[Engine-devel] Floating Disks implementation in REST-API

[Ori Liel]

>
>
>On 04/09/2012 05:14 PM, Ori Liel wrote:
>> The "Floating Disks" feature makes disks into stand-alone entities: a given disk may be attached to a VM (as all disks are today), or it may be not attached to any VM, which makes it a floating disk (http://www.ovirt.org/wiki/Features/FloatingDisk)
>>
>> To implement attach/detach of disk to/from VM in REST-API, we intend to introduce two new actions:
>>
>>    POST .../api/vms/{vm:id}/disks/{disk:id}/attach
>>    POST .../api/vms/{vm:id}/disks/{disk:id}/detach
>>
>> Since we try not to add new actions unless we have to, I want to explain why I believe that these actions are necessary.
>> The other implementation would use existing add/remove flows:
>>
>>    POST   .../api/vms/{vm:id}/disks               - if the disk was passed with an ID, attach it to this VM. If no id - create a new disk.
>>    DELETE .../api/vms/{vm:id}/disks/{disk:id}     - *ambiguity problem, need to add a flag*
>>
>> We can't break existing API, so regular DELETE must remove the disk, as it does today. To detach a disk using DELETE we'd have to add a flag to the DETELE command. This is quite risky, because if the user forgets to pass this flag, the disk which he wanted to detach will actually be deleted.
>>
>> Theoretically, if we could break the API, the following modelling would resolve the ambiguity and perhaps be ideal:
>> - POST/DELETE disk in root context means create or delete it.
>> - POST/DELETE disk in VM context means attach or detach it.
>> But we don't have the privilege of breaking the API.
>>
>> Considering all of the above - and the fact that attach/detach nics to/from host is also implemented using actions - I believe that the new actions are justifiable.
>>
>> Any comments?
>
>I assume that a floating disk can be attached to 0 VMs as well, right?
>So i would assume we get a top-level /disks collection correct?? And I
>assume that collection would only list floating disks?

Indeed, there will be a root collection. but I do not see a reason why this collection should only show floating disks. I tend to think it should show all disks in the system. Keep in mind that we're going towards Shared disks, meaning the same disk may be used by two VMs, so it makes sense to show such a disk in the root collection, along with the information of which VMs it's attached to. 

>
>In my view, backwards compatible DELETE semantics for a floating disk
>aren't that bad:
>
>  DELETE /vms/{vm:id}/disks/{disk:id}
>
>=> Accept a <detach>true|false</detach> argument.
>=> Defaults to "false" for compatibility.
>
>This means that DELETE will by default really DELETE any non-floating
>disk. That is compatibility with today. For safety, implement this:
>
>=> When deleting a disk that is floating, fail unless <force> is also
>set to "true". And always fail if one of the other VMs is running
>(obviously).
>

I Agree with Eoghan that the drawbacks outweigh the benefits in this case, 
for the reasons Eoghan said. 

>To attach a disk and make it float, use POST and you and Eoghan described.
>
>To detach a disk, use DELETE with <detach>true</detach>.
>
>To delete a non-attached, floating disks, use:
>
>  DELETE /disks/{disk:id}
>
>To delete an attached, floating disk, use:
>
>  DELETE /vm/{vm:id}/disks/{disk:id} with <force>true</force>; OR
>  DELETE /disks/{disk:id}
>
>To create a non-attached, floating, disk, use:
>
>  POST /disks

Worth mentioning that creation of disk that is attached to a VM is still possible by POST to /vms/{vm:id}/disks/ without supplying disk-Id. 

>
>Regards,
>Geert

Ori