http://docs.basho.com/riak/latest/tutorials/fast-track/What-is-Riak/
Riak Automatically Re-Distributes Data When Capacity is Added
When you add machines, data is rebalanced automatically with no downtime. New machines take responsibility for their share of data by assuming ownership of some of the partitions; existing cluster members hand off the relevant partitions and the associated data. The new node continues claiming partitions until data ownership is equal, updating a picture of which nodes own what as it goes. This picture of cluster state is shared to every node using a gossip protocol and serves as a guide to route requests. This is what makes it possible for any node in the cluster to receive requests - developers don't need to deal with the underlying complexity of what data is where.
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