FastestPingTimeReadPreference.java

package com.mongodb;

import com.mongodb.connection.ClusterDescription;
import com.mongodb.connection.ServerDescription;
import org.bson.BsonDocument;
import org.bson.BsonString;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;

import static java.lang.Float.compare;
import static java.util.Collections.emptyList;
import static java.util.Collections.singletonList;

public class FastestPingTimeReadPreference extends ReadPreference {

  private static final Logger LOGGER = LoggerFactory.getLogger(FastestPingTimeReadPreference.class);

  @Override
  public boolean isSlaveOk() {
    return true;
  }

  @Override
  public String getName() {
    return "nearestNode";
  }

  @Override
  public BsonDocument toDocument() {
    return new BsonDocument("mode", new BsonString(getName()));
  }

  @Override
  public List<ServerDescription> choose(ClusterDescription clusterDescription) {
    List<ServerDescription> nodes = clusterDescription.getAnyPrimaryOrSecondary();

    ServerDescription nearestNode = selectNearestQueryableNode(nodes);

    if (nearestNode == null) {
      LOGGER.debug("Could not find any mongodb server.");
      return emptyList();
    }

    if (LOGGER.isTraceEnabled()) {
      StringBuilder buffer = new StringBuilder();
      for (ServerDescription node : nodes) {
        if (!node.equals(nearestNode)) {
          buffer.append("[");
          buffer.append(node.getAddress().getHost());
          buffer.append("/");
          buffer.append(node.getRoundTripTimeNanos());
          buffer.append("] ");
        }
      }
      String choosenNode = nearestNode.getAddress().getHost() + "/" + nearestNode.getRoundTripTimeNanos();
      LOGGER.trace("take {} as mongodb host. other {}", choosenNode, buffer.toString());
    } else {
      if (LOGGER.isDebugEnabled()) {
        LOGGER.debug("take {} as mongodb host", nearestNode.getAddress().getHost());
      }
    }
    return singletonList(nearestNode);
  }


  private static ServerDescription selectNearestQueryableNode(final List<ServerDescription> nodeSet) {
    ServerDescription nearest = null;
    for (ServerDescription node : nodeSet) {
      if (node.isOk()) {
        nearest = calculateNearest(nearest, node);
      }
    }
    return nearest;
  }

  private static ServerDescription calculateNearest(final ServerDescription nodeA, final ServerDescription nodeB) {
    if (nodeA == null) {
      return nodeB;
    }
    if (compare(nodeA.getRoundTripTimeNanos(), nodeB.getRoundTripTimeNanos()) > 0) {
      return nodeB;
    }
    return nodeA;
  }
}