Sync-groups and preemption

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Hi,

I've been working on issue #285, for which I now have a proposal.
During the development I noticed a number of issues with sync-groups,
particularly when "preemption" is permitted on instance members. Its
relatively trivial to setup a scenario where conflicting priorities on
instances on each peer cause state instability and oscillation of
sync-group state between peers.

I'm currently investigating a mechanism which would disable preemption
whenever a instance is added to a sync-group. Alternatively its
possible that we could limit preemption within a sync-group to one
control instance which could be identified in configuration.

Does anyone have any opinions on this subject?

Thanks,
Colin.
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