order

This paper focuses on the routing service of elastic search

RoutingService

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/cluster/routing/RoutingService.java

public class RoutingService extends AbstractLifecycleComponent {
    private static final Logger logger = LogManager.getLogger(RoutingService.class);

    private static final String CLUSTER_UPDATE_TASK_SOURCE = "cluster_reroute";

    private final ClusterService clusterService;
    private final AllocationService allocationService;

    private AtomicBoolean rerouting = new AtomicBoolean();

    @Inject
    public RoutingService(ClusterService clusterService, AllocationService allocationService) {
        this.clusterService = clusterService;
        this.allocationService = allocationService;
    }

    @Override
    protected void doStart() {
    }

    @Override
    protected void doStop() {
    }

    @Override
    protected void doClose() {
    }

    /**
     * Initiates a reroute.
     */
    public final void reroute(String reason) {
        try {
            if (lifecycle.stopped()) {
                return;
            }
            if (rerouting.compareAndSet(false, true) == false) {
                logger.trace("already has pending reroute, ignoring {}", reason);
                return;
            }
            logger.trace("rerouting {}", reason);
            clusterService.submitStateUpdateTask(CLUSTER_UPDATE_TASK_SOURCE + "(" + reason + ")",
                new ClusterStateUpdateTask(Priority.HIGH) {
                    @Override
                    public ClusterState execute(ClusterState currentState) {
                        rerouting.set(false);
                        return allocationService.reroute(currentState, reason);
                    }

                    @Override
                    public void onNoLongerMaster(String source) {
                        rerouting.set(false);
                        // no biggie
                    }

                    @Override
                    public void onFailure(String source, Exception e) {
                        rerouting.set(false);
                        ClusterState state = clusterService.state();
                        if (logger.isTraceEnabled()) {
                            logger.error(() -> new ParameterizedMessage("unexpected failure during [{}], current state:\n{}",
                                source, state), e);
                        } else {
                            logger.error(() -> new ParameterizedMessage("unexpected failure during [{}], current state version [{}]",
                                source, state.version()), e);
                        }
                    }
                });
        } catch (Exception e) {
            rerouting.set(false);
            ClusterState state = clusterService.state();
            logger.warn(() -> new ParameterizedMessage("failed to reroute routing table, current state:\n{}", state), e);
        }
    }
}

• The constructors of RoutingService require input of clusterService and allocationService. The reroute method is to submit ClusterStateUpdateTask to clusterService, and the execute method is to delegate to allocationService.reroute

AllocationService.reroute

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/cluster/routing/allocation/AllocationService.java

public class AllocationService {
	//......

    /**
     * Reroutes the routing table based on the live nodes.
     * <p>
     * If the same instance of ClusterState is returned, then no change has been made.
     */
    public ClusterState reroute(ClusterState clusterState, String reason) {
        return reroute(clusterState, reason, false);
    }

    /**
     * Reroutes the routing table based on the live nodes.
     * <p>
     * If the same instance of ClusterState is returned, then no change has been made.
     */
    protected ClusterState reroute(ClusterState clusterState, String reason, boolean debug) {
        ClusterState fixedClusterState = adaptAutoExpandReplicas(clusterState);

        RoutingNodes routingNodes = getMutableRoutingNodes(fixedClusterState);
        // shuffle the unassigned nodes, just so we won't have things like poison failed shards
        routingNodes.unassigned().shuffle();
        RoutingAllocation allocation = new RoutingAllocation(allocationDeciders, routingNodes, fixedClusterState,
            clusterInfoService.getClusterInfo(), currentNanoTime());
        allocation.debugDecision(debug);
        reroute(allocation);
        if (fixedClusterState == clusterState && allocation.routingNodesChanged() == false) {
            return clusterState;
        }
        return buildResultAndLogHealthChange(clusterState, allocation, reason);
    }

    private void reroute(RoutingAllocation allocation) {
        assert hasDeadNodes(allocation) == false : "dead nodes should be explicitly cleaned up. See disassociateDeadNodes";
        assert AutoExpandReplicas.getAutoExpandReplicaChanges(allocation.metaData(), allocation.nodes()).isEmpty() :
            "auto-expand replicas out of sync with number of nodes in the cluster";

        // now allocate all the unassigned to available nodes
        if (allocation.routingNodes().unassigned().size() > 0) {
            removeDelayMarkers(allocation);
            gatewayAllocator.allocateUnassigned(allocation);
        }

        shardsAllocator.allocate(allocation);
        assert RoutingNodes.assertShardStats(allocation.routingNodes());
    }

	//......
}

• The reroute method of AllocationService is to build RoutingAllocation, and then conduct gatewayAllocator.allocateUnassigned and shardsAllocator.allocate(allocation)

BalancedShardsAllocator.allocate

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/cluster/routing/allocation/allocator/BalancedShardsAllocator.java

public class BalancedShardsAllocator implements ShardsAllocator {
	//......

    public void allocate(RoutingAllocation allocation) {
        if (allocation.routingNodes().size() == 0) {
            /* with no nodes this is pointless */
            return;
        }
        final Balancer balancer = new Balancer(logger, allocation, weightFunction, threshold);
        balancer.allocateUnassigned();
        balancer.moveShards();
        balancer.balance();
    }

	//......
}

• For the allocate method of balancedshards allocator, create the Balancer, and then execute the allocateUnassigned(), moveShards(), and balance() methods of Balancer

Summary

• The constructors of RoutingService require input of clusterService and allocationService. The reroute method is to submit ClusterStateUpdateTask to clusterService, and the execute method is to delegate to allocationService.reroute
• The reroute method of AllocationService is to build RoutingAllocation, and then conduct gatewayAllocator.allocateUnassigned and shardsAllocator.allocate(allocation)
• For the allocate method of balancedshards allocator, create the Balancer, and then execute the allocateUnassigned(), moveShards(), and balance() methods of Balancer

doc

• RoutingService