Class ShardCoordinator.LeastShardAllocationStrategy

  • All Implemented Interfaces:
    NoSerializationVerificationNeeded, ShardCoordinator.ShardAllocationStrategy, java.io.Serializable
    Enclosing class:
    ShardCoordinator

    public static class ShardCoordinator.LeastShardAllocationStrategy
    extends java.lang.Object
    implements ShardCoordinator.ShardAllocationStrategy, java.io.Serializable
    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.

    See Also:
    Serialized Form
    • Constructor Summary

      Constructors 
      Constructor Description
      LeastShardAllocationStrategy​(int rebalanceThreshold, int maxSimultaneousRebalance)  
    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      scala.concurrent.Future<ActorRef> allocateShard​(ActorRef requester, java.lang.String shardId, scala.collection.immutable.Map<ActorRef,​scala.collection.immutable.IndexedSeq<java.lang.String>> currentShardAllocations)  
      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)
      Invoked periodically to decide which shards to rebalance to another location.
      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
    • Constructor Detail

      • LeastShardAllocationStrategy

        public LeastShardAllocationStrategy​(int rebalanceThreshold,
                                            int maxSimultaneousRebalance)
    • 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.
        Specified by:
        rebalance in interface ShardCoordinator.ShardAllocationStrategy
        Parameters:
        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
        Returns:
        a Future of the shards to be migrated, may be empty to skip rebalance in this round