The default implementation of LeastShardAllocationStrategy allocates new shards to the ShardRegion with least number of previously allocated shards. It picks shards for rebalancing handoff from the ShardRegion with most number of previously allocated shards. They will then be allocated to the ShardRegion with least number of previously allocated shards, i.e. new members in the cluster. There is a configurable threshold of how large the difference must be to begin the rebalancing. The number of ongoing rebalancing processes can be limited.

[SerializableAttribute]
public class LeastShardAllocationStrategy : IShardAllocationStrategy, 
	INoSerializationVerificationNeeded

[<SerializableAttribute>]
type LeastShardAllocationStrategy =  
    class
        interface IShardAllocationStrategy
        interface INoSerializationVerificationNeeded
    end