Comment 6 for bug 1296913

Actually, let me try to summarize the change better ...

In Havana:

If you configured the filter scheduler to include the GroupAntiAffintyFilter, you could boot with the group=foo scheduler hint. The group would get stored in the instance's system_metadata. Any other instances with that same group in their system metadata would be put into a list to be used by the filter for enforcing affinity.

In Icehouse:

You can enable both GroupAffinityFilter and GroupAntiAffinityFilter for the filter scheduler at the same time. You must define an instance group named 'foo' with a policy of 'anti-affinity'. Then, you can boot an instance with a scheduler hint of group=foo. That instance will get added to the group and the anti-affinity filter will automatically get used since it's in a group with that policy set.

This is certainly a change in behavior. The problem really is that we released incomplete group support without the API in Havana. Technically you could have used it, and the changes we've made break you if you used it the way it was.