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On 04/28/2014 11:22 AM, Dan Smith wrote:
>>> 2. There's no way to add an existing server to this "group".
>>
>> In the original API there was a way to add existing servers to the
>> group.  This didn't make it into the code that was submitted.  It is
>> however supported by the instance group db API in nova.
>>
>>> 3. There's no way to remove members from the group
>>
>> In the original API there was a way to remove members from the group.
>> This didn't make it into the code that was submitted.
>
> Well, it didn't make it in because it was broken. If you add an instance
> to a group after it's running, a migration may need to take place in
> order to keep the semantics of the group. That means that for a while
> the policy will be being violated, and if we can't migrate the instance
> somewhere to satisfy the policy then we need to either drop it back out,
> or be in violation. Either some additional states (such as being queued
> for inclusion in a group, etc) may be required, or some additional
> footnotes on what it means to be in a group might have to be made.

I think your comment actually applies to adding existing instances to a 
group.  There's no good reason not to allow removing instances from a group.


As for the case of addition, we could start with something simple...if 
adding an instance to a group would violate the group scheduling policy, 
then raise an exception.

> It was for the above reasons, IIRC, that we decided to leave that bit
> out since the semantics and consequences clearly hadn't been fully
> thought-out. Obviously they can be addressed, but I fear the result will
> be ... ugly. I think there's a definite possibility that leaving out
> those dynamic functions will look more desirable than an actual
> implementation.

Your idea of "pending group membership" doesn't sound too ugly.

That said, I would expect "adding existing instances to a group" to be 
something that would be done under fairly well-controlled circumstances. 
  In that case I think it would be reasonable to push the work of 
managing any migrations onto whoever is trying to create a group from 
existing instances.

Chris