ConsistentHashResolver.java

/*
 * Copyright 2014-2016 CyberVision, Inc.
 *
 * Licensed 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 org.kaaproject.kaa.server.hash;

import org.kaaproject.kaa.server.common.zk.gen.OperationsNodeInfo;
import org.kaaproject.kaa.server.resolve.OperationsServerResolver;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Comparator;
import java.util.List;
import java.util.SortedMap;
import java.util.concurrent.ConcurrentSkipListMap;

/**
 * Implementation of {@link OperationsServerResolver} based on consistent hash
 * function and MD5 digest.
 *
 * @author Andrew Shvayka
 */
public class ConsistentHashResolver implements OperationsServerResolver {

  private static final Logger LOG = LoggerFactory.getLogger(ConsistentHashResolver.class);

  private static final int SIZE_OF_INT = 4;
  private static final String MD5 = "MD5";
  private static final Charset UTF8 = Charset.forName("UTF-8");
  private static final ThreadLocal<MessageDigest> md5 = new ThreadLocal<MessageDigest>() { //NOSONAR
    @Override
    protected MessageDigest initialValue() {
      try {
        return MessageDigest.getInstance(MD5);
      } catch (NoSuchAlgorithmException exception) {
        throw new RuntimeException(exception);
      }
    }
  };
  private final int replicas;
  private final SortedMap<byte[], OperationsNodeInfo> circle;

  /**
   * Create new instance of <code>ConsistentHashResolver</code>.
   *
   * @param nodes is list of nodes
   * @param replicas is replicas
   */
  public ConsistentHashResolver(List<OperationsNodeInfo> nodes, int replicas) {
    this.replicas = replicas;
    this.circle = new ConcurrentSkipListMap<byte[], OperationsNodeInfo>(new ByteArrayComparator());
    for (OperationsNodeInfo node : nodes) {
      onNodeAdded(node);
    }
  }

  @Override
  public OperationsNodeInfo getNode(String user) {
    if (user == null) {
      throw new RuntimeException("user id is null");
    }
    return getNearest(hash(user));
  }

  @Override
  public void onNodeAdded(OperationsNodeInfo node) {
    for (int i = 0; i < replicas; i++) {
      LOG.trace("Adding node {} replica {} to the circle", node.getConnectionInfo(), i);
      circle.put(hash(node, i), node);
    }
  }

  @Override
  public void onNodeRemoved(OperationsNodeInfo node) {
    for (int i = 0; i < replicas; i++) {
      LOG.trace("Removing node {} replica {} from the circle", node.getConnectionInfo(), i);
      circle.remove(hash(node, i));
    }
  }

  @Override
  public void onNodeUpdated(OperationsNodeInfo node) {
    onNodeRemoved(node);
    onNodeAdded(node);
  }

  private OperationsNodeInfo getNearest(byte[] hash) {
    if (circle.isEmpty()) {
      return null;
    }
    if (circle.size() == 1) {
      return circle.get(circle.firstKey());
    }
    SortedMap<byte[], OperationsNodeInfo> tailMap = circle.tailMap(hash);
    hash = tailMap.isEmpty() ? circle.firstKey() : tailMap.firstKey();
    return circle.get(hash);
  }

  private byte[] hash(OperationsNodeInfo node, int replica) {
    byte[] key = node.getConnectionInfo().getPublicKey().array();
    ByteBuffer data = ByteBuffer.wrap(new byte[key.length + SIZE_OF_INT]);
    data.put(key);
    data.putInt(replica);
    return md5.get().digest(data.array());
  }

  private byte[] hash(String data) {
    return md5.get().digest(data.getBytes(UTF8));
  }

  private final class ByteArrayComparator implements Comparator<byte[]> {
    @Override
    public int compare(byte[] a1, byte[] a2) {
      if (a1 == a2) {
        return 0;
      } else {
        if (a1 == null) {
          return -1;
        }
        if (a2 == null) {
          return 1;
        }
        if (a1.length == a2.length) {
          for (int i = 0; i < a1.length; i++) {
            if (a1[i] != a2[i]) {
              return a1[i] - a2[i];
            }
          }
          return 0;
        } else {
          return a1.length - a2.length;
        }
      }
    }
  }
}