Mechanism 1: Clone Groups

No motivation in the paper where these mechanisms specifically came. We are seeing the "results" of the work, but not the work in getting here.

Clone is a virtual node. A clone group is a virtual cluster.

Clones and clone groups are mapped to real nodes.

Clone group mechanisms: membership, broadcast and barrier synchronization (to ensure all nodes are at the same stage).

Replica group is a subset of a partition.