org.apache.druid.server.coordinator

Interface BalancerStrategy

All Known Implementing Classes:

CachingCostBalancerStrategy, CostBalancerStrategy, DiskNormalizedCostBalancerStrategy, RandomBalancerStrategy

public interface BalancerStrategy

This interface describes the coordinator balancing strategy, which is responsible for making decisions on where to place DataSegments on historical servers (described by ServerHolder). The balancing strategy is used by LoadRule to assign and drop segments, and by BalanceSegments to migrate segments between historicals.

Method Summary

Modifier and Type	Method and Description
void	emitStats(String tier, CoordinatorStats stats, List<ServerHolder> serverHolderList) Add balancing strategy stats during the 'balanceTier' operation of BalanceSegments to be included
ServerHolder	findNewSegmentHomeBalancer(DataSegment proposalSegment, List<ServerHolder> serverHolders) Find the best server to move a DataSegment to according the balancing strategy.
ServerHolder	findNewSegmentHomeReplicator(DataSegment proposalSegment, List<ServerHolder> serverHolders) Find the best server on which to place a DataSegment replica according to the balancing strategy
default Iterator<BalancerSegmentHolder>	pickSegmentsToMove(List<ServerHolder> serverHolders, Set<String> broadcastDatasources, int reservoirSize, double percentOfSegmentsToConsider) Pick the best segments to move from one of the supplied set of servers according to the balancing strategy.
default Iterator<ServerHolder>	pickServersToDrop(DataSegment toDropSegment, NavigableSet<ServerHolder> serverHolders) Returns an iterator for a set of servers to drop from, ordered by preference of which server to drop from first for a given drop strategy.

Method Detail

findNewSegmentHomeBalancer

@Nullable
ServerHolder findNewSegmentHomeBalancer(DataSegment proposalSegment,
                                                  List<ServerHolder> serverHolders)

Find the best server to move a DataSegment to according the balancing strategy.

Parameters:

proposalSegment - segment to move

serverHolders - servers to consider as move destinations

Returns:

The server to move to, or null if no move should be made or no server is suitable

findNewSegmentHomeReplicator

@Nullable
ServerHolder findNewSegmentHomeReplicator(DataSegment proposalSegment,
                                                    List<ServerHolder> serverHolders)

Find the best server on which to place a DataSegment replica according to the balancing strategy

Parameters:

proposalSegment - segment to replicate

serverHolders - servers to consider as replica holders

Returns:

The server to replicate to, or null if no suitable server is found

pickSegmentsToMove

default Iterator<BalancerSegmentHolder> pickSegmentsToMove(List<ServerHolder> serverHolders,
                                                           Set<String> broadcastDatasources,
                                                           int reservoirSize,
                                                           double percentOfSegmentsToConsider)

Pick the best segments to move from one of the supplied set of servers according to the balancing strategy.

Parameters:

serverHolders - set of historicals to consider for moving segments

broadcastDatasources - Datasources that contain segments which were loaded via broadcast rules. Balancing strategies should avoid rebalancing segments for such datasources, since they should be loaded on all servers anyway. NOTE: this should really be handled on a per-segment basis, to properly support the interval or period-based broadcast rules. For simplicity of the initial implementation, only forever broadcast rules are supported.

reservoirSize - the reservoir size maintained by the Reservoir Sampling algorithm.

percentOfSegmentsToConsider - The percentage of the total number of segments that we will consider when choosing which segment to move. CoordinatorDynamicConfig defines a config percentOfSegmentsToConsiderPerMove that will be used as an argument for implementations of this method.

Returns:

Iterator for set of BalancerSegmentHolder containing segment to move and server they currently reside on, or empty if there are no segments to pick from (i. e. all provided serverHolders are empty).

pickServersToDrop

default Iterator<ServerHolder> pickServersToDrop(DataSegment toDropSegment,
                                                 NavigableSet<ServerHolder> serverHolders)

Returns an iterator for a set of servers to drop from, ordered by preference of which server to drop from first for a given drop strategy. One or more segments may be dropped, depending on how much the segment is over-replicated.

Parameters:

toDropSegment - segment to drop from one or more servers

serverHolders - set of historicals to consider dropping from

Returns:

Iterator for set of historicals, ordered by drop preference

emitStats

void emitStats(String tier,
               CoordinatorStats stats,
               List<ServerHolder> serverHolderList)

Add balancing strategy stats during the 'balanceTier' operation of BalanceSegments to be included

Parameters:

tier - historical tier being balanced

stats - stats object to add balancing strategy stats to

serverHolderList - servers in tier being balanced