Class LeastShardAllocationStrategy

  • All Implemented Interfaces:
    NoSerializationVerificationNeeded, ShardCoordinator.ActorSystemDependentAllocationStrategy, ShardCoordinator.ShardAllocationStrategy

    public class LeastShardAllocationStrategy
    extends AbstractLeastShardAllocationStrategy
    INTERNAL API: Use ShardCoordinator.ShardAllocationStrategy.leastShardAllocationStrategy factory method.

    ShardAllocationStrategy that allocates new shards to the ShardRegion (node) with least number of previously allocated shards.

    When a node is added to the cluster the shards on the existing nodes will be rebalanced to the new node. The LeastShardAllocationStrategy picks shards for rebalancing from the ShardRegions 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. The amount of shards to rebalance in each round can be limited to make it progress slower since rebalancing too many shards at the same time could result in additional load on the system. For example, causing many Event Sourced entites to be started at the same time.

    It will not rebalance when there is already an ongoing rebalance in progress.

    param: absoluteLimit the maximum number of shards that will be rebalanced in one rebalance round param: relativeLimit fraction (< 1.0) of total number of (known) shards that will be rebalanced in one rebalance round

    • Constructor Detail

      • LeastShardAllocationStrategy

        public LeastShardAllocationStrategy​(int absoluteLimit,
                                            double relativeLimit)
    • Method Detail

      • rebalance

        public scala.concurrent.Future<scala.collection.immutable.Set<java.lang.String>> rebalance​(scala.collection.immutable.Map<ActorRef,​scala.collection.immutable.IndexedSeq<java.lang.String>> currentShardAllocations,
                                                                                                   scala.collection.immutable.Set<java.lang.String> rebalanceInProgress)
        Description copied from interface: ShardCoordinator.ShardAllocationStrategy
        Invoked periodically to decide which shards to rebalance to another location.

        currentShardAllocations - all actor refs to ShardRegion and their current allocated shards, in the order they were allocated
        rebalanceInProgress - set of shards that are currently being rebalanced, i.e. you should not include these in the returned set
        a Future of the shards to be migrated, may be empty to skip rebalance in this round
      • toString

        public java.lang.String toString()
        toString in class java.lang.Object