package com.jackiesteed.algorithms;import java.util.*;/*** 虚拟节点个数需要事先确定好, 而且不能修改.* 核心代码模拟了一下remapping的逻辑, 如何做到均衡分配.* 如何保证一致性呢? 每次重新分配的时候, 只有被移动的虚拟节点才会收到影响, 保持了一致性.* 一致性hash 简单实现, 单线程使用* key 固定为String, value 支持泛型* 感觉只是走了下流程, 离实际使用在性能, 并发方面还要改进.* Created by jackie on 5/24/15.*/
public class ConsistentHashMap <T>{/*** 存储机器id对应的真实ip, 用来做远程调用.*/private Map<String, Stack<Integer>> realNodeMap;/*** 虚拟节点到真实节点的映射* 虚拟节点数根据现实情况来权衡.* 节点数极大的话, 可以保证趋向于决定均匀分布, 但是remapping的时间开销交大.* 节点数少, 压力不一定非常均匀, 但是remapping速度回块一些.* 但是虚拟节点数, 一定要是服务所需的机器数的N倍之多.*/private String[] virtualRealMap;/*** 用来模拟分布式系统里面的每台机器* key 是ip, value mock机器里面的存储, 是个java.util.Map*/private Map<String, Map<String, T>> hostMock;/*** 构造函数里面初始化虚拟节点个数.* @param virtualNodeCount*/public ConsistentHashMap(int virtualNodeCount){virtualRealMap = new String[virtualNodeCount];realNodeMap = new HashMap<String, Stack<Integer>>();hostMock = new HashMap<String, Map<String, T>>();}/*** 对外接口, 写入数据到分布式存储里面.* @param key* @param value*/public void put(String key, T value){put(getRealHashCode(key), key, value);}/*** 对外暴露的接口, 从分布式存储提取数据.* @param key* @return*/public T get(String key){return get(getRealHashCode(key), key);}/*** 根据key计算需要映射的真是机器的id.* @param key* @return*/private String getRealHashCode(String key){//字符串原本的hash值int oriHashCode = key.hashCode();//key对应的虚拟节点的hash值int virtualHashCode = (oriHashCode % virtualRealMap.length + virtualRealMap.length) % virtualRealMap.length;//映射出来的真实String ip  = virtualRealMap[virtualHashCode];return ip;}/*** 这个其实就是和真是的数据存储进行交互了, ip用来访问真实机器.* @param ip* @param key* @param value*/private void put(String ip, String key, T value){hostMock.get(ip).put(key, value);}/*** realHashCode表示对应机器的id, 从这台机器上面获取数据.* @param* @param key* @return*/private T get(String ip, String key){return hostMock.get(ip).get(key);}/*** 添加一台机器.* @param ip*/public void addHost(String ip){/*** 如果已经存在了, 就不加入*/if(realNodeMap.containsKey(ip))return;hostMock.put(ip, new HashMap<String, T>());//目前还没有机器加入, 那么需要走特殊的初始化流程if(realNodeMap.isEmpty()){Stack<Integer> virtualIds = new Stack<Integer>();for(int i = 0; i < virtualRealMap.length; i++){virtualRealMap[i] = ip;virtualIds.push(i);}realNodeMap.put(ip, virtualIds);return;}int expectCount = virtualRealMap.length / (realNodeMap.size() + 1);Stack<Integer> virtualIds = new Stack<Integer>();while(virtualIds.size() < expectCount){for(Map.Entry<String, Stack<Integer>> entry : realNodeMap.entrySet()){Stack<Integer> oldVirtualIds = entry.getValue();while(oldVirtualIds.size() > expectCount){int virtualId = oldVirtualIds.pop();virtualRealMap[virtualId] = ip;virtualIds.push(virtualId);}if(virtualIds.size() >= expectCount)break;}}realNodeMap.put(ip, virtualIds);}/*** 从分布式系统里删除一台机器* @param ip*/public void deleteHost(String ip){if(!realNodeMap.containsKey(ip))return;hostMock.remove(ip);Stack<Integer> virtualIds = realNodeMap.get(ip);int expectCount = virtualRealMap.length / (realNodeMap.size() - 1);realNodeMap.remove(ip);//说明这是最后一个ip, 没有办法把这个ip映射的虚拟节点重新映射了, 直接全部都删掉.if(realNodeMap.isEmpty()){for(int i = 0; i < virtualRealMap.length; i++){virtualRealMap[i] = null;}return;}while(!virtualIds.isEmpty()){for(Map.Entry<String, Stack<Integer>> entry : realNodeMap.entrySet()){Stack<Integer> oldVirtualIds = entry.getValue();while(oldVirtualIds.size() < expectCount && !virtualIds.isEmpty()){int virtualId = virtualIds.pop();virtualRealMap[virtualId] = ip;oldVirtualIds.push(virtualId);}if(virtualIds.isEmpty())break;}//如果不增加, 有可能永远分配不出去.expectCount++;}}public void dump(){System.out.println("=====================================================");System.out.println("VirtualNodeCount : " + virtualRealMap.length);for(Map.Entry<String, Stack<Integer>> entry : realNodeMap.entrySet()){System.out.print(entry.getKey() + " : | ");for(Integer virtualId : entry.getValue()){System.out.format("%2d | ", virtualId);}System.out.println();}System.out.println("=====================================================");}public static void main(String[] args){ConsistentHashMap<String> consistentHashMap = new ConsistentHashMap<String>(30);consistentHashMap.addHost("192.168.0.1");consistentHashMap.addHost("192.168.0.2");consistentHashMap.addHost("192.168.0.4");consistentHashMap.addHost("192.168.0.3");consistentHashMap.addHost("192.168.0.8");consistentHashMap.addHost("192.168.0.13");consistentHashMap.addHost("192.168.0.14");consistentHashMap.deleteHost("192.168.0.4");consistentHashMap.deleteHost("192.168.0.1");consistentHashMap.deleteHost("192.168.0.2");consistentHashMap.addHost("123.123.123.123");consistentHashMap.addHost("123.123.123.124");consistentHashMap.addHost("123.123.123.125");consistentHashMap.dump();consistentHashMap.put("jackie", "abc");System.out.println(consistentHashMap.get("jackie"));}
}