Reid, I don't think you can ensure that local node is primary for all updates. In theory you can implement affinity function having only 1 primary node for all partitions, but once you initiate replicated cache update from another node then primary node becomes remote.
As far as LOCAL_SYNC - I am not sure that this option will be ever introduced. In some scenarios it will be counter-intuitive. Can you please provide more details on your usecase? Frequent updates to replicated cache maybe very harmful for application from ops latency standpoint. --Yakov
