xmemcached中的一致性hash实现

1.内部使用TreeMap<Long, List<Session>>实现session存储和定位，其中key为hash值，value为这个hash值对应的session存储集合。其中并不是有多少session就有几个键值，而是会有NUM_REPS（默认为160）*session.size个键值。还需要与session的权重相乘

2.使用treemap的tailMap方法高效定位session位置（为的是与jdk1.5兼容，jdk6使用的是ceilingKey，metaq做消费者负载均衡默认使用的就是ceilingKey）。

3.性能与hash算法密切相关，因为使用的是sessionList.get(this.random.nextInt(size))方式，如果在buildmap时，多个session的hash映射到同一个key，使用get方式获取时，可能得不到正确的缓存位置。

4.使用虚拟节点，默认个数为session的个数的160倍，也跟session的权重有关，具体一致性hash的虚拟节点介绍，可以参考http://blog.csdn.net/x15594/article/details/6270242

/** *Copyright [2009-2010] [dennis zhuang([e-mail])] *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 net.rubyeye.xmemcached.impl;import java.net.InetSocketAddress;import java.util.ArrayList;import java.util.Collection;import java.util.List;import java.util.Random;import java.util.SortedMap;import java.util.TreeMap;import net.rubyeye.xmemcached.HashAlgorithm;import net.rubyeye.xmemcached.networking.MemcachedSession;import com.google.code.yanf4j.core.Session;/** * ConnectionFactory instance that sets up a ketama compatible connection. * * <p> * This implementation piggy-backs on the functionality of the * <code>DefaultConnectionFactory</code> in terms of connections and queue * handling. Where it differs is that it uses both the <code> * KetamaNodeLocator</code> * and the <code>HashAlgorithm.KETAMA_HASH</code> to provide consistent node * hashing. * * @see http://www.last.fm/user/RJ/journal/2007/04/10/392555/ * * </p> *//** * Consistent Hash Algorithm implementation,based on TreeMap.tailMap(hash) * method. *  * @author dennis *  */public class KetamaMemcachedSessionLocator extendsAbstractMemcachedSessionLocator {static final int NUM_REPS = 160;private transient volatile TreeMap<Long, List<Session>> ketamaSessions = new TreeMap<Long, List<Session>>();private final HashAlgorithm hashAlg;private volatile int maxTries;private final Random random = new Random();/** * compatible with nginx-upstream-consistent,patched by wolfg1969 */static final int DEFAULT_PORT = 11211;private final boolean cwNginxUpstreamConsistent;public KetamaMemcachedSessionLocator() {this.hashAlg = HashAlgorithm.KETAMA_HASH;this.cwNginxUpstreamConsistent = false;}public KetamaMemcachedSessionLocator(boolean cwNginxUpstreamConsistent) {this.hashAlg = HashAlgorithm.KETAMA_HASH;this.cwNginxUpstreamConsistent = cwNginxUpstreamConsistent;}public KetamaMemcachedSessionLocator(HashAlgorithm alg) {this.hashAlg = alg;this.cwNginxUpstreamConsistent = false;}public KetamaMemcachedSessionLocator(HashAlgorithm alg,boolean cwNginxUpstreamConsistent) {this.hashAlg = alg;this.cwNginxUpstreamConsistent = cwNginxUpstreamConsistent;}public KetamaMemcachedSessionLocator(List<Session> list, HashAlgorithm alg) {super();this.hashAlg = alg;this.cwNginxUpstreamConsistent = false;this.buildMap(list, alg);}private final void buildMap(Collection<Session> list, HashAlgorithm alg) {TreeMap<Long, List<Session>> sessionMap = new TreeMap<Long, List<Session>>();for (Session session : list) {String sockStr = null;if (this.cwNginxUpstreamConsistent) {InetSocketAddress serverAddress = session.getRemoteSocketAddress();sockStr = serverAddress.getAddress().getHostAddress();if (serverAddress.getPort() != DEFAULT_PORT) {sockStr = sockStr + ":" + serverAddress.getPort();}} else {if (session instanceof MemcachedTCPSession) {// Always use the first time resolved address.sockStr = ((MemcachedTCPSession) session).getInetSocketAddressWrapper().getRemoteAddressStr();}if (sockStr == null) {sockStr = String.valueOf(session.getRemoteSocketAddress());}}/** * Duplicate 160 X weight references */int numReps = NUM_REPS;if (session instanceof MemcachedTCPSession) {numReps *= ((MemcachedSession) session).getWeight();}if (alg == HashAlgorithm.KETAMA_HASH) {for (int i = 0; i < numReps / 4; i++) {byte[] digest = HashAlgorithm.computeMd5(sockStr + "-" + i);for (int h = 0; h < 4; h++) {long k = (long) (digest[3 + h * 4] & 0xFF) << 24| (long) (digest[2 + h * 4] & 0xFF) << 16| (long) (digest[1 + h * 4] & 0xFF) << 8| digest[h * 4] & 0xFF;this.getSessionList(sessionMap, k).add(session);}}} else {for (int i = 0; i < numReps; i++) {long key = alg.hash(sockStr + "-" + i);this.getSessionList(sessionMap, key).add(session);}}}this.ketamaSessions = sessionMap;this.maxTries = list.size();}private List<Session> getSessionList(TreeMap<Long, List<Session>> sessionMap, long k) {List<Session> sessionList = sessionMap.get(k);if (sessionList == null) {sessionList = new ArrayList<Session>();sessionMap.put(k, sessionList);}return sessionList;}public final Session getSessionByKey(final String key) {if (this.ketamaSessions == null || this.ketamaSessions.size() == 0) {return null;}long hash = this.hashAlg.hash(key);Session rv = this.getSessionByHash(hash);int tries = 0;while (!this.failureMode && (rv == null || rv.isClosed())&& tries++ < this.maxTries) {hash = this.nextHash(hash, key, tries);rv = this.getSessionByHash(hash);}return rv;}public final Session getSessionByHash(final long hash) {TreeMap<Long, List<Session>> sessionMap = this.ketamaSessions;if (sessionMap.size() == 0) {return null;}Long resultHash = hash;if (!sessionMap.containsKey(hash)) {// Java 1.6 adds a ceilingKey method, but xmemcached is compatible// with jdk5,So use tailMap method to do this.SortedMap<Long, List<Session>> tailMap = sessionMap.tailMap(hash);if (tailMap.isEmpty()) {resultHash = sessionMap.firstKey();} else {resultHash = tailMap.firstKey();}}//// if (!sessionMap.containsKey(resultHash)) {// resultHash = sessionMap.ceilingKey(resultHash);// if (resultHash == null && sessionMap.size() > 0) {// resultHash = sessionMap.firstKey();// }// }List<Session> sessionList = sessionMap.get(resultHash);if (sessionList == null || sessionList.size() == 0) {return null;}int size = sessionList.size();return sessionList.get(this.random.nextInt(size));}public final long nextHash(long hashVal, String key, int tries) {long tmpKey = this.hashAlg.hash(tries + key);hashVal += (int) (tmpKey ^ tmpKey >>> 32);hashVal &= 0xffffffffL; /* truncate to 32-bits */return hashVal;}public final void updateSessions(final Collection<Session> list) {this.buildMap(list, this.hashAlg);}}