package com.github.ltsopensource.core.support;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.util.List;
import java.util.SortedMap;
import java.util.TreeMap;
/**
* 一致性hash算法
* Robert HG (254963746@qq.com) on 3/25/15.
*/
public class ConsistentHashSelector<S> { // S类封装了机器节点的信息
private TreeMap<Long, S> nodes; // 虚拟节点
private final int NODE_NUM = 100; // 每个机器节点关联的虚拟节点个数
public ConsistentHashSelector(List<S> shards) {
super();
nodes = new TreeMap<Long, S>();
for (int i = 0; i != shards.size(); ++i) { // 每个真实机器节点都需要关联虚拟节点
final S shardInfo = shards.get(i);
for (int n = 0; n < NODE_NUM; n++)
// 一个真实机器节点关联NODE_NUM个虚拟节点
nodes.put(hash("SHARD-" + i + "-NODE-" + n), shardInfo);
}
}
public S selectForKey(String key) {
SortedMap<Long, S> tail = nodes.tailMap(hash(key)); // 沿环的顺时针找到一个虚拟节点
if (tail.size() == 0) {
return nodes.get(nodes.firstKey());
}
return tail.get(tail.firstKey()); // 返回该虚拟节点对应的真实机器节点的信息
}
/**
* MurMurHash算法,是非加密HASH算法,性能很高,
* 比传统的CRC32,MD5,SHA-1(这两个算法都是加密HASH算法,复杂度本身就很高,带来的性能上的损害也不可避免)
* 等HASH算法要快很多,而且据说这个算法的碰撞率很低.
* http://murmurhash.googlepages.com/
*/
private Long hash(String key) {
ByteBuffer buf = ByteBuffer.wrap(key.getBytes());
int seed = 0x1234ABCD;
ByteOrder byteOrder = buf.order();
buf.order(ByteOrder.LITTLE_ENDIAN);
long m = 0xc6a4a7935bd1e995L;
int r = 47;
long h = seed ^ (buf.remaining() * m);
long k;
while (buf.remaining() >= 8) {
k = buf.getLong();
k *= m;
k ^= k >>> r;
k *= m;
h ^= k;
h *= m;
}
if (buf.remaining() > 0) {
ByteBuffer finish = ByteBuffer.allocate(8).order(
ByteOrder.LITTLE_ENDIAN);
// for big-endian version, do this first:
// finish.position(8-buf.remaining());
finish.put(buf).rewind();
h ^= finish.getLong();
h *= m;
}
h ^= h >>> r;
h *= m;
h ^= h >>> r;
buf.order(byteOrder);
return h;
}
}