- All Implemented Interfaces:
- Enclosing class:
public static class ShardCoordinator.LeastShardAllocationStrategy extends java.lang.Object implements ShardCoordinator.ShardAllocationStrategy, java.io.SerializableThe default implementation of
ShardCoordinator.LeastShardAllocationStrategyallocates new shards to the
ShardRegionwith 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
ShardRegionwith most number of previously allocated shards. They will then be allocated to the
ShardRegionwith 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
rebalanceThresholdfor the rebalance to occur.
rebalanceThresholdof 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=10can 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
- See Also:
- Serialized Form
Constructors Constructor Description
LeastShardAllocationStrategy(int rebalanceThreshold, int maxSimultaneousRebalance)
All Methods Instance Methods Concrete Methods Modifier and Type Method Description
allocateShard(ActorRef requester, java.lang.String shardId, scala.collection.immutable.Map<ActorRef,scala.collection.immutable.IndexedSeq<java.lang.String>> currentShardAllocations)
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.
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.ShardAllocationStrategyInvoked periodically to decide which shards to rebalance to another location.
- Specified by:
currentShardAllocations- all actor refs to
ShardRegionand 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
Futureof the shards to be migrated, may be empty to skip rebalance in this round