Type Members

1.  abstract class AbstractShardAllocationStrategy extends ShardAllocationStrategy

   Java API: Java implementations of custom shard allocation and rebalancing logic used by the ShardCoordinator should extend this abstract class and implement the two methods.

2.  class LeastShardAllocationStrategy extends ShardAllocationStrategy with Serializable

   The default implementation of ShardCoordinator.LeastShardAllocationStrategy allocates new shards to the ShardRegion with least number of previously allocated shards.

   The default implementation of ShardCoordinator.LeastShardAllocationStrategy allocates new shards to the ShardRegion with least number of previously allocated shards.

   When a node is removed from the cluster the shards on that node will be started on the remaining nodes, evenly spread on the remaining nodes (by picking regions with least shards).

   When a node is added to the cluster the shards on the existing nodes will be rebalanced to the new node. It picks shards for rebalancing 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 difference between number of shards in the region with most shards and the region with least shards must be greater than the rebalanceThreshold for the rebalance to occur.

   A rebalanceThreshold of 1 gives the best distribution and therefore typically the best choice. A higher threshold means that more shards can be rebalanced at the same time instead of one-by-one. That has the advantage that the rebalance process can be quicker but has the drawback that the the number of shards (and therefore load) between different nodes may be significantly different. Given the recommendation of using 10x shards than number of nodes and rebalanceThreshold=10 can result in one node hosting ~2 times the number of shards of other nodes. Example: 1000 shards on 100 nodes means 10 shards per node. One node may have 19 shards and others 10 without a rebalance occurring.

   The number of ongoing rebalancing processes can be limited by maxSimultaneousRebalance.

   Annotations

   @SerialVersionUID()

3.  trait ShardAllocationStrategy extends NoSerializationVerificationNeeded

   Interface of the pluggable shard allocation and rebalancing logic used by the ShardCoordinator.

   Interface of the pluggable shard allocation and rebalancing logic used by the ShardCoordinator.

   Java implementations should extend AbstractShardAllocationStrategy.