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>> 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.

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.

--Dan