CLUSTER SET-CONFIG-EPOCH implemented.
Initially Redis Cluster accepted that after cluster creation all the nodes were at configEpoch 0, evolving from zero as failovers happen. However later the semantic was made more strict in order to make sure a cluster has always all the master nodes with a different configEpoch, which is more robust in some corner case (especially resulting from errors by the system administrator). To assign different configEpochs to different nodes at startup was a task performed naturally by the config conflicts resolution algorithm (see the Cluster specification). However this works well only for small clusters or when there are actually just a few collisions, since it is designed for exceptional cases. When a large cluster is created hundred of nodes can be at epoch 0, so the conflict resolution code is slow to provide an unique config to each node. For this reason this new command was introduced. It can be called only when a node is totally fresh: no other nodes known, and configEpoch set to zero, so it is safe even against misuses. redis-trib will use the new command in order to start the cluster already setting an incremental unique config to every node.
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