content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package com.datastax.oss.driver.api.core.loadbalancing;
  
  import com.datastax.oss.driver.api.core.metadata.Node;
  import com.datastax.oss.driver.api.core.metadata.NodeState;
  import com.datastax.oss.driver.api.core.session.Request;
  import com.datastax.oss.driver.api.core.session.Session;
  import com.datastax.oss.driver.api.core.tracker.RequestTracker;
  import edu.umd.cs.findbugs.annotations.NonNull;
  import edu.umd.cs.findbugs.annotations.Nullable;
  import java.util.Collections;
  import java.util.Map;
  import java.util.Optional;
  import java.util.Queue;
  import java.util.UUID;
  
  /** Decides which Cassandra nodes to contact for each query. */
  public interface LoadBalancingPolicy extends AutoCloseable {
  
    /**
     * Returns an optional {@link RequestTracker} to be registered with the session. Registering a
     * request tracker allows load-balancing policies to track node latencies in order to pick the
     * fastest ones.
     *
     * <p>This method is invoked only once during session configuration, and before any other methods
     * in this interface. Note that at this point, the driver hasn't connected to any node yet.
     *
     * @since 4.13.0
     */
    @NonNull
    default Optional<RequestTracker> getRequestTracker() {
      return Optional.empty();
    }
  
    /**
     * Initializes this policy with the nodes discovered during driver initialization.
     *
     * <p>This method is guaranteed to be called exactly once per instance, and before any other
     * method in this interface except {@link #getRequestTracker()}. At this point, the driver has
     * successfully connected to one of the contact points, and performed a first refresh of topology
     * information (by default, the contents of {@code system.peers}), to discover other nodes in the
     * cluster.
     *
     * <p>This method must call {@link DistanceReporter#setDistance(Node, NodeDistance)
     * distanceReporter.setDistance} for each provided node (otherwise that node will stay at distance
     * {@link NodeDistance#IGNORED IGNORED}, and the driver won't open connections to it). Note that
     * the node's {@link Node#getState() state} can be either {@link NodeState#UP UP} (for the
     * successful contact point), {@link NodeState#DOWN DOWN} (for contact points that were tried
     * unsuccessfully), or {@link NodeState#UNKNOWN UNKNOWN} (for contact points that weren't tried,
     * or any other node discovered from the topology refresh). Node states may be updated
     * concurrently while this method executes, but if so this policy will get notified after this
     * method has returned, through other methods such as {@link #onUp(Node)} or {@link
     * #onDown(Node)}.
     *
     * @param nodes all the nodes that are known to exist in the cluster (regardless of their state)
     *     at the time of invocation.
     * @param distanceReporter an object that will be used by the policy to signal distance changes.
     *     Implementations will typically store this in a field, since new nodes may get {@link
     *     #onAdd(Node) added} later and will need to have their distance set (or the policy might
     *     change distances dynamically over time).
     */
    void init(@NonNull Map<UUID, Node> nodes, @NonNull DistanceReporter distanceReporter);
  
    /** Returns map containing details that impact C* node connectivity. */
    @NonNull
    default Map<String, ?> getStartupConfiguration() {
      return Collections.emptyMap();
    }
  
    /**
     * Returns the coordinators to use for a new query.
     *
     * <p>Each new query will call this method, and try the returned nodes sequentially.
     *
     * @param request the request that is being routed. Note that this can be null for some internal
     *     uses.
     * @param session the session that is executing the request. Note that this can be null for some
     *     internal uses.
     * @return the list of coordinators to try. <b>This must be a concurrent queue</b>; {@link
     *     java.util.concurrent.ConcurrentLinkedQueue} is a good choice.
     */
    @NonNull
    Queue<Node> newQueryPlan(@Nullable Request request, @Nullable Session session);
  
    /**
     * Called when a node is added to the cluster.
     *
     * <p>The new node will be at distance {@link NodeDistance#IGNORED IGNORED}, and have the state
     * {@link NodeState#UNKNOWN UNKNOWN}.
     *
     * <p>If this method assigns an active distance to the node, the driver will try to create a
     * connection pool to it (resulting in a state change to {@link #onUp(Node) UP} or {@link
     * #onDown(Node) DOWN} depending on the outcome).
     *
     * <p>If it leaves it at distance {@link NodeDistance#IGNORED IGNORED}, the driver won't attempt
     * any connection. The node state will remain unknown, but might be updated later if a topology
     * event is received from the cluster.
     *
     * @see #init(Map, DistanceReporter)
     */
    void onAdd(@NonNull Node node);
  
    /** Called when a node is determined to be up. */
    void onUp(@NonNull Node node);
  
    /** Called when a node is determined to be down. */
    void onDown(@NonNull Node node);
  
    /** Called when a node is removed from the cluster. */
    void onRemove(@NonNull Node node);
  
    /** Called when the cluster that this policy is associated with closes. */
    @Override
    void close();
  
    /** An object that the policy uses to signal decisions it makes about node distances. */
    interface DistanceReporter {
      void setDistance(@NonNull Node node, @NonNull NodeDistance distance);
    }
  }
  