Open Stack

On 09/10/2014 04:16 PM, Russell Bryant wrote:
>
>
>> On Sep 10, 2014, at 2:03 PM, Joe Cropper <cropper.joe at gmail.com>
>> wrote:
>>
>> I would like to craft up a blueprint proposal for Kilo to add two
>> simple extensions to the existing server group APIs that I believe
>> will make them infinitely more usable in any ‘real world’ scenario.
>> I’ll put more details in the proposal, but in a nutshell:
>>
>> 1. Adding a VM to a server group Only allow it to succeed if its
>> policy wouldn’t be violated by the addition of the VM
>>
>
> I'm not sure that determining this at the time of the API request is
> possible due to the parallel and async nature of the system. I'd love
> to hear ideas on how you think this might be done, but I'm really not
> optimistic and would rather just not go down this road.

I can see a possible race against another instance booting into the 
group, or another already-running instance being added to the group.  I 
think the solution is to do the update as an atomic database transaction.

It seems like it should be possible to create a database operation that 
does the following in a single transaction:
--look up the hosts for the instances in the group
--check that the scheduler policy would be satisfied (at least for the 
basic affinity/anti-affinity policies)
--add the instance to the group

Chris