Module org.elasticsearch.server
Package org.elasticsearch.cluster.routing.allocation.allocator

Interface ShardsAllocator

All Known Implementing Classes:
BalancedShardsAllocator, DesiredBalanceShardsAllocator
public interface ShardsAllocator

A ShardsAllocator is the main entry point for shard allocation on nodes in the cluster. The allocator makes basic decision where a shard instance will be allocated, if already allocated instances need to relocate to other nodes due to node failures or due to rebalancing decisions.

  • Method Details

    • allocate

      void allocate(RoutingAllocation allocation)
      Allocates shards to nodes in the cluster. An implementation of this method should: - assign unassigned shards - relocate shards that cannot stay on a node anymore - relocate shards to find a good shard balance in the cluster
      Parameters:
      allocation - current node allocation

    • allocate

      default void allocate(RoutingAllocation allocation, ActionListener<Void> listener)
      Allocates shards to nodes in the cluster. An implementation of this method should: - assign unassigned shards - relocate shards that cannot stay on a node anymore - relocate shards to find a good shard balance in the cluster
      Parameters:
      allocation - current node allocation
      listener - listener to be executed once async allocation is completed

    • execute

      default RoutingExplanations execute(RoutingAllocation allocation, AllocationCommands commands, boolean explain, boolean retryFailed)
      Execute allocation commands

    • explainShardAllocation

      ShardAllocationDecision explainShardAllocation(ShardRouting shard, RoutingAllocation allocation)
      Returns the decision for where a shard should reside in the cluster. If the shard is unassigned, then the AllocateUnassignedDecision will be non-null. If the shard is not in the unassigned state, then the MoveDecision will be non-null. If an implementation of this interface does not support explaining decisions for a single shard through the cluster explain API, then this method should throw a UnsupportedOperationException.

    • explainShardAllocationFunction

      default Function<ShardRouting,ShardAllocationDecision> explainShardAllocationFunction(RoutingAllocation allocation)
      Similar to explainShardAllocation, but returns a Function that is more efficient for explaining many shards in a bulk circumstance. Internally, an allocator builds up internal data structures for simulating its balancing algorithm. In some implementations, such as the BalancedShardsAllocator (also used within the DesiredBalanceAllocator), this internal context is computationally expensive. When run over many shards, it can be Order(shards * nodes). Instead of taking a shard and explaining it, explainShardAllocationFunction returns a Function that can be called repeatedly to explain multiple shards while reusing the context in the Function's closure. This reduces the computational cost of explain.