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 edu.umd.cs.findbugs.annotations.NonNull;
  import edu.umd.cs.findbugs.annotations.Nullable;
  
  /**
   * A pluggable {@link NodeDistance} evaluator.
   *
   * <p>Node distance evaluators are recognized by all the driver built-in load balancing policies.
   * They can be specified {@linkplain
   * com.datastax.oss.driver.api.core.session.SessionBuilder#withNodeDistanceEvaluator(String,
   * NodeDistanceEvaluator) programmatically} or through the configuration (with the {@code
   * load-balancing-policy.evaluator.class} option).
   *
   * @see com.datastax.oss.driver.api.core.session.SessionBuilder#withNodeDistanceEvaluator(String,
   *     NodeDistanceEvaluator)
   */
  @FunctionalInterface
  public interface NodeDistanceEvaluator {
  
    /**
     * Evaluates the distance to apply to the given node.
     *
     * <p>This method will be invoked each time the {@link LoadBalancingPolicy} processes a topology
     * or state change, and will be passed the node being inspected, and the local datacenter name (or
     * null if none is defined). If it returns a non-null {@link NodeDistance}, the policy will
     * suggest that distance for the node; if it returns null, the policy will assign a default
     * distance instead, based on its internal algorithm for computing node distances.
     *
     * @param node The node to assign a new distance to.
     * @param localDc The local datacenter name, if defined, or null otherwise.
     * @return The {@link NodeDistance} to assign to the node, or null to let the policy decide.
     */
    @Nullable
    NodeDistance evaluateDistance(@NonNull Node node, @Nullable String localDc);
  }
  