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A driver for DataStax Enterprise (DSE) and Apache Cassandra 1.2+ clusters that works exclusively with the Cassandra Query Language version 3 (CQL3) and Cassandra's binary protocol, supporting DSE-specific features such as geospatial types, DSE Graph and DSE authentication.

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/*
 * Copyright DataStax, Inc.
 *
 * This software can be used solely with DataStax Enterprise. Please consult the license at
 * http://www.datastax.com/terms/datastax-dse-driver-license-terms
 */
package com.datastax.driver.dse;

import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Host;
import com.datastax.driver.core.HostDistance;
import com.datastax.driver.core.Statement;
import com.datastax.driver.core.policies.ChainableLoadBalancingPolicy;
import com.datastax.driver.core.policies.LoadBalancingPolicy;
import com.google.common.collect.AbstractIterator;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Adds DSE-specific load balancing abilities on top of another policy.
 *
 * Currently, the only special processing performed by this policy is to route graph OLAP queries
 * to the graph analytics master.
 */
public class DseLoadBalancingPolicy implements ChainableLoadBalancingPolicy {

  private static final Logger logger = LoggerFactory.getLogger(DseLoadBalancingPolicy.class);

  private final LoadBalancingPolicy childPolicy;
  private Set upHosts = Collections.newSetFromMap(new ConcurrentHashMap());

  /**
   * Build a new instance.
   *
   * @param childPolicy the policy to add the DSE-specific behavior to.
   */
  public DseLoadBalancingPolicy(LoadBalancingPolicy childPolicy) {
    this.childPolicy = childPolicy;
  }

  @Override
  public void init(Cluster cluster, Collection hosts) {
    upHosts.addAll(hosts);
    childPolicy.init(cluster, hosts);
  }

  @Override
  public Iterator newQueryPlan(String loggedKeyspace, Statement statement) {
    final Host preferredHost =
        (statement instanceof HostTargetingStatement)
            ? ((HostTargetingStatement) statement).preferredHost
            : null;

    final Iterator childPlan = childPolicy.newQueryPlan(loggedKeyspace, statement);

    if (preferredHost == null) {
      return childPlan;
    } else if (!upHosts.contains(preferredHost)) {
      logger.debug(
          "Received statement targeted to {}, but it's not available at the moment", preferredHost);
      return childPlan;
    } else {
      logger.debug(
          "Received statement targeted to {}, moving it to the beginning of the query plan",
          preferredHost);
      return new AbstractIterator() {
        volatile boolean first = true;

        @Override
        protected Host computeNext() {
          if (first) {
            first = false;
            return preferredHost;
          } else if (childPlan.hasNext()) {
            Host next = childPlan.next();
            if (next.equals(preferredHost)) {
              // Already returned at the beginning, skip
              return childPlan.hasNext() ? childPlan.next() : endOfData();
            } else {
              return next;
            }
          } else {
            return endOfData();
          }
        }
      };
    }
  }

  @Override
  public HostDistance distance(Host host) {
    return childPolicy.distance(host);
  }

  @Override
  public void onAdd(Host host) {
    upHosts.add(host);
    childPolicy.onAdd(host);
  }

  @Override
  public void onUp(Host host) {
    upHosts.add(host);
    childPolicy.onUp(host);
  }

  @Override
  public void onDown(Host host) {
    upHosts.remove(host);
    childPolicy.onDown(host);
  }

  @Override
  public void onRemove(Host host) {
    upHosts.remove(host);
    childPolicy.onRemove(host);
  }

  @Override
  public LoadBalancingPolicy getChildPolicy() {
    return childPolicy;
  }

  @Override
  public void close() {
    childPolicy.close();
  }
}