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 extends
AbstractMemcachedSessionLocator {

	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);
	}
}