: OS::Cinder::Volume
properties:
name: test
size: 1
volume_type: { if: ["voltype_given" , {get_param:[typename]} , null] }
Is there any way to get the default volume type , when you have the "volume_type" property defined?
Alternatively, is there any way to have the "volume_type" property itself behind a conditional? I tried several ways, but no luck. Something like:
type: OS::Cinder::Volume
properties:
if: ["voltype_given" , [ volume_type: {get_param:[typename]} ] , ""]
name: test
size: 1
ERROR: TypeError: : resources.kk-test-vol: : 'If' object is not iterable

Answer 1

Could you do something like this?
---
parameters:
typename:
type: string
conditions:
use_default_type: {equals: [{get_param: typename}, '']}
resources:
MyVolumeWithDefault:
condition: use_default_type
type: OS::Cinder::Volume
properties:
name: test
size: 1
MyVolumeWithExplicit:
condition: {not: use_default_type}
type: OS::Cinder::Volume
properties:
name: test
size: 1
volume_type: {get_param: typename}
# e.g. if you need to refer to the volume from another resource
MyVolumeAttachment:
type: OS::Cinder::VolumeAttachment
properties:
instance_uid: some-instance-uuid
volume_id:
if:
- use_default_type
- get_resource: MyVolumeWithDefault
- get_resource: MyVolumeWithExplicit